alkanes and their chemical reactions

As a class, alkanes are singularly unreactive. The name saturated hydrocarbon (or " paraffin," which literally means " little affinity " [L. par(um), little, + afins, affinity1)arises because their chemical affinity for most common reagents may be regarded as saturated or satisfied. Thus none of the C-H or C-C bonds in a typical saturated hydrocarbon such as ethane are attacked at ordinary temperatures by a strong acid such as sulfuric, or by powerful oxidizing agents such as potassium permanganate, or by vigorous reducing agents such as lithium aluminum hydride (LiAIH₄,).

We have seen that methane and other hydrocarbons are attacked by oxygen at elevated temperatures and, if oxygen is in excess, complete combustion occurs to give carbon dioxide and water with the evolution of large amounts of heat. Vast quantities of hydrocarbons from petroleum are utilized as fuels for the production of heat and power, as will be described in the next section.

A. PETROLEUM AND COMBUSTION OF ALKANES

The liquid mixture of hydrocarbons delivered by oil wells is called petroleum. Its composition varies according to the location of the field but the major components are invariably alkanes. Natural gas is found in association with petroleum and also alone as trapped pockets of underground gas. Natural gas is chiefly methane, while crude petroleum is an astonishing mixture of hydrocarbons up to C,, in size. This dark, viscous oil is present in interstices in porous rock and is usually under great pressure.

Petroleum is believed to arise from the decomposition of the remains of marine organisms over the ages and new fields are continually sought to satisfy the enormous world demand. The effect of advanced technology on our environment is shown by the fact that combustion of fossil fuels, chiefly petroleum, has increased the carbon dioxide content of the atmosphere by 10% in the past century and an increase of 25 % has been predicted by the year 2000. These increases would be even more marked were it not for the fact that the rate of photosynthesis by plants becomes more efficient at utilizing carbon dioxide as the concentration increases.

In addition to serving as a source of power-and being the only natural sources of suitable fuel for the internal combustion engine-petroleum and natural gas are extremely useful as starting materials for the synthesis of other organic compounds. These are often called petrochemicals to indicate their source but they are, of course, identical with compounds prepared in other ways or found in nature.

Petroleum refining involves separation into fractions by distillation. Each of these fractions with the exception of the first, which contains only a few components, is still a complex mixture of hydrocarbons. The main petroleum fractions are given below in order of decreasing volatility.

1. Natural Gas. Natural gas varies considerably in composition depending on the source but methane is always the major component, mixed with smaller amounts of ethane, propane, butane, and 2-methylpropane (isobutane). These are the only alkanes with boiling points below 0°C. Methane and ethane cannot be liquefied by pressure at room temperature (their critical temperatures are too low) but propane, butane, and isobutane can. Liquid propane (containing some of the C₄, compounds) can be easily stored in cylinders and is a convenient source of gaseous fuel. It is possible to separate natural gas into its pure components for sale as pure chemicals although the mixture is, of course, perfectly adequate as a fuel.

2. Gasoline. Gasoline is a complex liquid mixture of hydrocarbons composed mainly of C₅, to C₁₀,, compounds. Accordingly, the boiling range of gasoline is usually very wide, from approximately 40" to 180". Because of the large number of isomers possible with alkanes of this size, it is much more difficult to separate gasoline into its pure components by fractional distillation than is the case with natural gas. Using a technique known as gas chromatography (Section 7.1), this separation can be done on an analytical scale. It has been shown that well over 100 compounds are present in appreciable amounts in ordinary gasoline. These include, besides the open-chain alkanes, cyclic alkanes (cycloalkanes, Section 3.4) and alkylbenzenes (arenes, Section 20-1).

