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# Alkane

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Alkane

A term used in organic chemistry to denote a type of hydrocarbon, in which the molecule has the maximum possible number of hydrogen atoms, and so has no double bonds. The generic formula for non-cyclic alkanes is CnH2n+2; the simplest possible alkane is methane (CH4)

The atoms in alkanes with more than three carbon atoms can be arranged in multiple ways, forming different isomers. "Normal" alkanes have the most linear, unbranched configuration, and are denoted with an n.

Those alkanes, and their derivatives, with four or fewer carbons have non-systematic common names, established by long precedence.

methane CH4

ethane? C2H6

propane CH3CH2CH3

n-butane? CH3CH2CH2CH3

n-pentane? CH3CH2CH2CH2CH3

n-hexane? CH3CH2CH2CH2CH2CH3

n-heptane? CH3CH2CH2CH2CH2CH2CH3

n-octane? CH3CH2CH2CH2CH2CH2CH2CH3

and so on . . . .

Branched alkanes have some non-systematic (or "trivial") names in common use, but there is also a systematic way of naming most such compounds, which starts from identifying the longest non-branched parent alkane in the molecule, counting up from one sequentially starting from the carbon involved in the most prominent functional group (or, more formally, attached to the collection of heteroatoms with highest priority according to some rules), and then numbering the side chains according to this sequence.

i-butane CH3CH(CH3)2 (or "isobutane")

is the only other C4 alkane isomer possible, aside from n-butane. It's formal name is 2-methylpropane.

Pentane, however, has two branched isomers, in addition to its strictly linear, normal form:

CH3C(CH3)2CH3 or 2,2-dimethylpropane

and

CH3CH2CH(CH3)2 or 2-methylbutane

Cycloalkanes have a single ring of carbons to which hydrogens are attached according to the formula CnH2n. They are named analogously to their normal alkane counterpart of the same carbon count: cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc.

The naming of polycyclic alkanes is more complex, with the base name indicating the number of carbons in the ring system, a prefix indicating the number of rings (eg, "bicyclo"), and a numeric prefix before that indicating the number of carbons in each part of each ring, exclusive of vertices. For instance, a bicyclooctane which consists of a six-member ring and a four member ring, which share two adjacent carbon atoms which form a shared edge, is [4.2.0]-bicyclooctane. That part of the six-member ring, exclusive of the shared edge has 4 carbons. That part of the four-member ring, exclusive of the shared edge, has 2 carbons. The edge itself, exclusive of the two vertices that define it, has 0 carbons.

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Last edited August 5, 2001 6:08 pm (diff)
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