6.1.3 (c,d,) Esters

Syllabus

(c) Esterification of:

(i) Carboxylic Acids with Alcohols in the presence of an acid catalyst (e.g. concentrated H₂SO₄)

(ii) Acid Anhydrides with Alcohols

(d) Hydrolysis of Esters:

(i) in hot aqueous acid to form Carboxylic Acids and Alcohols

(ii) in hot aqueous alkali to form Carboxylate salts and Alcohols

What does this mean?

Esterification with Carboxylic acids

An ester looks like a Carboxylic acid with an alkyl group where the H⁺ ion would be in the acid.

And Esters are generally made from Carboxylic acids, usually by reacting with an alcohol.

Any ester made from Methanoaic Acid will be a Methanoate

Any ester made from Ethanoaic Acid will be an Ethanoate

Etc

This reaction requires warming (not heating strongly or the product will evaporate) and a concentrated acid catalyst.

The product will generally have a fruity smell (although a few are less pleasant).

Slightly confusingly, whereas you'd expect the Acid to lose a Hydrogen ion when water is formed it's actually the alcohol that loses an H while the acid loses an OH.

Notice that the acid that was used to make the ester provides the end of its name.

So Ethanoic acid will always make Ethanoate esters.

For example;

Ethanoic Acid + Propanol make Propyl Ethanoate

but

Propanoic Acid + Ethanol make Ethyl Propanoate

Esterification with Acid derivates

Acyl Chlorides (or Acid Chlorides) have most of their chemical properties in common with Carboxylic Acids.

But they're usually more reactive so make a good alternative to make an Ester from since they don't need a catalyst.

The acyl chloride to the left is based on Ethanoic Acid

- it is called Ethanoyl Chloride.

The one on the right is based on Propanoic Acid

- it is called Propanoyl Chloride.

It's no surprise that they also react with Alcohols to make Esters, the difference being that HCl is made instead of water.

And this can be a problem as, if the reaction gets hot, the HCl will be a choking, acidic gas.

Image result for esterification acyl chloride

One advantage of using an acid chloride is that the reaction is faster, due to the increased reactivity of acid chlorides compared to acids.

For the same reason, it's possible to use acid chlorides to make Esters that Carboxylic acids are usually not strong enough to produce.

For A level, this means with Phenols.

Phenols are not unlike alcohols but don't esterify with carboxylic acids.

But they do with Acid Chlorides and no Acid Catalyst is required.

An examiner may expect you to know this

Image result for phenol and acyl chloride

Acid Anhydrides

The same is true of another type of Acid derivate - the Acid Anhydride.

If we dehydrate two Ethanoic Acid molecules we get Ethanoic Anhydride.

Ethanoic Anhydride would also be reactive enough to esterify phenols with a catalyst, but has the advantage of not producing HCl in the process.

Image result for ethanoic anhydride esterification

Hydrolysis of Esters

When an ester forms from an alcohol and an acid water is produced.

If we heat up an ester in the presence of a catalyst water molecules break up to reform the original reactants - Hydrolysis.

Note that an acid catalyst or a base catalyst can be used.

However, if a base is used once the Carboxylic acid has been produced it would react with the base.

So, although the NaOH is a catalyst in the reaction above it is also a reactant and must be in the equation.

Since the H+ catalyst doesn't react with the products we can show the hydrolysis as a reaction with water