The efficiency of gasoline as a fuel in modern high-compression internal combustion engines varies greatly with composition. Gasolines containing large amounts of branched-chain alkanes such as 2,2,4-trimethylpentane have high octane ratings and are in great demand, while those containing large amounts of continuous-chain alkanes such as octane or heptane have low octane ratings and perform poorly in a modern high-compression automobile engine. The much greater efficiency of branched-chain alkanes is not the result of greater heat of combustion but of the smoothness with which they burn. The heats of combustion of octane and 2,2,4-trimethylpentane can be calculated from the data in Table 2.1 and, since each contains the same number of carbon-carbon bonds (seven) and carbon-hydrogen bonds (18), we would expect their heats of combustion to be identical. The calculated value is - 1218 kcal/mole, and the experimental values are close to this.

The combustion of vaporized branched-chain alkanes is slower and less explosive than that of continuous-chain compounds. In an automobile engine, too rapid combustion leads to dissipation of the combustion energy as heat to the engine block, rather than as movement of the piston. You can clearly hear the "knock" in an engine that is undergoing too rapid combustion of the fuel vapor in the cylinders. The problem is aggravated in highcompression engines. These will often continue to run (though inefficiently) on low-octane gasoline even when the ignition switch has been turned off, the heat of compression in the cylinder being sufficient to ignite the fuel mixture.

Combustion of hydrocarbons occurs by a complex chain reaction (Section 2-5B). It is possible to slow the propagation of the chain by adding volatile compounds such as tetraethyllead, (CH₃,CH₂) ₄   Pb, to gasoline. The fine particles of solid lead oxide formed by oxidation of tetraethyllead moderate the chain-carrying reactions and reduce the tendency for knock to occur., The octane rating of a gasoline is measured by comparing its knock with that of blends of 2,2,4-trimethylpentane whose octane rating is set at 100, and heptane whose octane rating is set at zero. Octane itself has a rating of -20. The higher the octane rating, the smoother the ignition and, with high-compression engines, the more efficient the gasoline. With low-compression engines the effect is negligible and it is wasteful or worse (see footnote) to use gasoline of higher rating than is needed to eliminate knocking.

The steadily increasing use of hydrocarbons in internal combustion engines has led to increasingly serious pollution problems. Some of these are associated with waste products from the refining operations used to produce suitable fuels from crude petroleum, others with spillage of petroleum in transit to the refineiies, but possibly the worst is atmospheric pollution from carbon monoxide and of the type known as "smog." The chemical processes in the production of smog are complex but appear to involve hydrocarbons (especially branched-chain hydrocarbons), sunlight, and oxides of nitrogen. The products of the reactions are ozone, which produces rubber cracking and plant damage, particulate matter, which produces haze, oxides of nitrogen, which color the atmosphere, and virulent eye irritants (one being acetyl pernitrite, CH,-C-0-0-N=O). The hydrocarbons in the atmosphere which produce smog come principally from incomplete combustion in gasoline engines, although sizable amounts arise from evaporation and spillage. Whether smog can be eliminated without eliminating the internal combustion engine is not yet known, but the prognosis is rather unfavorable.

Pollution of the atmosphere from carbon monoxide is already so severe in heavy downtown traffic in large cities as to pose immediate health problems. The main reason for high concentrations of carbon monoxide in automobile exhaust is that the modern gasoline engine runs most efficiently on a slight deficiency in the ratio of oxygen to hydrocarbon which would produce complete combustion. A current solution to this problem is to introduce air and complete the combustion process in the exhaust manifold.

3. Kerosene. Kerosene consists chiefly of Cll and Cl, hydrocarbons, compounds that do not vaporize well in automobile engines. It now finds considerable use as fuel for jet engines. It is also used in small heating units and can, if necessary, be converted to gasoline by a process known as cracking. This involves catalytic decomposition to smaller molecules, one of whichis an alkene :

4. Diesel Oil. The petroleum fraction which boils between about 250" and 400" (C₁₃, to C₂₅,) is known as diesel oil or fuel oil. Large amounts are used in oil-burning furnaces, some is cracked to gasoline, and much is used as f~iel for diesel engines. These engines operate with a very high compression ratio and no spark system, so that they depend on compression to supply the heat for ignition of the fine spray of liquid fuel that is injected into the cylinder near the top of the compression stroke. Branched-chain compounds turn out to be too unreactive to ignite and for this reason diesel and automobile engines have quite different fuel requirements.

