Alkanes, their structure and properties. What are alkanes: structure and chemical properties
Alkanes (saturated hydrocarbons, paraffins) - saturated hydrocarbons, in the molecules of which carbon and hydrogen atoms are linked only by -bonds.
The general formula for alkanes is FROM n H 2 n +2 .
In alkane molecules, carbon atoms are in the sp 3 -hybrid state and each of them forms 4 -bonds with carbon and (or) hydrogen atoms. The sp 3 -hybridization state is characterized by the tetrahedral configuration of the carbon atom.
Natural sources
Natural sources of alkanes are oil, associated petroleum gases and natural gas.
Oil is of the utmost importance. Oil is a complex mixture of organic compounds, mainly hydrocarbons. It also contains small amounts of oxygen-, nitrogen- and sulfur-containing compounds. Depending on the oil field, the hydrocarbon composition can be represented by both alkanes and other groups of hydrocarbons. Oil is used as a fuel and a valuable raw material for the chemical industry.
Currently, there are several ways of industrial oil refining.
Distillation. It is a physical way of refining oil. Petroleum hydrocarbons differ in molecular weight and, consequently, in boiling points. Therefore, by simple distillation, oil can be divided into a number of fractions that differ in boiling points. In this way, petroleum ether is obtained (up to 60 ° C; C 5 –C 6); aviation gasoline (60–180 ° С; С 6 –С 10); gasoline (up to 200 ° С; С 11 –C 12); kerosene and jet fuel (175–280 ° С; С 7 –С 14); diesel fuel (200-350 ° C; C 13 -C 18). The fraction with a boiling point above 360 \u200b\u200b° C is called fuel oil (C 18 –C 25). The fraction containing hydrocarbons C 25 and higher hydrocarbons is not distilled; heavy oils, petroleum jelly, paraffin are obtained from it.
Catalytic cracking. Unlike distillation, cracking (from the English cracking - cleavage) is a process of chemical refining of oil, which consists in the splitting of higher hydrocarbons and obtaining more valuable lower alkanes, constituting, for example, a gasoline fraction, etc. When oil is heated to a temperature of 500 ° C in the presence of aluminosilicate catalysts (aluminum oxide Al 2 O 3 on silica gel SiO 2) the bonds between the carbon atoms in the chain are broken and alkanes with a lower number of carbon atoms (C 5 –C 10) and a branched chain are formed.
Natural gas consists mainly of methane (up to 95%) with minor admixtures of ethane and propane. Associated petroleum gas, in addition to methane, contains a significant amount of ethane, propane, butane. Natural and petroleum gases are used as a high-calorific fuel, as well as feedstock for a number of large chemical industries. Methane is the raw material for the most important chemical processes for the production of carbon and hydrogen, acetylene, oxygen-containing organic compounds - alcohols, aldehydes, acids.
The finely dispersed carbon (soot) obtained by thermal decomposition (pyrolysis) of methane (1) is used as a filler in the production of rubber and printing inks. Hydrogen is used in various syntheses, including the synthesis of ammonia. High temperature cracking of methane (2) produces acetylene. The required high temperature (1400–1600 ° C) is created by an electric arc. Mild oxidation of methane with atmospheric oxygen in the presence of catalysts (3) yields substances valuable for organic synthesis and the production of plastics: methyl alcohol, formaldehyde, and formic acid.
CH 4 C + 2H 2 (1)
2CH 4 
НССН + 3Н 2 (2)
Methods of obtaining
The most important synthetic methods of obtaining:
1. Hydrogenation of unsaturated hydrocarbons.
CH 3 –CH \u003d CH – CH 3 + H 2 
CH 3 -CH 2 -CH 2 -CH 3
butene-2 \u200b\u200bbutane
(the catalyst is nickel, platinum or palladium).
2. Obtaining from halogen derivatives (reaction of Sh.A. Wurtz, 1854).
2СН 3 I + 2Na CH 3 –CH 3 + 2NaI.
iodomethane ethane
3. Obtaining from salts of carboxylic acids (laboratory method).
CH 3 COONa (s) + NaOH (s). 
CH 4 + Na 2 CO 3.
sodium acetate methane
In these reactions, as a rule, not pure sodium hydroxide is taken, but its mixture with calcium hydroxide, called soda lime ... The starting materials are pre-calcined.
4. Direct fusion from carbon and hydrogen.
A significant amount of methane is generated in an electric arc burning in a hydrogen atmosphere:
C + 2H 2 CH 4.
The same reaction takes place at a temperature of 400–500C, a pressure of about 30 MPa, and in the presence of a catalyst - iron or manganese. This reaction is of great practical importance for obtaining a mixture of hydrocarbons - synthetic gasoline.
5. Production from synthesis gas.
nCO + (2n + 1) H 2 
C n H 2n + 2 + nH 2 O.
synthesis gas
Physical properties
Under normal conditions (at 25 ° C and atmospheric pressure), the first four members of the homologous series of alkanes (C 1 - C 4) are gases. Normal alkanes from pentane to heptadecane (C 5 - C 17) are liquids, starting from C 18 and above are solids. As the number of carbon atoms in the chain increases, i.e. with an increase in the relative molecular weight, the boiling and melting points of alkanes increase. With the same number of carbon atoms in the molecule, branched alkanes have lower boiling points than normal alkanes.
Alkanes are practically insoluble in water, since their molecules are low-polarity and do not interact with water molecules. Liquid alkanes mix easily with each other. They dissolve well in non-polar organic solvents such as benzene, carbon tetrachloride (carbon tetrachloride), diethyl ether, etc.
Chemical properties
Chemically, alkanes are inactive, for which they were called paraffins (from lat. parum affinis - devoid of affinity). This is explained by the strength of -bonds, the stability of which is due to the small size of the C atom and its tetrahedral configuration, which contributes to the maximum concentration of the electron density between the atomic nuclei. The C – H-bond is slightly polarized due to the proximity of the electronegativities of the C and H atoms. Because of this, alkanes are low-polarity substances and difficult to polarize; acids, alkalis, common oxidants (for example, KMnO 4), metals do not act on them.
For alkanes, a homolytic (radical) rupture of bonds is possible, with the reactions of substitution of H atoms, cleavage of the carbon skeleton (cracking), and oxidation (partial or complete).
1. Substitution reactions.
– Chlorination.
As an example of substitution reactions, let us consider the methane chlorination reaction, which belongs to radical chain reactions. The process proceeds vigorously in the light or when heated (up to 250–400 о С). The first stage is called inception, or initiating, chains:
(under the action of the energy of light quanta or high temperature, some chlorine molecules are homolytically split into 2 atoms with unpaired electrons - free radicals).
