A2 Chemistry 9701: Reaction of an Acyl Chloride with Phenol

Pentanoyl chloride, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{COCl}$ , is added dropwise to liquid phenol, $\text{C}_6\text{H}_5\text{OH}$ , and the mixture is gently warmed.

(a) Write an equation for the reaction and name the ester formed. [3]

(b) The corresponding reaction between pentanoyl chloride and ethanol proceeds immediately in the cold, with no need for warming. Explain, in terms of the availability of the oxygen atom's lone pair, why the reaction with phenol is slower and often needs warming (or the phenol first converted into sodium phenoxide) to obtain a good yield. [3]

(c) Describe a simple test, using concentrated aqueous ammonia, that could be used to confirm that hydrogen chloride gas is released during the reaction in (a), stating the observation expected. [2]

Part (a): Reaction with phenol

Acyl chlorides react with phenol by nucleophilic addition-elimination, in the same general way as with water, an alcohol or an amine, releasing hydrogen chloride and forming an ester:

$\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{COCl} + \text{C}_6\text{H}_5\text{OH} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{COOC}_6\text{H}_5 + \text{HCl}$

The organic product is the ester phenyl pentanoate (the “phenyl” part comes from phenol, the “pentanoate” part from pentanoyl chloride).

Part (b): Why the reaction with phenol is slower

The mechanism is the same nucleophilic addition-elimination in both cases: the nucleophile’s oxygen lone pair attacks the electrophilic carbonyl carbon, then chloride ion leaves. What differs is how readily that lone pair is available:

In ethanol, the oxygen’s lone pairs are localised entirely on the oxygen atom, making it a strongly nucleophilic centre that attacks the acyl chloride immediately, even in the cold.
In phenol, one of the oxygen’s lone pairs overlaps with (is delocalised into) the aromatic ring’s $\pi$ system. This lowers the electron density on the oxygen itself, making it a weaker nucleophile than an alcohol’s oxygen. The reaction with the acyl chloride is consequently slower, and gentle warming is often used to give a reasonable rate.

An alternative, and generally more efficient, approach is to first convert phenol into sodium phenoxide using aqueous sodium hydroxide. Removing the acidic O-H proton leaves the oxygen bearing a full negative charge, which, despite some of this charge still being delocalised into the ring, makes the phenoxide ion considerably more nucleophilic than neutral phenol, so it reacts with the acyl chloride far more readily.

Part (c): Testing for hydrogen chloride gas

Hydrogen chloride gas reacts with ammonia gas in a simple acid-base (neutralisation) reaction to form a solid smoke of ammonium chloride:

$\text{NH}_3\text{(g)} + \text{HCl(g)} \rightarrow \text{NH}_4\text{Cl(s)}$

Holding a glass rod that has been dipped in concentrated aqueous ammonia close to the mouth of the reaction vessel (without letting it touch the liquid mixture) produces dense white smoke/fumes of solid ammonium chloride particles, confirming that hydrogen chloride gas is being evolved.

Final answers

(a) Phenyl pentanoate; $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{COCl} + \text{C}_6\text{H}_5\text{OH} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{COOC}_6\text{H}_5 + \text{HCl}$ .
(b) Phenol’s oxygen lone pair is partly delocalised into the ring, making it less nucleophilic than ethanol’s oxygen, so the reaction is slower and benefits from warming or converting phenol to sodium phenoxide first.
(c) Dense white smoke/fumes of $\text{NH}_4\text{Cl}$ form when concentrated ammonia (on a glass rod) is brought near the vessel.