Indiana University Purdue University Indianapolis
Search:     Advanced search

Student Question: What if a halogen and an alkyl substituent are on the same numbered carbons? The book and the lecture slides tell us that they are simply equal and numbering is seemingly random.

Article ID: 149
Last updated: 06 Oct, 2012
Add comment
Views: 276
Comments: 0

Halogens typically do receive priority over alkyl substituents when you can number a parent chain either of two ways resulting in the same locants (numbers) on the substituents.  See the examples below:

The left compound would not be 2-methyl-4-chloropentane because chlorine is alphabetically before methyl.  The middle compound is not 2-chloro-4-bromopentane for the same reason.  The rightmost example would lead some to argue which receives priority, the higher atomic number substituent or the earliest alphabetical substituent.  The Chemical Abstracts Service (CAS), an authority in archiving all chemical compounds known, lists this molecule as 1-chloro-2-iodoethane, which would necessitate alphabetical priority being used.

This article was:   Helpful | Not Helpful Add comment
Also read
Alkane Nomenclature

Also listed in
Organic Chemistry -> Alkyl Halides: Substitutions and Eliminations

Prev   Next
Polycyclic Molecules     Alkane Structure, Stability, and Reactivity

Department of Chemistry & Chemical Biology