\(C_6H_5COOC_2H_5\) on acid-catalyzed hydrolysis produces:

Benzoic acid + Ethanol

The correct answer is option 1. Benzoic acid + Ethanol.

When phenyl propanoate is heated under reflux with a dilute acid such as dilute hydrochloric acid or dilute sulfuric acid, the ester reacts with the water present to produce benzoic acid and ethanol. Because the reaction is reversible, an equilibrium mixture is produced containing all four of the substances in the equation. In order to get as much hydrolysis as possible, a large excess of water can be used. The dilute acid provides both the acid catalyst and the water.

The mechanism

Step 1

The actual catalyst in this case is the hydroxonium ion, \(H_3O^+\), present in all solutions of acids in water. In the first step, the ester takes a proton (a hydrogen ion) from the hydroxonium ion. The proton becomes attached to one of the lone pairs on the oxygen which is double-bonded to the carbon.

The transfer of the proton to the oxygen gives it a positive charge, but the charge is actually delocalized (spread around) much more widely than this shows.

One way of representing this delocalization is to draw a number of structures called resonance structures or canonical forms joined by double-headed arrows. You could, if you wished, put in some curly arrows to show the movements of electrons which change one of these structures into the next.

None of these formulae represents the true structure of the ion - but each gives you some information about it. For example, notice where the positive charge is in the three structures. What this means in reality is that the positive charge is spread around over those three atoms - the two oxygens and the carbon.

Step 2

The positive charge on the carbon atom is attacked by one of the lone pairs on the oxygen of a water molecule.

Step 3

What happens next is that a proton (a hydrogen ion) gets transferred from the bottom oxygen atom to one of the others. It gets picked off by one of the other substances in the mixture (for example, by attaching to a lone pair on a water molecule), and then dumped back onto one of the oxygens more or less at random. Eventually, by chance, it will join the oxygen with the ethyl group attached. When that happens, the net effect is:

Step 4

Now a molecule of ethanol is lost from the ion. That's one of the products of the reaction.

The structure for the latest ion is just like the one we discussed at length back in step 1. The positive charge is actually delocalized all over that end of the ion. The real structure will be a hybrid of these:

Step 5

The hydrogen is removed from the oxygen by reaction with a water molecule.