Cl + CH 4 HCl + CH 3 (methyl); СН 3 + Сl 2 СН 3 Сl + Сl, etc.
Open circuit occurs when two free radicals collide; in this case, molecules C 2 H 6, CH 3 Cl, Cl 2 can be formed.
In the chlorination of other alkanes, the substitution is easiest at the tertiary carbon atom (the atom that spends 3 valences on the C – C bond), then at the secondary, and lastly at the primary. In the formation of polyhalogenated compounds, chlorine atoms predominantly replace hydrogen atoms at the same or at neighboring carbon atoms. - The patterns of V.V. Markovnikov.
– Nitration (reaction of M.I.Konovalov, 1888).
The reaction is carried out with 10% nitric acid at a temperature of about 140 ° C and low pressure.
2. Oxidation.
In industry, alkanes are oxidized with atmospheric oxygen on manganese catalysts at a temperature of about 200 ° C. In this case, C – C bonds are cleaved and low molecular weight oxygen-containing compounds are obtained - alcohols, aldehydes, ketones, carboxylic acids (see. Natural sources). Alkanes burn in air with the release of a large amount of heat. Therefore, they are widely used in everyday life and technology as a high-calorie fuel.
CH 4 + 2O 2 CO 2 + 2H 2 O + 890 kJ.
Alkenes
Alkenes (ethylene hydrocarbons) - unsaturated hydrocarbons, the molecules of which contain one double bond C \u003d C.
General formula of alkenes - FROM n H 2 n .
The double bond is formed by two pairs of generalized electrons. Carbon atoms linked by a double bond are in the state of sp 2 -hybridization, each of them forms 3 -bonds lying in one plane at an angle of 120. Non-hybrid p-orbitals of carbon atoms are located perpendicular to the plane of the -bonds and are parallel to each other and, due to "lateral" overlap, form an-bond, the electron cloud of which is located partly above and partly below the plane of the molecule.
Methods of obtaining
1. Alkane pyrolysis.
Pyrolysis - the processes of chemical transformations of organic compounds that occur during high temperature.
The pyrolysis of alkanes is the most important industrial method for producing alkenes from high-boiling oil fractions. Under the action of strong heating (up to 700 o C) in alkane molecules, the C – C and C – H -bonds are homolytically cleaved. In this case, various free radicals are formed. As a result of the interaction of radicals with alkane molecules and with each other, a mixture of low molecular weight alkanes, alkenes and hydrogen is formed.
2. Dehydrogenation of alkanes.
3. Dehydration of alcohols.
CH 3 –CH 2 –OH 
CH 2 \u003d CH 2 + H 2 O
Reaction mechanism:
CH 3 –CH 2 –OH + HO – SO 3 H CH 3 –CH 2 –O – SO 3 H + H 2 O
ethanol sulfuric acid ethyl sulfur acid
CH 3 –CH 2 –O – SO 3 H 
CH 2 \u003d CH 2 + HO – SO 3 H
During the dehydration of alcohols, hydrogen is preferentially removed by rule A.M. Zaitseva (1875) - from the one of the neighboring carbon atoms, which is poorer in hydrogen (least hydrogenated):
4. Dehydrohalogenation of halogenated derivatives.
This process also proceeds according to the rule of A.M. Zaitseva:
5. Dehalogenation of dihalogenated derivatives.
Physical properties
In terms of physical properties, alkenes differ little from alkanes with the same number of carbon atoms in a molecule. Lower homologues С 2 –С 4 under normal conditions. - gases; С 5 –С 17 - liquids; higher homologues are solids. Alkenes are insoluble in water. Well soluble in organic solvents.
Chemical properties
A characteristic feature of p-electrons is their mobility; they are less firmly held by atomic nuclei than -electrons. Therefore, the -bond under the action of electrophilic reagents readily polarizes and breaks heterolytically, and therefore addition reactions are characteristic of alkenes.
In addition, alkenes easily enter into oxidation and double bond polymerization reactions.
1. Electrophilic addition reactions.
– Hydrohalogenation (addition of hydrogen halide).
СН 3 –СН \u003d СН – СН 3 + НСl СН 3 –СН 2 –СССl – СН 3
If an unsymmetrical alkene (for example, propene) participates in the reaction, then the preferred direction of the reaction is determined the rule of V.V. Markovnikova (1869): when molecules of the HX type (where X is a halogen atom, an OH group, etc.) are attached to asymmetric alkenes, the H atom is attached to the more hydrogenated C atom of the alkene molecule:
CH 3 –CH \u003d CH 2 + HBr CH 3 –CHBr – CH 3.
propene 2-bromopropane
This can be explained on the basis of the following considerations:
one). The distribution of electron density in a non-reactive molecule ( static factor) - the methyl radical repels the electron density of the C – C bond. As a result, the -bond polarization occurs:
The polar HBr molecule is oriented by the H atom to the propene molecule due to electrostatic attraction to the double bond electrons; ionic cleavage of the Н – Вr bond occurs, and the Н + ion is attracted mainly to the first C atom of the propene molecule and attaches to it due to the пары-bond electron pair. A “+” charge appears on the middle C atom of the propene molecule (the molecule turns into a carbocation):
Then the bromide ion is attached to the positively charged C atom of the carbocation:
2). The stability of the resulting carbocation ( dynamic factor). The interaction of the H + ion with a propene molecule can theoretically lead to the formation of a carbocation 
(propenia ion-1). However, this does not happen because due to the fact that alkyl radicals (CH 3 , CH 3 –CH 2 ) repel the electron density from themselves and quench the positive charge of the carbon atom, the propenia-2 ion 
turns out to be more stable than propenia-1 ion. This also speaks in favor of directing the reaction according to the Markovnikov rule.
– Hydration.
This process also proceeds according to the rule of V.V. Markovnikova:
– Halogenation (qualitative double bond reaction ).
Discoloration of bromine water is observed.
2. Oxidation and reduction reactions.
– The reaction of E.E. Wagner(1888) - a qualitative reaction to a double bond.
3СН 2 \u003d СН 2 + 2KMnO 4 + 4H 2 O 3HO – CH 2 –CH 2 –OH + 2KOH + 2MnO 2
ethylene glycol
Isolation of a brown precipitate of manganese (IV) oxide and discoloration of the potassium permanganate solution are observed.
