Further carboxylic acids and derivatives: esters and acyl chlorides

carboxylic acids - review

esters

To recap: Esters, like acyl chlorides and amides can be classed as derivatives of carboxylic acids, as they contain a C = O group in common.

For a comparison of esters, acyl chlorides and amides check the amide page

or use the side navigation panel or scan the QR code.

Then scroll down.

esters - properties

boiling point

Even though esters are polar molecules they cannot hydrogen bond with other ester molecules, due to the lack of a hydrogen atom bonding to a F, O, or N.

This means they have much lower boiling points than the carboxylic acids which have the same formula as them, ie they are isomers of each other.

Like all polar molecules they have permanent dipole-dipole attractions, as well as temporarily induced dipole attractions (London Dispersion Forces)

Being polar they have higher boiling points than corresponding hydrocarbons with the same number of carbons,

solubility

The table shows the solubility of low mass esters, and how quickly solubility drops as the length of chain increases.

esters - naming

Esters can be recognised from the ester link (-COO-) in which the CO is derived from either the carboxylic acid or the acyl chloride and the O comes from the alcohol; this is important in naming:

When naming esters the following points are necessary to understand:

Identify the ester link to determine which part of the ester has come from the carboxylic acid / acyl chloride (C=O) and which part has come from the alcohol (O-CH₂)

The alcohol part is named first and is treated like a branched carbon group off a main chain, that is it ends in the suffix -yl after the number of carbons in the alcohol. Fro example if the the alcohol was ethanol this part of the ester name would be ethyl

The last part of the ester name, treated as the 'root' comes from the carboxylic acid / acyl chloride and ends in -anoate. For example if the carboxylic acid was methanoic acid (or the acyl chloride was methanoyl chloride) this part of the ester name would be methanoate.

esters - key points

For Level 3 chemistry

Esterification reactions

Hydrolysis and saponification

Reactions with ammonia (alcoholic), along with amines

Polymerisation reactions

esterification - preparation of esters

From a carboxylic acid and an alcohol
From an acyl chloride and an alcohol

esters - hydrolysis and saponification

Hydrolysis is a reaction with water that 'cleaves / splits' the ester link (-COO-) to form the constituent molecules: carboxylic acid and an alcohol. This can be done under acidic or basic conditions. Think acid - base reactions!
Saponfication is hydrolysis of large esters to produce soaps

The fats / oils made of a triol and 3 fatty acids are known as triglycerides. These are the large esters used to make soaps

The fatty acids are long chained carboxylic acids

esters - reactions with ammonia and amines

The reaction of an ester with NH₃ produces a primary amide
The reaction between an ester and an amine produces a secondary amide

esters - polymerisation

Esters form polyesters in condensation polymerisation reactions

acyl chlorides

acyl chlorides - naming

Acyl chlorides, or acid chlorides can be viewed as carboxylic acids which have had the -OH replaced in a nucleophilic substitution reaction with a -Cl group. They are not classed as being soluble in water due to their vigorous reaction when exposed to water, including vapour, producing HCl.

When naming acyl chlorides think of the corresponding carboxylic acid and replace the -oic with -oyl and add chloride for the -Cl group. For example butanoic acid, with 4 carbons in the chain would be butanoyl chloride

Remember the functional group is always at the terminal carbon, which would be number C # 1. This is important to remember when naming branched acyl chlorides:

acyl chlorides - key points

For level 3 chemistry

Acyl chlorides are not found in nature due to their extreme reactivity, due to the highly polar -COCl group. Due to electronegativity differences between the C = O and the C-Cl there is a strong pull of electrons away from the C, making this C positive and very attractive to nucleophiles (atoms attracted to positive charges) which substitute with the Cl

Methanoyl chloride does not really exist as it decomposes rapidly at room temperature to form CO and HCl

They need to be chemically prepared, generally by nucleophilic substitution of carboxylic acids using chlorine containing compounds

Reactions involving acyl chlorides to produce esters and amides, as well as carboxylic acids

All reactions are highly exothermic and produce HCl gas

Due to production of HCl they turn blue litmus red

acyl chlorides - preparation

acyl chloride - hydrolysis

acyl chlorides - ester and amide preparation

The preparation of esters and amides is preferably done using acyl chlorides and not carboxyic acids, due to

the faster and more vigorous reaction
the production of HCl (g) driving the reaction to completion
the lack of need of a catalyst and / or dehydrating agent

At Level 3 chemistry you do not need to name 2⁰ amides

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