If we have an alkyl substituent containing a double bond(s) or triple bonds(s), what is it's priority in comparison to other functional groups and substituents?

For example, take the compound given below

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What would it's IUPAC name be?

We know that the cyclohexane ring will be the parent chain (as it contains two functional groups), and the ethenyl substituent as well as the $\ce{-Cl}$ will be prefixes, while the $\ce{-OH}$ will be the suffix. This means we start numbering from $\ce{C1}$ carbon atom, which the $\ce{-OH}$ is attached to.

But exactly how would we go about numbering the carbons? And what is the priority of the ethenyl group and alkenyl or alkynyl groups in general (so that we know how to number it such that the higher priority prefix receives the lower locant, in accordance with set rule)?


1 Answer 1


There is no priority order for functional groups that are expressed as prefixes. When assigning the lowest set of locants, they are all considered together.

The corresponding section in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:


When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:


(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;

(g) lowest locants for the substituent cited first as a prefix in the name;


Note that Rule f takes precedence over Rule g.

Therefore, the example is named as 2-chloro-6-ethenylcyclohexan-1-ol rather than 6-chloro-2-ethenylcyclohexan-1-ol since ‘chloro’ is cited first as a prefix in the name.


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