In an acidic medium, the reaction of alkenes with potassium permanganate proceeds with the cleavage of the C \u003d C bond, therefore the corresponding carboxylic acids or ketones are formed:
5Н 3 C – CH \u003d CH – CH 3 + 8KMnO 4 + 12H 2 SO 4 10H 3 C – COOH + 8MnSO 4 + 4K 2 SO 4 + 12H 2 O
butene-2 acetic acid
5Н 3 C – CH \u003d C (СH 3) –CH 3 + 6KMnO 4 + 9H 2 SO 4
2-methylbutene-2
5H 3 C – COOH + 5H 3 C – CO – CH 3 + 6MnSO 4 + 3K 2 SO 4 + 9H 2 O
acetic acid propanone (acetone)
– Hydrogenation.
H 3 C – CH \u003d CH – CH 3 + H 2 
Н 3 C – CH 2 –CH 2 –CH 3
In addition to nickel, platinum and palladium can be used as catalysts in this reaction.
3. Polymerization reactions.
Polymerization reaction - This is the sequential attachment of molecules of unsaturated compounds to each other with the formation of a high-molecular compound - a polymer.
The most common hydrocarbon polymers are polyethylene and polypropylene.
nН 2 С \u003d СН 2 (–СН 2 –СН 2 -) n
ethylene polyethylene
Limit hydrocarbons, or paraffins, are those biocompounds in whose molecules the carbon atoms are connected by a simple (single) bond, and all other valence units are saturated with hydrogen atoms.
Alkanes: physical properties
The elimination of hydrogen from the alkane molecule, or dehydrogenation, in the presence of catalysts and heating (up to 460 ° C) allows obtaining the necessary alkenes. Methods for the oxidation of alkanes at low temperatures in the presence of catalysts (magnesium salts) have been developed. This allows you to directly influence the course of the reaction and obtain the necessary oxidation products in the process of chemical synthesis. For example, the oxidation of higher alkanes produces a variety of higher alcohols or higher fatty acids.
The cleavage of alkanes also occurs under other conditions (combustion, cracking). Saturated hydrocarbons burn with a blue flame, generating enormous amounts of heat. These properties allow them to be used as a high-calorie fuel both in everyday life and in industry.
The content of the article
ALKANES AND CYCLOALCANS- hydrocarbons, in which all carbon atoms are connected to each other and to hydrogen atoms by simple (single) bonds. Alkanes (synonyms - saturated hydrocarbons, saturated hydrocarbons, paraffins) - hydrocarbons with general formula C n H 2 n+2, where n - the number of carbon atoms. The familiar polyethylene has the same formula, only the value n it is very large and can reach tens of thousands. In addition, polyethylene contains molecules of different lengths. In cycloalkanes, carbon atoms form a closed chain; if there is only one cycle, the cycloalkane formula C n H 2 n .
Depending on the order in which carbon atoms are joined into a chain, alkanes are divided into linear and branched. Accordingly, for alkanes with n і 4 more than one substance with the same formula may exist. Such substances are called isomers (from the Greek. isis - equal, equal and meros - share, part.
Alkane names.
The word "alkane" is of the same origin as "alcohol" ( see below). The outdated term "paraffin" comes from the Latin parum - little, little, and affinis - related; paraffins have low reactivity with respect to most chemical reagents. Many paraffins are homologous; in the homologous series of alkanes, each subsequent member differs from the previous one by one CH 2 methylene group. The term comes from the Greek homologos - appropriate, similar.
Nomenclature (from lat. nomenclatura - list of names) the names of alkanes are built according to certain rules, which are not always unambiguous. So, if there are various substituents in the alkane molecule, then in the name of the alkane they are listed in alphabetical order. However, in different languages this order may vary. For example, hydrocarbon СН 3 –СН (СН 3) –СН (С 2 Н 5) –СН 2 –СН 2 –СН 3 in accordance with this rule in Russian will be called 2-methyl-3-ethylhexane, and in English 3-ethyl-2-methylhexane ...
In accordance with the name of the hydrocarbon, alkyl radicals are also called: methyl (CH 3 -), ethyl (C 2 H 5 -), isopropyl (CH 3) 2 CH-, sec-butyl C 2 H 5 -CH (CH 3) -, rubs-butyl (CH 3) 3 C - etc. Alkyl radicals are included as a whole in many organic compounds; in a free state, these particles with an unpaired electron are extremely active.
Some alkane isomers also have trivial names ( cm... TRIVIAL NAMES OF SUBSTANCES), e.g. isobutane (2-methylpropane), isooctane (2,2,4-trimethylpentane), neopentane (2,3-dimethylpropane), squalane (2,6,10,15,19,23-hexamethyltetracosane) , whose name comes from lat squalus - shark (unsaturated derivative of squalane - squalene, an important compound for metabolism, was first discovered in shark liver). The trivial name of the pentyl radical (C 5 H 11) - amyl is also often used. It comes from the Greek. amylon - starch: once isoamyl alcohol C 5 H 11 OH (3-methylbutanol-1) was called "amyl alcohol of fermentation", since it forms the basis of fusel oil, and it is formed as a result of fermentation of sugary substances - products of starch hydrolysis.
The simplest member of the series of cycloalkanes C n H 2 n - cyclopropane ( n \u003d 3). Its homologues are named the same as alkanes with the addition of the prefix "cyclo" (cyclobutane, cyclopentane, etc.). In cycloalkanes, isomerism is possible, associated with the presence of side alkyl groups and their arrangement in the ring. For example, cyclohexane, methylcyclopentane, 1,1-, 1,2- and 1,3-dimethylcyclobutanes, 1,1,2- and 1,2,3-trimethylcyclopropanes are isomeric.
The number of alkane isomers increases sharply with an increase in the number of carbon atoms. The names of some alkanes, as well as the theoretical number of their possible isomers, are given in the table.
| Formula | Name | Number of isomers | Formula | Name | Number of isomers |
| CH 4 | Methane | 1 | S 11 N 24 | Undecane | 159 |
| C 2 H 6 | Ethane | 1 | C 12 H 26 | Dodecane | 355 |
| C 3 H 8 | Propane | 1 | S 13 N 28 | Tridecan | 802 |
| C 4 H 10 | Butane | 2 | S 14 N 30 | Tetradecan | 1858 |
| C 5 H 12 | Pentane | 3 | S 15 N 32 | Pentadecane | 4347 |
| C 6 H 14 | Hexane | 5 | S 20 N 42 | Eicosan | 366319 |
| C 7 H 16 | Heptane | 9 | S 25 N 52 | Pentacosan | 36797588 |
| C 8 H 18 | Octane | 18 | C 30 N 62 | Triacontan | 4111846763 |
| C 9 H 20 | Nonan | 35 | C 40 N 82 | Tetrakontan | 62481801147341 |
| S 10 N 22 | Dean | 75 | S 100 N 202 | Hektan | about 5.921 10 39 |
Dealing with most of the nomenclature names of saturated hydrocarbons is not very difficult even for those who did not study Greek in a classical gymnasium. These names come from Greek numerals with the addition of the suffix -an. It is more difficult with the first members of the series: they use not numerals, but other Greek roots associated with the names of the corresponding alcohols or acids. These alcohols and acids were known long before the discovery of the corresponding alkanes; an example is ethyl alcohol and ethane (obtained only in 1848).
