# Are acid chlorides really acidic?

If I test a pure solution of an acid chloride with a pH-meter, will I get a reading of pH 7?

When I add water to it, I know $\ce{HCl}$ gas will form, but this gas will dissolve in water to give an acidic solution.

My main confusion is that we call ethanoyl chloride an "acid" chloride, but is its pure solution acidic?

• As long as it is water free it should not be acidic as there are no free protons. – Martin - マーチン Apr 2 '14 at 8:43
• Just think of it as the chloride derivative of a carboxylic acid ;) – Klaus-Dieter Warzecha Apr 2 '14 at 9:08
• @Klaus Warzecha: Hmm... so that's where the "acid" part came from :P – Eliza Apr 2 '14 at 9:15
• @Eliza Exactly :) – Klaus-Dieter Warzecha Apr 2 '14 at 9:18
• @G M: when acid chloride reacts with water, carboxylic acid and hydrogen chloride gas is formed... well that's what I learned – Eliza Apr 2 '14 at 9:21

An acid chloride is a covalent molecule. It has no pH. If you put a pH electrode into a reactive acid chloride you will dehydrate and acylate its hydrated glass surface ($\ce{#Si-OH}$) and damage its future response when properly used. pH electrodes are stored sitting in buffer or saline solution.

An acid chloride is an oxo-acid whose acidic $\ce{-OH}$ has been replaced with $\ce{-Cl}$. Hydrolysis restores the oxo-acid, displacement makes derivatives. Hydrochloric acid is not an acid chloride.

pH-values only truely make sense in a solution whose solvent has acidic (as in ‘abstractable’) hydrogens. Apart from the obvious water this includes alcohols, enoliseable ketones and more. This means that a pure acid chloride cannot have a pH value, because there is no $\ce{[H+]}$ around.

However, all those solvents that can liberate a proton in a Brønsted manner have then created an anion. This anion, whatever its composition may be, is likely to be nucleophilic enough to attack the acid chloride and liberate $\ce{HCl}$ thereby reducing the pH value of the solution.

So a pure acid chloride or one dissolved in non-protic solvents has no pH as the solvent has none. Those that are dissolved in protic solvents are likely already liberating $\ce{HCl}$ due to solvolysis.

Is there such a thing as a completely pure solution ? Would there not always be some water contamination and hence would you not always have some protons floating around ?

However, as a thought experiment I imagine that in a solution of 'pure' ethanoyl chloride you would have a pH of 7 as there are no free protons moving around in solution.

• pH = 7 says [H+] = 10^(-7) molar. Your statement is incorrect. There is no water in a bottle of acyl chloride. If there were, it would react to give HCl and the acyl anhydride. – Uncle Al Apr 2 '14 at 14:28