Answer to Question #120289 in Organic Chemistry for Benjamin ogonyo

Question

Arrange the compounds in each of the following sets in increasing order of acidities and give the reasons in each :
3-chloropentanoic acid , 2-chloropentanoic acid , 4-chloropentanoic acid , pentanoic acid , 5-chloropentanoic acid

Answer 1

pentanoic acid < 5-chloropentanoic acid < 4-chloropentanoic acid < 3-chloropentanoic acid < 2-chloropentanoic acid

RCOOH $\leftrightarrow$ RCOO^- + H⁺

RCOO^- is conjugate base. More stable conjugate base $\implies$stronger acid. Acceptors near COO^- will stabilize it.

Cl possess +M<-I effects. In that case the conjugation between Cl and COOH is absent. So, only -I effect should be taken into account. Therefore, Cl is strong acceptor in that case.

Since inductive effect operates only through single bonds and are dependent on distance, closer the acceptor to COOH , stronger the acid.