Methane (as well as methanol, methyl, methylene, etc.) have a common root "meth", which in chemistry means a group containing one carbon atom: methyl CH 3, methylene (methylidene) CH 2, methine (methylidine) CH. Historically, the first such substance was methyl (aka wood) alcohol, methanol, which was previously obtained by dry distillation of wood. Its name comes from the Greek words methy - to intoxicate wine and hile - forest (so to speak, "woody wine"). The most striking thing here is that methane, amethyst and honey share a common root! In ancient times gems endowed magical properties (and many still believe this). So, it was believed that beautiful purple stones protect from intoxication, especially if a drinking cup is made of this stone. Together with the negative prefix it turned out amethystos - counteracting intoxication. The word honey is present, it turns out, in almost all European languages: English. mead - honey (as a drink), German Met (in Old German metu), Dutch mede, Swedish mjöd, Danish mjød, Lithuanian and Latvian medus, not to mention the Slavic languages. All of these words, including Greek, come from the Indo-European medhu, meaning sweet drink. The Greek brandy Metaxa has gone not far from them, although it is not at all sweet.
Ethane (as well as ether, ethanol, alcohol, alkane) have a common origin. The ancient Greek philosophers called aither a certain substance that permeates the cosmos. When the alchemists in the 8th century. obtained an easily evaporating liquid from tartaric alcohol and sulfuric acid, it was called sulfuric ether. In the 19th century. found out that sulfuric ether (in English ether) refers to the so-called ethers and contains a group of two carbon atoms - the same as ethyl alcohol (ethanol); this grouping was named ethyl (ethyl). Thus, the name of the substance “ethyl ether” (C 2 H 5 –O – C 2 H 5) is essentially “oil oil”.
The name ethane comes from "ethyl". One of the names for ethanol, alcohol, is of the same origin as the word alkane (and also alkene, alkyne, alkyl). In Arabic al-kohl means powder, powder, dust. At the slightest breath, they rise into the air, like wine vapors - the "alcohol of wine", which eventually turned into just alcohol.
Why is the letter "t" in "ethane" and "ethanol", and "f" in "ether"? After all english language, unlike Russian, the words "ether" (ether) and "ethyl" (ethyl) have a similar spelling and sound. Th comes from the Greek letter q (theta); in Russian until 1918, the letter "phita" had the same style, which, however, was pronounced as "f" and was used for the sole purpose of distinguishing words in which this letter comes from the Greek q and 247 ("fi"). In Western European languages, Greek. j went to ph and q went to th. In Russian, in many words "fit" as early as the 18th century. was replaced by the letter "f": theater instead of "qeaftr", mathematics instead of "mathematics", theory instead of "theory" ... In this regard, it is interesting that in Dahl's dictionary, published in 1882, it is written eqir, and in encyclopedic dictionary Brockhaus and Efron (1904) - "ether".
By the way, esters in Western languages \u200b\u200bare ester, not ether. But there is no word "ester" in Russian, so any chemist is struck by the illiterate translation of English polyester on textile labels as "polyester" instead of "polyester", "polyester fiber" (polyesters include, for example, lavsan, terylene, dacron).
The names "propane" and "butane" come from the names of the corresponding acids - propionic and butanoic (butyric). Propionic acid is the "first" (ie with the shortest chain) that is found in fats ( cm... FATS AND OILS), and its name is derived from the Greek. protos - first and pion - fat. Butane and butanoic acid (rus. butyric acid) - from the Greek. butyron - butter; in Russian, butyrates are salts and esters of butyric acid. This acid is released when the oil is rancid.
Further, starting with pentane, the names are derived from Greek numerals. A rare exception is cetane, one of the names for C 16 hexadecane. This word comes from the name of cetyl alcohol, which was obtained in 1823 by the French chemist Michel Eugene Chevreul. Chevreul isolated this substance from spermacet, a wax-like substance from the head of a sperm whale. The word spermaceti comes from the Greek sperma - seed and ketos - a large marine animal (whale, dolphin). From the Latin spelling of the second word (cetus), cetyl alcohol C 16 H 33 OH (hexadecanol) and cetane come from.
There are many words in the Russian language with the same roots as those of alkanes: Pentagon, heptahord (scale of 7 steps), dodecaphony (method of musical composition), octave, decima and undecima (musical intervals), octet and nonet (ensembles of 8 and 9 musicians), pentode, hexode and heptode (radio tubes); hexameter (verse size), octahedron, decade, decan, hectare, October, December, etc. etc.
The alkane with the longest molecules was synthesized by British chemists in 1985. It is nonacontatricthane C 390 H 782, containing a chain of 390 carbon atoms. The researchers were interested in how such long chains would pack during crystallization (flexible hydrocarbon chains can fold easily).
The number of isomers of alkanes.
The problem of the theoretically possible number of alkane isomers was first solved by the English mathematician Arthur Cayley (1821-1895), one of the founders of an important branch of mathematics - topology (in 1879 he published the first article on the famous "problem of four colors": are they enough for coloring any geographic maps; this problem was only solved in 1976). It turned out that there is no formula according to which the number of carbon atoms in the alkane C n H 2 n+2 calculate the number of its isomers. There are only so-called recurrent formulas (from the Latin recurrens - returning), which allow you to calculate the number of isomers n-th term of the series, if the number of isomers of the previous term is already known. Therefore, calculations for large n were obtained relatively recently with the help of computers and brought to the hydrocarbon C 400 N 802; for it, taking into account the spatial isomers, a value is obtained that is difficult to imagine: 4.776 · 10 199. And already starting with the alkane C 167 H 336, the number of isomers exceeds the number of elementary particles in the visible part of the Universe, which is estimated as 10 80. The number of isomers indicated in the table for most alkanes will increase significantly if we also consider mirror-symmetric molecules - stereoisomers ( cm... OPTICAL ISOMERY): for heptane - from 9 to 11, for decane - from 75 to 136, for eicosane - from 366 319 to 3 396 844, for hectane - from 5.921 · 10 39 to 1.373 · 10 46, etc.
