I had this confusion related to nomenclature of halo substituted compounds.

Why is it that 1-bromo-3-methylbut-2-ene is named like that and not as 2-(1-bromomethyl)but-2-ene? why do we prefer the carbon chain including the carbon attached to the bromine and not the chain including the carbon of the methyl group?

Which IUPAC rule was used in making this preference between those two carbon chains of equal length?

  • $\begingroup$ Regardless of the correct IUPAC nomenclature, the proposed names “1-bromo-3-methylbut-2-ene” and “2-(bromomethyl)but-2-ene” describe different compounds. $\endgroup$
    – user7951
    Mar 22, 2017 at 16:53

1 Answer 1


Here's 1-bromo-3-methylbut-2-ene's structure:


You have only two possible chains to choose from: a 3-carbon chain and a 4-carbon chain (doesn't matter which methyl group would be part of that chain, the other one will become a substituent on the chain).

You won't choose the 3-C chain for two reasons:

  1. It is not the longest chain.
  2. It does not contain the double bond in the molecule.

Thus, you are left with a 4-C chain, which necessarily includes the bromine atom.

  • $\begingroup$ But why does the numbering start from Br and not the other side if C=C is the functional group here, Br is a substituent. $\endgroup$
    – Rick
    Jan 19, 2018 at 7:09
  • $\begingroup$ @Loong can you please help? (See my other comment on this answer) $\endgroup$
    – Rick
    Jan 19, 2018 at 7:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.