17C.14-17C.16 - Acyl Chlorides, Esters And Polyesters

Why Acyl Chlorides React So Readily

Acyl chlorides have the functional group $\ce{RCOCl}$. The carbonyl carbon is strongly $\delta^{+}$ because both oxygen and chlorine withdraw electron density from it, so electron-pair donors such as water, alcohols, ammonia and amines can attack this carbon.

The key reaction pattern is nucleophilic acyl substitution. A nucleophile attacks the $\ce{C=O}$ carbon, the carbonyl π bond temporarily breaks, then the carbonyl reforms and chloride leaves. Chloride is a much better leaving group than hydroxide, $\ce{OH-}$, so acyl chlorides react more vigorously than carboxylic acids and usually do not need the acid catalyst used for esterification of carboxylic acids.

This shorthand hides an important exam detail: if the nucleophile is ammonia or an amine, the $\ce{HCl}$ formed is normally neutralised by another molecule of ammonia or amine to give an ammonium salt. Always give both the organic product and the small inorganic or salt product.

Four Reactions Of Acyl Chlorides

Use $\ce{RCOCl}$ as the acyl chloride. The group attached to the carbonyl, $\ce{R}$, is unchanged; the chlorine is replaced by the group supplied by the nucleophile.

The water reaction can produce steamy acidic fumes because $\ce{HCl}$ is formed. With an alcohol, the product is an ester, so acyl chlorides provide a faster route to esters than reacting a carboxylic acid with an alcohol. With ammonia and amines, the nitrogen lone pair makes the molecule a nucleophile; because ammonia and amines are also bases, they also remove $\ce{H+}$ or neutralise $\ce{HCl}$.

Worked example: writing an acyl chloride equation

Propanoyl chloride reacts with ethylamine. Write an equation showing the organic product and the salt formed.

1. Keep the acyl part from propanoyl chloride: $\ce{CH3CH2CO -}$.
2. Replace $\ce{Cl}$ by $\ce{NHCH2CH3}$ to make the amide: $\ce{CH3CH2CONHCH2CH3}$.
3. Use a second ethylamine molecule to neutralise the $\ce{HCl}$: $\ce{CH3CH2NH3Cl}$.

The chemical interpretation is that one ethylamine molecule acts as the nucleophile and becomes part of the amide, while another ethylamine molecule acts as a base and forms the salt.

Hydrolysis Of Esters In Acid And Alkali

Hydrolysis breaks an ester link using water or hydroxide. For a simple ester, the bond broken is the acyl-oxygen bond in $\ce{RCOOR'}$, so the acyl side becomes a carboxylic acid or carboxylate, and the alkoxy side becomes an alcohol.

In acidic solution, the reaction is catalysed by acid and is reversible:

$\ce{RCOOR' + H2O <=> RCOOH + R'OH}$ (dilute acid catalyst, heat under reflux)

In alkaline solution, hydroxide ions are reactants and the reaction is treated as effectively irreversible because the carboxylic acid product is converted into a carboxylate ion:

$\ce{RCOOR' + OH- -> RCOO- + R'OH}$ (aqueous alkali, heat under reflux)

or, with sodium hydroxide:

$\ce{RCOOR' + NaOH -> RCOO- Na+ + R'OH}$

If the carboxylic acid is required after alkaline hydrolysis, the mixture must be acidified afterwards. That acidification is a separate step, not the direct product of the alkaline hydrolysis stage.

Worked comparison: methyl propanoate

Acidic hydrolysis:

Alkaline hydrolysis with sodium hydroxide:

A strong compare-and-contrast answer should name both sides of each comparison: both methods break the ester and produce methanol; acid hydrolysis produces propanoic acid and is reversible with acid as a catalyst; alkaline hydrolysis produces sodium propanoate and is effectively irreversible with hydroxide as a reactant.