From the point of view of a chemist, the number of structural isomers of saturated hydrocarbons is of practical interest only for the first members of the series. Even for a relatively simple alkane containing only fifteen carbon atoms, the overwhelming majority of isomers have not been obtained and are unlikely to be ever synthesized. For example, the last of the theoretically possible 75 decane isomers were synthesized only by 1968. And this was done for practical purposes - in order to have a more complete set of standard compounds by which various hydrocarbons, for example, those found in oil, can be identified. By the way, all 18 possible octane isomers were found in various types of oil.
But the most interesting thing is that, starting with heptadecane C 17 H 36, at first only some of the theoretically possible number of isomers, then many, and finally, almost all are a vivid example of "paper chemistry", i.e. cannot really exist. The point is that as the number of carbon atoms in the molecules of branched isomers increases, serious problems of spatial packing arise. After all, mathematicians considered carbon and hydrogen atoms as points, when in fact they have a finite radius. So, the methane "ball" has 4 hydrogen atoms on the "surface", which are freely located on it. In neopentane C (CH 3) 4 on the "surface" there are already 12 hydrogen atoms located much closer to each other; but there is still room for them to place. But for alkane 4 (C 17 H 36), there is little space on the surface to accommodate all 36 hydrogen atoms in 12 methyl groups; it is easy to check if we draw a flat image (or, even better, mold a volumetric model from plasticine and matches) for such isomers, keeping the constancy of the C – C and C – H bond lengths and all angles between them). With growth n placement problems also arise for carbon atoms. As a result, despite the fact that the number of possible isomers with increasing n increases very quickly, the proportion of "paper" isomers grows much faster. A computer-assisted assessment showed that as n the ratio of the number of really possible isomers to the number of "paper" ones quickly tends to zero. That is why the calculation exact number isomers of saturated hydrocarbons for large n, which once aroused considerable interest, now has only theoretical significance for chemists.
The structure and physical properties of alkanes.
In alkanes, four sp 3 -hybrid orbitals of the carbon atom ( cm... ORBITALS) are directed to the vertices of the tetrahedron with an angle between them of about 109 ° 28 "- in this case, the repulsion between the electrons and the energy of the system are minimal. As a result of the overlap of these orbitals with each other, as well as with s-orbitals of hydrogen atoms are formed s-bonds C – C and C – H. These bonds in alkane molecules are covalent non-polar or low-polarity.
In alkanes, primary carbon atoms are distinguished (they are bonded to only one neighboring C atom), secondary (bonded to two C atoms), tertiary (bonded to three C atoms) and quaternary (bonded to four C atoms). So, in 2,2-dimethyl-3-methylpentane CH 3 –C (CH 3) 2 –CH (CH 3) –CH 2 –CH 3 there is one quaternary, one tertiary, one secondary and five primary carbon atoms. The different environment of carbon atoms has a very strong effect on the reactivity of the hydrogen atoms associated with them.
The spatial arrangement of sp 3 orbitals leads, starting from propane, to a zigzag configuration of carbon chains. In this case, the rotation of molecular fragments around the C – C bonds is possible (in an ethane molecule at 20 ° C - at a speed of millions of revolutions per second!), Which makes the molecules of higher alkanes flexible. Straightening of such chains occurs, for example, when stretching polyethylene, which consists of a mixture of alkanes with long chains. Alkane molecules interact weakly with each other, so alkanes melt and boil at much lower temperatures than substances with polar molecules close to them in mass. The first 4 members of the homologous series of methane are gases under normal conditions, propane and butane are easily liquefied under low pressure (liquid propane-butane mixture is contained in household gas cylinders). Higher homologues are liquids with the smell of gasoline or solids that do not dissolve in water and float on its surface. The melting and boiling points of alkanes increase with an increase in the number of carbon atoms in the molecule, while the temperature rise gradually slows down, so C 100 H 202 melts at 115 ° C, C 150 H 302 - at 123 ° C. Melting and boiling points for the first 25 alkanes are given in the table - it can be seen that starting with octadecane, alkanes are solids.
| Table. MELTING AND BOILING TEMPERATURES OF ALKANES | ||
| Alcan | T pl | T bale |
| Methane | –182,5 | –161,5 |
| Ethane | –183,3 | –88,6 |
| Propane | –187,7 | –42,1 |
| Butane | –138,4 | –0,5 |
| Pentane | –129,7 | 36,1 |
| Hexane | –95,3 | 68,7 |
| Heptane | –90,6 | 98,4 |
| Octane | –56,8 | 125,7 |
| Nonan | –51,0 | 150,8 |
| Dean | –29,7 | 174,1 |
| Undecane | –25,6 | 195,9 |
| Dodecane | –9,6 | 216,3 |
| Tridecan | –5,5 | 235,4 |
| Tetradecan | +5,9 | 253,7 |
| Pentadecane | +9,9 | 270,6 |
| Hexadecane | 18,2 | 286,8 |
| Heptadecan | 22,0 | 301,9 |
| Octadecane | 28,2 | 316,1 |
| Nonadecan | 32,1 | 329,7 |
| Eicosan | 36,8 | 342,7 |
| Geneicosan | 40,5 | 356,5 |
| Dokosan | 44,4 | 368,6 |
| Tricosan | 47,6 | 378,3 |
| Tetracosan | 50,9 | 389,2 |
| Pentacosan | 53,7 | 399,7 |
Branching in the chain dramatically alters the physical properties, especially the melting point. So, if hexane of normal structure ( n-hexane) melts at –95.3 ° С, then its isomeric 2-methylpentane - at –153.7 ° С. This is due to the difficulty of packing branched molecules during crystallization. As a result, alkanes with chain branching do not crystallize upon rapid cooling, but pass into the glassy state of a supercooled liquid ( cm... GLASS). For example, if a thin ampoule with pentane is immersed in liquid nitrogen (temperature –196 ° C), the substance will turn into a white snow-like mass, while isopentane (2-methylbutane) solidifies into a transparent “glass”.
The original method of their separation is based on the difference in the geometric shape of linear and branched alkanes: in urea crystals there are channels in which alkanes with a straight chain can fit and branched ones do not.
