why 2,2-dimethylpropane is not called isopentane? since 2-methylpropane is called isobutane

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    $\begingroup$ ... because it is not the only isomer of n-pentane. $\endgroup$ – Karl Apr 7 '18 at 10:12
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    $\begingroup$ It's called neopentane, typically 2-(mono)methylalkanes are called iso. $\endgroup$ – Mithoron Apr 7 '18 at 14:29
  • $\begingroup$ @Mithoron Yes; except the famous “isooctane” which is used for the determination of the octane rating of fuel. This isomer of octane is actually 2,2,4-trimethylpentane. $\endgroup$ – Loong Apr 7 '18 at 16:30
  • $\begingroup$ @Loong Indeed, calling in isoneo would be complicated ;D $\endgroup$ – Mithoron Apr 7 '18 at 18:44

In common nomenclature, the prefix 'iso-' is applied when the chain of carbons is continuous bar one methyl group. The example you provided, 2-methylpropane, is observed to be continuous with exception to one methyl group bound to the central carbon atom.

However, 2,2-dimethylpropane does not feature an analogous structure: because two methyl groups are bound to the central carbon, the same classification is inapplicable. In place, the molecule could be labelled as 'neopentane', in which the prefix specifies a continuous chain excepting two alkyl groups.


The above image is 2-methylbutane, or isopentane.


The above-shown image is 2,2-dimethylpropane, or neopentane

If you desire further information concerning the nomenclature, I would advise the following link, from which these images were extracted: http://www.chem.ucla.edu/~harding/IGOC/C/common_name.html

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    $\begingroup$ Which is "isohexane"? 2-methylpentane or 3-methylpentane? If I am not mistaken "iso-n-ane" is generally applied to 2-methyl-(n-1)-ane. $\endgroup$ – Oscar Lanzi Apr 7 '18 at 11:13

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