Two unlabelled colourless liquids, J and K, are known to be either propanoyl chloride, CH3CH2COCl, or ethyl propanoate, CH3CH2COOCH2CH3, but their labels have been lost. (a) Describe a simple test, using only water, that would allow J and K to be distinguished, stating the obser…

Answer: ((a)) Add a few drops of each liquid to separate portions of cold water. The liquid that reacts vigorously, giving off steamy white fumes of hydrogen chloride, is propanoyl chloride. The liquid that shows no visible reaction (esters do not hydrolyse noticeably in cold water without a… ((b)) CH3CH2COCl + H2O → CH3CH2COOH + HCl (no reaction occurs for ethyl propanoate under these conditions). ((c)) The mixture from propanoyl chloride now contains chloride ions (released as HCl during hydrolysis); adding dilute nitric acid then aqueous silver nitrate gives an immediate white precipitate of silver chloride: Ag^+(aq) + Cl^-(aq) → AgCl(s). The mixture from ethyl propanoate con…. Part (a): Testing with water Acyl chlorides are far more reactive than esters towards nucleophiles such as water, because chloride ion is a much better leaving group than the alkoxide/alcohol that would have to leave from an ester, and the…

A2 Chemistry 9701: Distinguishing an Acyl Chloride from an Ester

Two unlabelled colourless liquids, J and K, are known to be either propanoyl chloride, $\text{CH}_3\text{CH}_2\text{COCl}$ , or ethyl propanoate, $\text{CH}_3\text{CH}_2\text{COOCH}_2\text{CH}_3$ , but their labels have been lost.

(a) Describe a simple test, using only water, that would allow J and K to be distinguished, stating the observation expected for each liquid. [3]

(b) Write an equation for any reaction that occurs during the test in (a). [1]

(c) The resulting mixtures from (a) are then treated with a few drops of dilute nitric acid followed by aqueous silver nitrate. State and explain the observation expected for the mixture derived from each liquid, giving an ionic equation for any reaction. [4]

Part (a): Testing with water

Acyl chlorides are far more reactive than esters towards nucleophiles such as water, because chloride ion is a much better leaving group than the alkoxide/alcohol that would have to leave from an ester, and the electronegative chlorine also makes the acyl chloride’s carbonyl carbon strongly electrophilic. Adding a few drops of each unknown liquid to separate portions of cold water:

Propanoyl chloride reacts immediately and vigorously, with steamy white fumes of hydrogen chloride gas given off.
Ethyl propanoate shows no visible reaction, esters hydrolyse only extremely slowly in cold water without an acid or alkali catalyst and heating under reflux.

Part (b): Equation for the reaction

$\text{CH}_3\text{CH}_2\text{COCl} + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{CH}_2\text{COOH} + \text{HCl}$

No equation is needed for ethyl propanoate, since it does not react under these conditions.

Part (c): Confirming with silver nitrate

After part (a), the mixture that started as propanoyl chloride now contains free chloride ions in solution (from the hydrogen chloride formed on hydrolysis), while the mixture that started as ethyl propanoate contains no chloride ions at all, since the ester did not react.

Adding a few drops of dilute nitric acid (to acidify the mixture and remove interference from any other ions) followed by aqueous silver nitrate gives:

$\text{Ag}^+\text{(aq)} + \text{Cl}^-\text{(aq)} \rightarrow \text{AgCl(s)}$

The mixture from propanoyl chloride gives an immediate white precipitate of silver chloride, confirming the presence of chloride ions.
The mixture from ethyl propanoate gives no precipitate, since there are no chloride ions present to react with the silver ions.

This combination of tests (reactivity towards cold water, followed by a silver nitrate test for the chloride ion released) unambiguously identifies J as propanoyl chloride and K as ethyl propanoate (or vice versa, depending on which liquid reacted).

Final answers

(a) The liquid giving steamy white fumes with cold water is propanoyl chloride; the liquid showing no reaction is ethyl propanoate.
(b) $\text{CH}_3\text{CH}_2\text{COCl} + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{CH}_2\text{COOH} + \text{HCl}$ .
(c) The propanoyl-chloride-derived mixture gives a white precipitate of AgCl with $\text{AgNO}_3\text{(aq)}$ (after dilute $\text{HNO}_3$ ); the ethyl-propanoate-derived mixture gives no precipitate, as it contains no chloride ions.