Cycloalkanes with n \u003d 2, 3 - gases, higher - liquids or solids. The largest cycle that chemists have managed to synthesize is the cyclooctaoctacontadictan C 288 H 576. The different shape of the molecules of cycloalkanes with an even and an odd number of carbon atoms in the molecule leads to a strong even-odd effect relative to the melting point, which can be seen from the table. This effect is explained by the difference in the "convenience" of packing molecules of different shapes in a crystal: the more compact the packing, the stronger the crystal and the higher its melting point. For example, cyclododecane melts almost 70 ° above its closest homologue, cycloundecane. Of course, the mass of the molecule also matters: light molecules melt at a lower temperature.
| C 3 H 6 | –127,5 |
| C 4 H 8 | –50 |
| C 5 H 10 | –93,9 |
| C 6 H 12 | +6,5 |
| C 7 H 14 | –12 |
| S 8 N 16 | 14,3 |
| C 9 H 18 | 9,7 |
| S 10 N 20 | 10,8 |
| S 11 N 22 | –7,2 |
| C 12 H 24 | 61,6 |
| S 13 N 26 | 23,5 |
| C 14 H 28 | 54 |
| S 15 N 30 | 62,1 |
The ease of rotation around the C – C bond leads to the fact that the cycloalkane molecules are not planar (with the exception of cyclopropane), in this way they avoid a strong distortion of the bond angles. Thus, in cyclohexane and its higher homologues, the bond angles are unstressed and close to tetrahedral (109 °), while in the hexagon the angles are 120 °, in the octagon - 135 °, etc. Individual carbon atoms in such cycloalkanes do not occupy a rigidly fixed position: the ring is, as it were, in constant wave-like motion. So, a cyclohexane molecule can be in the form of different geometric structures (conformers) that can transform into each other (cycle inversion). For their outward resemblance, they were called “bathtub” and “armchair” (in English literature, “bathtub” is called a “boat”):
The shape of the chair is more stable; at ambient temperatures, 99.9% cyclohexane exists in a more stable chair form. The transition between the two forms is carried out through an intermediate "twist-conformation" (from the English. twist - twist).
In cyclopropane, the angle decreases from 108 ° to 60 °, which leads to strong stress and "bending" of the bonds, which occupy an intermediate position between the usual s and p bonds; because of their shape, these bonds are called "banana". In this case, the sp 3 orbitals of carbon atoms overlap only partially. The result is a duality in the chemical properties of cyclopropane. On the one hand, substitution of hydrogen atoms is possible in it (a reaction typical of alkanes), on the other hand, addition with ring opening is possible (a reaction typical of alkenes, for example: cyclo-C 3 H 6 + Br 2 ® BrCH 2 CH 2 CH 2 Br).
Cycloalkanes with two rings and one common carbon atom are called spiroalkanes. If there are more than two total carbon atoms, then bicycloalkanes, tricycloalkanes, etc. are formed. As a result of this "crosslinking" of several cycles at once, chemists managed to obtain hydrocarbons, the spatial structure of which corresponds to various polyhedra: tetrahedron, cube, prism, etc. Bicyclic derivatives cyclohexane is contained in essential oils, coniferous resin, turpentine. A cycle of six and five carbon atoms is contained in camphor, cholesterol, saccharin, piperine (it gives a burning taste to black pepper), nitrogenous bases - nucleotides, and other compounds (while some carbon atoms in the cycles can be double bonds, and some are replaced other atoms such as saccharin). A cycle of 17 carbon atoms (two of them are double-bonded) is contained in civetone, a scent, a component of musk, which is used in perfumery. The beautiful adamantane molecule contains three six-membered rings and corresponds in structure to the crystal lattice of diamond. The adamantane structure is contained in the antiviral drug rimantadine, in hexamethylenetetramine (in the latter compound, 4 carbon atoms are replaced by nitrogen atoms, which are connected to each other by methylene bridges –CH 2 -). Below are the structures of some cycloalkanes with more than one differently linked ring in their molecules.
Bicyclodecane (tetrahydronaphthalene, decalin)
Adamantane
Chemical properties of alkanes.
Alkanes are the least chemically active organic compounds. All C – C and C – H bonds in alkanes are single, therefore alkanes are incapable of addition reactions. Alkanes are characterized by reactions of replacement of hydrogen atoms with other atoms and groups of atoms. Thus, chlorination of methane produces methyl chloride CH 3 Cl, methylene chloride CH 2 Cl 2, trichloromethane (chloroform) CHCl 3 and carbon tetrachloride (carbon tetrachloride) CCl 4. These reactions follow a chain mechanism with the intermediate formation of free radicals.
In the chlorination of alkanes, starting with propane, the very first chlorine atom can replace different hydrogen atoms. The direction of substitution depends on the strength of the C – H bond: the weaker it is, the faster the substitution of this particular atom. Primary С – Н bonds, as a rule, are stronger than secondary bonds, and secondary bonds are stronger than tertiary ones. As a result, chlorination at 25 ° С at the secondary bond (СН 3) 2 СН – Н occurs 4.5 times faster than at the primary bond С 2 Н 5 –Н, and the tertiary bond (СН 3) 3 С – Н - in 6.7x faster. The different reactivity of primary, secondary and tertiary hydrogen atoms can lead to the fact that out of several possible chlorination products, only one will prevail. For example, chlorination of 2,3-dimethylbutane in a solution of carbon disulfide (CS 2) produces 95% of the 2-chloro derivative and only 5% of the 1-chloro derivative, i.e. 19 times less. If we take into account that in the initial alkane there are 6 times more primary hydrogen atoms than tertiary ones, then the ratio of their reactivity will be even greater (19 ґ 6 \u003d 114). As a solvent, carbon disulfide lowers the reactivity of chlorine atoms and, accordingly, increases its selectivity. Lowering the temperature works similarly.
Bromine atoms are less active; the appreciable activation energy of this reaction leads to the fact that the bromination of alkanes, although proceeding according to a chain mechanism, is much slower than chlorination, and only at elevated temperatures or in the light. The lower activity of bromine atoms also leads to an increase in the selectivity of bromination. So, if the relative rate of photochemical bromination of ethane at 40 ° C is taken equal to 1, then the rate of propane bromination (at the secondary H atom) will be 220 under the same conditions, and the rate of isobutane bromination (at the tertiary H atom) - 19000
Iodine atoms are the least active, therefore the reaction of iodination of alkanes RH + I 2 ® RI + HI is endothermic, possible only at high temperatures and proceeds with very short chains. Moreover, the reverse exothermic reaction RI + HI ® RH + I 2 proceeds very easily. When alkanes are iodized, unsaturated compounds are also formed. For example, at 685 ° C ethane, reacting with iodine, forms 72% ethylene and 10% acetylene. The same results were obtained with propane, butane and pentane.
The fluorination reaction of alkanes proceeds at a very high, often explosive, rate with the formation of all possible polyfluorinated derivatives of the initial alkane. The energy released during the fluorination of alkanes is so great that it can lead to the decomposition of product molecules into radicals, which start new chains. As a result, the reaction rate increases like an avalanche and this leads to an explosion even at low temperatures. The peculiarity of fluorination of alkanes is the possibility of destruction of the carbon skeleton by fluorine atoms with the formation of CF 4 with other halogens as the final product, such a reaction does not occur.
