Which of the following reaction sequences represents the best synthesis of the carboxylic acid shown below?

Step-by-step solution

The target molecule is 2,2-dimethylpropanoic acid, (CH₃)₃C-COOH . Evaluating the third sequence: CH₃COCH₃ + CH₃MgBr (CH₃)₃C-O^-MgBr^+ (CH₃)₃C-O^-MgBr^+ + H₃O^+ (CH₃)₃C-OH (CH₃)₃C-OH + SOBr₂ (CH₃)₃C-Br (CH₃)₃C-Br + Mg (CH₃)₃C-MgBr (CH₃)₃C-MgBr + CO₂ (CH₃)₃C-COO^-MgBr^+ (CH₃)₃C-COO^-MgBr^+ + H₃O^+ (CH₃)₃C-COOH This sequence successfully produces the target molecule. The first sequence is incorrect because using KCN on the tertiary halide (CH₃)₃C-Br leads predominantly to an elimination reaction, forming isobutylene i