Question

2 chlorobutane to acetic acid​

Answer 1

CO2 takes the place of the MgCl at the 2 position of butane, and once you add a proton source (H3O+) it becomes a carboxylic acid. The carbon of the newly added COOH becomes part of the backbone of the molecule and using the proper nomenclature rules, now you have 2-methylbutanoic acid.