5. Lubricating Oils and Waxes. The high-molecular-weight hydrocarbons (C₂₆, to C₂₈,) in petroleum have very high boiling points and can only be obtained in a reasonably pure state by distillation at reduced pressure. Thermal decomposition (pyrolysis) occurs if distillation is attempted at atmospheric pressure because the thermal energy acquired by collision of these compounds at their boiling points (400") is sufficient to rupture carboncarbon bonds. Almost all alkanes higher than C₂₀, are solids at room temperature, and you might be wondering why lubricating oil is liquid. This is because it is a complex mixture whose melting point is much below that of its pure components. Indeed, as the temperature is lowered, its viscosity simply increases although this undesirable property can often be corrected by special additives such as chlorinated hydrocarbons.

Paraffin wax used in candles is a mixture of very high-molecular-weight hydrocarbons similar enough in structure to pack together to give a semicrystalline solid. Vaseline is a mixture of paraffin wax and low-melting oils.

6. Residue. After removal of all volatile components from petroleum, a black, tarry material remains which is a mixture of minerals and complex high-molecular-weight organic compounds; it is known as asphalt.

B. SUBSTITUTION OF HALO AND NITRO GROUPS IN ALKANES

negative and moderate for chlorine and bromine, and positive for iodine (see Table 3.5). With fluorine, the reaction evolves so much heat that it is difficult to control, and products from cleavage of carbon-carbon as well as of carbonhydrogen bonds are obtained. Indirect methods for preparation of fluorinesubstituted hydrocarbons will be discussed later. Bromine is generally much less reactive toward hydrocarbons than chlorine, both at high temperatures and with activation by light. Nonetheless, it is usually possible to brominate saturated hydrocarbons successfully. Iodine is unreactive.

As we have seen, the chlorination of methane does not have to stop with the formation of chloromethane, and it is possible to obtain the higher chlorination products: dichloromethane (methylene chloride), trichloromethane (chloroform), and tetrachloromethane (carbon tetrachloride). In practice, all the substitution products are formed to some extent, depending on the

chlorine-to-methane ratio employed. If monochlorination is desired, a large excess of hydrocarbon is advantageous.

For propane and higher hydrocarbons, where more than one monosubstitution product is generally possible, difficult separation problems may arise when a particular product is desired. For example, the chlorination of 2-methylbutane at 300" gives all four possible monosubstitution products, [3], [4], 151, and [6]. On a purely statistical basis, we might expect the ratio of products to correlate with the number of available hydrogens at the various positions of substitution; that is, [3], [4], [5], and [6] would be formed in the ratio 6 : 3 : 2 : 1. However, in practice, the product composition is substantially different, because the different kinds of hydrogens are not attacked at equal rates. Actually, the approximate ratios of the rates of attack of chlorine atoms on hydrogens located at primary, secondary, and tertiary positions are 1.0 : 3.3 : 4.4 at 300". These results indicate that dissociation energies of C-H bonds are not exactly the same but decrease in the order primary > secondary > tertiary.

Another reaction of commercial importance is the nitration of alkanes to give nitroalkanes. Reaction is usually carried out in the vapor phase at elevated temperatures using nitric acid or nitrogen tetroxide as the nitrating agent. All available evidence points to a radical-type mechanism for nitration

but many aspects of the reaction are not fully understood. Mixtures are obtained-nitration of propane gives not only 1- and 2-nitropropanes but nitroethane and nitromethane.

In commercial practice, the yield and product distribution in nitration of alkanes are controlled as far as possible by the judicious addition of catalysts (e.g., oxygen and halogens) which are claimed to raise the concentration of alkyl radicals. The product mixtures are separated by fractional distillation.

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