19. The Simple Compounds
                                   of Carbon
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As an example, the longest chain that can be traced through the isopentane molecule contains four carbon atoms, so in systematic nomenclature this molecule is a butane.

A methyl group (CH3-) branches off from the second carbon along the chain, so isopentane is called 2-methylbutane. One also could call it 3-methylbutane by beginning the backbone numbering from the other end, but there is no logical reason for using this name.

The numbering is begun at the end that leads to the simplest name with the smallest numbers. Notice that there is no 1-methylbutane or 4-methylbutane; these molecules simply would be the straight-chain pentane.

In systematic nomenclature, neopentane is classed as a propane because the longest carbon chain contains three atoms. Since two methyl groups branch off from the second carbon of the chain, the molecule is called 2,2-dimethylpropane.

Example: What is the systematic name for isooctane, shown at the top of the preceding page?
Solution: With five carbons as the longest continuous carbon chain, the molecule is called 2,2-dimethyl-4-methylpentane.  In this example the systematic name clearly is clumsier than the name isooctane, which is why the latter name ordinarily is used.                                               

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