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alkanes





the simplest alkanes
The simplest of organic compounds. Alkanes, also known as paraffins, are saturated hydrocarbons that form an homologous series with the general formula CnH2n+2. The first few in the series are methane, ethane, propane, and butane; from pentane on they are named after the number of carbon atoms in the molecule. Lower members in the series are gases; from pentane (C5H12) to heptadecane (C17H36) they are liquids; higher members are waxy solids.

Alkanes with four or more carbon atoms have several isomers, the straight-chain isomers being called normal alkanes (n-alkanes). Branched alkanes are named as derivatives of the longest straihght chain in the molecule.

Alkanes are present in natural gas and petroleum.; they may be synthesized by hydrogenation of alkenes or from carbon monoxide and hydrogen. The higher members can be cracked to form smaller alkanes and alkenes.

Alkanes are soluble in most organic solvents, but not in water. The lower alkanes are less reactive than the higher ones. Typical reactions include combustion in air, decomposition and rearrangement on heating, isomerization and condensation with alkenes (with acid catalyst), nitration, sulfonation, and halgenation by fluorine, chlorine, and bromine (with heat and light).

The monovalent radicals CnH2n+2 derived from alkanes by loss of one hydrogen atom are called alkyl groups (methyl, ethyl, etc.) usually denoted by the symbol R.

Alkanes are converted into other compounds by replacing a hydrogen with other functional groups. The most important substitutions for biochemistry are -OH (alcohol), -CHO (aldehyde), -COO-R- (ester, R=alkyl group), -COOH (carboxylic acid), -PO4 (organic phosphate), and -NH2 (amine).


Cracking

Cracking is a form of thermal decomposition, which breaks long chain hydrocarbons, many of which are not very useful, into shorter molecules by heating them. Many of the substances with longer molecules produced from the fractional distillation of crude oil are cracked; for example, naphtha is cracked to produce gasoline. Cracking also produces extra alkenes, which are needed for making plastics.

In industrial cracking, vaporized hydrocarbons are passed over powdered aluminum oxide catalyst at a temperature of 400°–700°C. Since any of the C–C bonds in a molecule can break, the result is a mixture of products, although the conditions for the reaction are chosen to favor some products over others.


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   • ORGANIC CHEMISTRY