Nitration of alkanes (Konovalov reaction) also proceeds according to the radical mechanism: RH + NO 2 ® R + HNO 2, R + NO 2 ® RNO 2. The source of NO 2 is nitric acid, which decomposes when heated. The reaction is carried out in solution at a temperature above 150 ° C or in vapors under a pressure of up to 10 atm and a temperature of 400 - 500 ° C. In the latter case, the C – C bonds in alkanes are also broken and a mixture of nitroalkanes is formed.
All alkanes burn with the release of heat, for example: C 5 H 12 + 8O 2 ® 5CO 2 + 6H 2 O. This reaction occurs, in particular, in the cylinders of internal combustion engines. To prevent the remnants of unburned alkanes from entering the atmosphere, their catalytic afterburning in the exhaust pipes is used (at the same time, CO is combusted and nitrogen oxides are converted into harmless nitrogen). The reaction of oxygen with higher alkanes (in the composition of paraffin) occurs when a candle is burning. Gaseous alkanes such as methane form explosive mixtures with air. Such mixtures can form in mines, as well as in residential buildings, when gas leaks, if its content in the air reaches 5%.
Considerable efforts of chemists were directed to a detailed study of the reaction of low-temperature oxidation of alkanes in order to stop it at the stage of formation of valuable intermediate products - aldehydes, ketones, alcohols, carboxylic acids. So, in the presence of Co (II), Mn (II) salts, butane can be oxidized to acetic acid, paraffin - to fatty acids C 12 - C 18. Oxidation of cyclohexane produces caprolactam - a monomer for the production of nylon and adipic acid.
An important industrial reaction is the photochemical sulfochlorination of alkanes: a joint radical-chain reaction with Cl 2 and SO 2 with the formation of acid chlorides of alkanesulfonic acids RSO 2 Cl. This reaction is widely used in the manufacture of detergents. When chlorine is replaced by oxygen, a radical chain reaction of alkanes sulfooxidation occurs with the formation of alkanesulfonic acids R – SO 2 –OH. Sodium salts of these acids are used as detergents and emulsifiers.
At high temperatures, decomposition (pyrolysis) of alkanes occurs, for example: CH 4 ® C + 2H 2 (1000 ° C), 2CH 4 ® C 2 H 2 + 3H 2 (1500 ° C), C 2 H 6 ® C 2 H 4 + H 2. The last reaction takes place at 500 ° C in the presence of a catalyst (Ni). Similarly, 2-butene CH 3 CH \u003d CHCH 3 can be obtained from butane, while a mixture of ethylene and ethane is formed. In contrast to this radical reaction, the catalytic cracking of alkanes proceeds according to the ionic mechanism and serves to obtain gasoline from heavier petroleum fractions. When heated in the presence of Lewis acids, for example, AlCl 3, isomerization occurs: unbranched (normal) alkanes are converted into branched ones with the same number of carbon atoms. This reaction is of great practical importance for obtaining high-quality motor fuel ( cm... OCTANE NUMBER). The dehydrogenation of alkanes can be accompanied by ring closure (dehydrocyclization). In the case of hexane dehydrocyclization, benzene is the main product.
Methane at a high temperature in the presence of a catalyst reacts with steam and carbon monoxide (IV) to form synthesis gas: CH 4 + H 2 O ® CO + 3H 2, CH 4 + CO 2 ® 2CO + 2H 2. Synthesis gas is used to produce motor fuels and methyl alcohol.
IN last years the efforts of chemists are aimed at creating catalysts that activate the C – H bonds in alkane molecules under mild conditions. Some microorganisms "are able" to carry out such reactions, whose enzymes are capable of "digesting" even paraffin with the formation of protein compounds. The challenge for chemists is to understand how natural catalysts work and to simulate enzymatic reactions that can occur at ordinary temperatures. In this case, various organometallic compounds are used as catalysts. For example, in the presence of some platinum compounds, it is possible to obtain methanol CH 3 OH directly from methane, and in the presence of the triphenylphosphine rhodium complex Rh [(C 6 H 5) 3 P] bound to CO molecules; In the course of the reaction, CO molecules are incorporated into the C – H bond of alkanes to form aldehydes.
Cycloalkanes are chemically similar to alkanes. So, they are flammable, can be halogenated by a radical mechanism, at elevated temperatures in the presence of catalysts they dehydrogenate - they split off hydrogen and turn into unsaturated hydrocarbons. Special propertiesis said to have cyclopropane. Unlike alkanes, cycloalkanes are hydrogenated, and the cycle opens and alkanes are formed, for example: cyclo-C 3 H 6 + H 2 ® C 3 H 8 (the reaction proceeds when heated in the presence of a platinum catalyst). With an increase in the size of the cycle, the reaction becomes more difficult - for example, cyclopentane is already hydrogenated (to pentane) with great difficulty and at high temperatures (300 ° C).
Being in nature and receiving.
The main sources of alkanes are oil and natural gas. Methane makes up the bulk of natural gas, with small amounts of ethane, propane and butane also present. Methane is contained in the excretions of swamps and coal seams. Along with light homologues, methane is present in associated petroleum gases. These gases are dissolved in oil under pressure and are also located above it. Alkanes make up a significant part of oil refined products. Contained in oil and cycloalkanes - they are called naphthenes (from the Greek. naphtha - oil). In nature, gas hydrates of alkanes, mainly methane, are also widespread; they occur in sedimentary rocks on continents and at the bottom of the oceans. Their reserves are likely to exceed the known reserves of natural gas and in the future may be a source of methane and its closest homologues.
Alkanes are also obtained by pyrolysis (coking) of coal and its hydrogenation (obtaining synthetic liquid fuel). Solid alkanes are found in nature in the form of deposits of mountain wax - ozokerite, in wax coatings of leaves, flowers and plant seeds, and are part of beeswax.
In industry, alkanes are obtained by catalytic hydrogenation of carbon oxides CO and CO 2 (the Fischer-Tropsch method). In the laboratory, methane can be obtained by heating sodium acetate with a solid alkali: CH 3 COONa + NaOH ® CH 4 + Na 2 CO 3, as well as by hydrolysis of some carbides: Al 4 C 3 + 12H 2 O ® 3CH 4 + 4Al (OH) 3. Homologues of methane can be obtained by the Wurtz reaction, for example: 2CH 3 Br + 2Na ® CH 3 –CH 3 + 2NaBr. In the case of dihaloalkanes, cycloalkanes are obtained, for example: Br – CH 2 - (CH 2) 4 –CH 2 Br + 2Na ® cyclo-C 6 H 12 + 2NaBr. Alkanes are also formed during the decarboxylation of carboxylic acids and during their electrolysis.
