Alkane

Alkane A compound with the general formula CnH2n+2· Alkanes are open-chain (aliphatic or noncyclic) hydrocarbons with no multiple bonds or functional groups. They consist of tetrahedral carbon atoms, up to 105 carbons or more in length. The C-C bonds are formed from sp3 orbitals, and there is free rotation around the bond axis.

Alkanes provide the parent names for all other aliphatic compounds in systematic nomenclature. Alkanes are designated by the ending-ane appended to a stem denoting the chain length. The straight-chain isomer is designated by the prefix C (normal); other isomers are named by specifying the size of the branch and its location . The number of isomers increases enormously in larger molecules; thus there are 75 isomers of C10H22 and over 4 billion for C30H62·

Alkanes with four or fewer carbons are gases at atmospheric pressure. Higher C alkanes are liquids or, above about 20 carbons, solids known as paraffin wax. Alkanes have densities lower than that of water and have very low water solubility; other prop­erties depend on the degree of branching. The heat of formation (t-.H1) is a measure of the energy content of a compound relative to the component elements in standard states. For example, comparison of heat of formation values for three alkane isomers with the molecular formula C5H12 indicates that the relative energy content of the three pentane isomers decreases with increased branching, that is, the branched isomer is thermodynamically most stable.

Alkanes are the major components of natural gas and petroleum, which are the only significant sources. Much smaller amounts of alkanes have been produced from coal at various times and locations, either indirectly by the Fischer-Tropsch process or by direct liquefaction.

Individual lower alkanes can be separated from the more volatile distillate fractions of petroleum, but beyond the C,C8 range the alkanes obtained are mixtures of many isomers. Compounds of a specific structure can be prepared in a laboratory scale by chemical synthesis. Various C-C bond-forming steps such as coupling or condensation are carried out to build up the desired carbon skeleton. The final step is usually removal of a functional group by some type of reduction.

Much of the chemistry of alkanes begins at the petroleum refinery, where sev­eral reactions are carried out to adjust the hydrocarbon composition of crude oil to that needed for a constantly changing set of applications. Major reactions are (1) isomerization of straight-chain alkanes to branched compounds; (2) cracking to produce smaller molecules; (3) alkylation, for example, combination of propylene and butane to give 2,3-dimethylpentane; and (4) cyclodehydrogenation (platforming), in which aromatizations occur. An important objective in some of these processes is to increase the yield of highly branched alkanes in the C6-C8 range needed for gasoline.

By far the most important end use of alkanes is combustion as fuel to provide heat and electric or motive power. In most cases, complete oxidation is not achieved, and varying amounts of incompletely oxidized fragments, carbon monoxide, and elemental carbon are produced.

Controlled partial oxidation is possible if all the C-H bonds in an alkane are equivalent or if one C-H bond is significantly weaker than all the others. An example of the latter situation is isobutane, which is converted to the hydroperoxide on industrial scale for the manufacture oft-butyl alcohol.

Alkanes have been referred to as paraffin hydrocarbons to indicate their low affinity or reactivity. They contain no unshared electron pairs or accessible empty bonding or­bitals, and they are unaffected by many reagents that attack c-c-bonds or other functional groups. One type of reaction that does occur is substitution by a radical chain pro­cess. Examples are chlorination and vapor-phase nitration, involving the odd-electron species Cl· and NO2-, respectively. Neither reaction is selective; when two or more types of C-H bonds are present in the alkane, mixtures of products are usually obtained. Thus propane gives rise to 1- and 2-chloropropanes as well as dichloro compounds. Nitration of propane leads to a mixture of 1- and 2-nitropropane, and also nitromethane and nitroethane by C-C bond cleavage.

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عن Akram Amir El Ali

استاذ الكيمياء التحليلية ومصمم غرافيك

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