Catalyzes the activation of fatty acids by CoA to produce an acyl-CoA, the first step in fatty acid metabolism (PubMed:10434065, PubMed:12616642). Capable of activating medium-chain fatty acids (e.g. butyric (C4) to decanoic (C10) acids), and certain carboxylate- containing xenobiotics, e.g. benzoate (PubMed:10434065, PubMed:12616642). {ECO:0000269|PubMed:10434065, ECO:0000269|PubMed:12616642}. This is the function of ACSM2B (acyl-CoA synthetase medium chain family member 2B, ENSG00000066813).