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For example, which is the right way to name this compound:

Compound

Is it 2-bromo-4-methylenehexane or 4-bromo-2-ethylpent-1-ene?

Does the larger number of carbon in a chain take priority, as in the former, or does the carbon-carbon double bond take priority, as in the latter?

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The IUPAC recommendations on this matter have been changed. In the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the order of seniority between unsaturation and length of chain given in earlier recommendations is reversed:

(…), the first criterion to be considered in choosing a preferred parent acyclic chain is the length of the chain; unsaturation is now the second criterion.

For example, a preferred IUPAC name is 3-methylidenehexane (not 2-ethylpent-1-ene) since the longer chain now supersedes a shorter unsaturated chain. (This example is given in the Blue Book.)

3-methylidenehexane

Besides, the prefix ‘methylene’ is no longer recommended in IUPAC nomenclature to designate the $\ce{=CH2}$ group. The preferred prefix is ‘methylidene’.

Therefore, the correct name for your example is 2-bromo-4-methylidenehexane.

2-bromo-4-methylidenehexane

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From wikipedia:

Identification of the parent hydrocarbon chain. This chain must obey the following rules, in order of precedence:

  1. It should have the maximum number of substituents of the suffix functional group. By suffix, it is meant that the parent functional group should have a suffix, unlike halogen substituents. If more than one functional group is present, the one with highest precedence should be used.

So the alkene takes priority over the alkane and 4-bromo-2-ethylpent-1-ene is the correct name.

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  • 2
    $\begingroup$ Gotta hate it when the gatekeepers change the rules right out from underneath one. $\endgroup$ – hBy2Py Nov 25 '16 at 12:34

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