What is the formula of 4-methylbutanoic acid?

Pentanoic acid is the proper name, and it looks like this:

![https://upload.wikimedia.org/]()

Its structural formula is therefore `"H"_3"C"("CH"_2)_3"COOH"`, and its molecular formula is `"C"_5"H"_(10)"O"_2`.

I would start from the parent name, then look at the suffix, then look at the substituent(s).

The parent name is "butane", and the new prefix, instead of "-ane", is "-anoic acid". That suffix indicates a carboxylic acid, which has a carboxyl group, `("C"="O")-"O"-"H"`, on one end.

Therefore, a main feature of this molecule is that it has four carbons in the main chain, and the fourth one is the one in the carboxyl group.

The aforementioned carbon is considered carbon-1, so carbon-4 is the terminal carbon, on which the methyl group apparently resides.

Since that is the case, "4-methylbutanoic acid" is not the proper name for this compound, as it specifies a `-"CH"_3` on carbon-4, extending the main chain by one carbon.

A better name is pentanoic acid (or valeric acid), and the structural formula is `"H"_3"C"("CH"_2)_3"COOH"`.