To provide a method for performing a cross-coupling reaction of a Grignard compound with an alkyl halide simply, efficiently and in high yield, a method for obtaining a ω-bromo long chaincarboxylic acid simply and efficiently using an easily obtainable raw material and a method for producing a useful branched fatty acid simply and efficiently.
[R
1
: an alkyl group having 1 to 15 carbon atoms, R
2
: an alkyl group having 1 to 30 carbon atoms with a carboxyl group and X and X′: a halogen atom]
[n: an integer of 9 to 17]
[n: an integer of 9 to 17, R
1a
: a branched alkyl group having 3 to 8 carbon atoms and X: a halogen atom]