Using an acid to aid attack on C=O

This has two uses:

1. It can protonate an electrophile, making it more open to attack.
2. It can protonate a leaving group - turning a poor leaving group into a good one

Both of these effects are shown in the most common example of acid catalysts - the formation of an ester:

Alternatively the OR in the tetrahedral intermediate could be protonated instead - which would lead back to the reactant. We drive the formation of product by removing water using distillation or a drying agent.

We can also use an excess of one of the reactants. If we want to fully esterificate an expensive carboxylic acid, we can use an excess of alcohol, and vice versa.

To reverse the reaction, we can use an excess of water.

The same principles can be used to convert the ester of one alcohol into the ester of another. For example, the reaction below can be driven to the right by distilling off methanol:

Another example of transesterifaction is in the commercial production of PET (polyethylene terephthalate) which is the polymer used in plastic drink bottles. It is the ester of terephalic acid and ethylene glycol.

It is produced commercially by transesterifying dimethyl terephthalate, distilling off methanol to drive the reaction.

I don't know why this process is cheaper than making it directly.