Chapter 19, Problem 54P

The Perkin condensation is an aldol-type condensation in which an aromatic aldehyde (ArCHO) reacts with a carboxylic acid anhydride, ${\left(RC{H}_{2}CO\right)}_{2}O$, to give an $\alpha ,\beta$ -unsaturated acid $\left(ArCH=CRC{O}_{2}H\right)$. The catalyst that is usually employed is the potassium salt of the carboxylic acid $\left(RC{H}_{2}C{O}_{2}K\right)$.

(a) Outline the Perkin condensation that takes place when benzaldehyde reacts with propanoic anhydride in the presence of potassium propanoate.

(b) How would you use a Perkin condensation to prepare p-chlorocinnamic acid, $\text{p-}Cl{C}_6{H}_{4}CH=CHC{O}_{2}H$ ?