The use of alkanes.
Alkanes in gasoline, kerosene, diesel oil, and fuel oil are used as fuel. Higher alkanes are found in lubricating oils, petroleum jelly and paraffin. A mixture of isomeric pentanes and hexanes is called petroleum ether and is used as a solvent. Cyclohexane is also widely used as a solvent and for the synthesis of polymers (nylon, nylon). Cyclopropane is used for general anesthesia. Squalane is a high quality lubricating oil, a component of pharmaceuticals and cosmetics, an adsorbent in gas-liquid chromatography.
Alkanes are used as raw materials for the production of many organic compounds, including alcohols, aldehydes, acids. Chlorine derivatives of alkanes are used as solvents, for example, trichloromethane (chloroform) CHCl 3, carbon tetrachloride CCl 4. A mixture of higher alkanes - paraffin is non-toxic and widely used in food Industry for the impregnation of containers and packaging materials (for example, milk bags), in the production of chewing gum. Pencils are impregnated with paraffin, the upper (near the head) part of the matches for their better burning. The heated paraffin is used for medicinal purposes (paraffin therapy). Oxidation of paraffin under controlled conditions in the presence of catalysts (organic salts of transition metals) leads to the production of oxygen-containing products, mainly organic acids.
Ilya Leenson
Literature:
A.A. Petrov Alkane chemistry... M., Science, 1974
Azerbaev I.N. and etc. Syntheses based on petroleum hydrocarbons... Alma-Ata, Science, 1974
Rudakov E.S. Reactions of alkanes with oxidants, metal complexes and radicals in solutions... Kiev, Naukova Dumka, 1985
Pereushanu V. Production and use of hydrocarbons... M., Chemistry, 1987
The table shows some representatives of a number of alkanes and their radicals.
|
Formula |
Name |
Radical name |
|||||||||||
|
CH3 methyl |
|||||||||||||
|
C3H7 kerf |
|||||||||||||
|
C4H9 butyl |
|||||||||||||
|
isobutane |
isobutyl |
||||||||||||
|
isopentane |
isopentyl |
||||||||||||
|
neopentane |
neopentyl |
||||||||||||
|
The table shows that these hydrocarbons differ from each other in the number of groups - CH2 -. Such a number of similar in structure, having similar chemical properties and differing from each other in the number of these groups is called a homologous series. And the substances that make up it are called homologues. Homologues - substances similar in structure and properties, but differing in composition by one or more homologous differences (- CH2 -)
Carbon chain - zigzag (if n ≥ 3) σ - bonds (free rotation around bonds) length (-C-C-) 0.154 nm binding energy (-С-С-) 348 kJ / mol All carbon atoms in alkane molecules are in the state of sp3 hybridization
angle between c-C links is 109 ° 28 ", so the molecules of normal alkanes with a large number of carbon atoms have a zigzag structure (zigzag). communication C-C in saturated hydrocarbons is 0.154 nm (1nm \u003d 1 * 10-9m). a) electronic and structural formulas; b) spatial structure
4. Isomerism - STRUCTURAL isomerism of the chain with C4 is characteristic One of these isomers ( n-butane) contains an unbranched carbon chain, and the other, isobutane, contains a branched (isostructural). The carbon atoms in a branched chain differ in the type of bonding to other carbon atoms. So, a carbon atom bonded to only one other carbon atom is called primary, with two other carbon atoms - secondary, with three - tertiary, with four - quaternary. With an increase in the number of carbon atoms in the composition of molecules, the possibilities for chain branching increase, i.e. the number of isomers increases with the number of carbon atoms. Comparative characteristics of homologues and isomers
1. They have their own nomenclature radicals(hydrocarbon radicals)
| |||||||||||||
The use of alkanes is quite diverse - they are used as a fuel, as well as in mechanics, medicine, etc. The role of these chemical compounds in life modern man difficult to overestimate.
Alkanes: properties and a brief description of
Alkanes are non-cyclic carbon compounds in which carbon atoms are linked by simple saturated bonds. These substances represent whole line with certain properties and characteristics. as follows:
N here represents the number of carbon atoms. For example, CH3, C2H6.
The first four representatives of the alkane series - gaseous substances - are methane, ethane, propane and butane. The following compounds (C5 to C17) are liquids. The series continues with compounds that are solids under normal conditions.
As for chemical properties, alkanes are inactive - they practically do not interact with alkalis and acids. By the way, it is the chemical properties that determine the use of alkanes.
However, these compounds are characterized by some reactions, including the replacement of hydrogen atoms, as well as the processes of splitting molecules.
- The most common reaction is considered to be halogenation, in which hydrogen atoms are replaced by halogens. Great importance have the reaction of chlorination and bromination of these compounds.
- Nitration - replacement of a hydrogen atom by a nitro group when reacting with a dilute (concentration 10%) Under normal conditions, alkanes do not interact with acids. In order to carry out such a reaction, a temperature of 140 ° C is needed.
- Oxidation - Under normal conditions, alkanes are not susceptible to oxygen. However, after being ignited in air, these substances enter into the final products of which are water and
- Cracking - this reaction takes place only if the necessary catalysts are available. In the process, the cleavage of persistent homologous bonds between carbon atoms occurs. For example, in the cracking of butane, the reaction can produce ethane and ethylene.
- Isomerization - as a result of the action of some catalysts, a certain rearrangement of the carbon skeleton of an alkane is possible.
Application of alkanes
The main natural sources of these substances are such valuable products as natural gas and oil. The fields of application of alkanes today are very wide and varied.
For instance, gaseous substances used as a valuable source of fuel. An example is methane, of which natural gas is composed, as well as a propane-butane mixture.
Another source of alkanes is oil , the importance of which for modern mankind is difficult to overestimate. Oil products include:
- gasolines - used as fuel;
- kerosene;
- diesel fuel, or light gas oil;
- heavy gas oil used as a lubricating oil;
- the remains are used to make asphalt.
Petroleum products are also used to make plastics, synthetic fibers, rubbers and some detergents.
Vaseline and liquid paraffin are products that consist of a mixture of alkanes. They are used in medicine and cosmetology (mainly for the preparation of ointments and creams), as well as in perfumery.
Paraffin is another well-known product, which is a mixture of solid alkanes. It is a solid white mass with a heating temperature of 50 - 70 degrees. In modern production, paraffin is used to make candles. Matches are impregnated with the same substance. In medicine, various kinds of thermal procedures are carried out with the help of paraffin.
