Alkanes

Alkanes

 

(also saturated hydrocarbons or paraffins), a homologous series of hydrocarbons of the general formula C_nH_2n + ₂; part of the class of acyclic compounds. The parent compound of the series is methane, CH₄. Each succeeding member is distinguished in composition from the preceding member by the constant difference CH₂. The names of the first four members of the series are methane, CH₄; ethane, C₂H₆; propane, C₃H₈; and butane, C₄H₁₀. The names of subsequent homologues are derived from the Greek cardinal numbers—for example, C₅H₁₂, pentane; C₈H₁₈, octane; C₁₀H₂₂, decane; and C₁₆H₃₄, cetane. The names of all alkanes end in “-ane.”

The carbon atoms in the molecules of alkanes are linked by single bonds to give straight unbranched chains or (starting with butane) branched chains. This is the reason for the existence of structural isomers in the alkane series. The number of isomers increases rapidly with the number of carbon atoms: pentane has three isomers, decane has 75, and eicosane (C₂₀H₄₂) has 366, 319. Starting with the heptanes, optical isomers also appear. The alkanes are colorless gases through butane and neopentane, liquids from C₅H₁₂ through C₁₇H₃₆, and solids thereafter. The boiling points of alkanes with branched chains are somewhat lower, and the melting points higher, than those of normal isomers. All alkanes are virtually insoluble in water but are readily soluble in numerous organic liquids.

Under normal conditions, alkanes are chemically the most inert hydrocarbons (hence the name “paraffins,” from the Latin parum, “little,” and affinitas, “affinity”). However, under relatively harsh conditions their hydrogen atoms may be replaced by other atoms and groups. Many such reactions are the basis of industrial processes for the manufacture of numerous important products. Chlorination of alkanes yields methyl chloride, methylene chloride, and chloroform; nitration yields nitroparaffins. Ultraviolet irradiation of a mixture of alkanes with sulfur dioxide and chlorine yields the sulfonyl chlorides, C_nH_2n + ₁SO₂CI. Oxidative sulfonation yields the sulfonic acids, C_nH_2n + ₁ SO₂-OH, and oxidation of the lower alkanes yields alcohols, aldehydes, ketones, and acids. Oxidation of solid alkanes to produce higher fatty acids is also of great industrial importance. Catalytic dehydrogenation of alkanes yields olefins (propylene, butenes, and amylenes) and diolefins (butadiene and isoprene); isomerization yields isobutane and isopentane. Alkylation of isobutane with olefins gives isooctane and neohexane. The lower alkanes are capable of forming inclusion compounds (clathrates) with water. Liquid and solid normal alkanes readily form clathrates with urea. This reaction is used in industry for deparaffination of petroleum products. Branched alkanes yield clathrates with thiourea.

Alkanes are present in petroleum (5–60 percent), which is the principal source for them. They are also produced during processing of hard coal, oil shale, and other materials. They are contained in plants and beeswax. The mineral wax ozokerite consists almost completely of higher alkanes. Natural gas contains up to 99 percent methane by volume.

Individual alkanes are prepared in the laboratory mainly by hydrogenation of olefins or by fusion of salts of fatty acids with caustic alkalies. Alkanes are important raw materials for the production of intermediates in making plastics, synthetic elastomers and fibers, and detergents. They are significant components of rocket and motor fuels and are used as solvents.

V. N. FROSIN