Why are acyl chlorides acidic

You end up with a mixture of the acyl chloride and phosphoric III acid old names: phosphorous acid or orthophosphorous acid , H 3 PO 3. For example:. Again, the ethanoyl chloride can be separated by fractional distillation. Sulfur dichloride oxide thionyl chloride is a liquid at room temperature and has the formula SOCl 2. Traditionally, the formula is written as shown, despite the fact that the modern name writes the chlorine before the oxygen alphabetical order. The sulfur dichloride oxide reacts with carboxylic acids to produce an acyl chloride, and sulfur dioxide and hydrogen chloride gases are given off.

The separation is simplified to an extent because the by-products are both gases. That is because methanoyl chloride is very unstable, decomposing to give carbon monoxide and HCl.

An acyl chloride like ethanoyl chloride is a colourless fuming liquid. The strong smell of ethanoyl chloride is a mixture of the smell of vinegar ethanoic acid and the acrid smell of hydrogen chloride gas. The smell and the fumes are because ethanoyl chloride reacts with water vapour in the air.

The reaction with water is given in detail on another page. Find it from the acyl chlorides menu - link at the bottom of this page. Acyl chlorides can't be said to dissolve in water because they react often violently with it. The strong reaction means that it is impossible to get a simple aqueous solution of an acyl chloride.

It is a polar molecule, and so has dipole-dipole attractions between its molecules as well as van der Waals dispersion forces. However, it doesn't form hydrogen bonds. Its boiling point is therefore higher than, say, an alkane of similar size which has no permanent dipoles , but not as high as a similarly sized alcohol which forms hydrogen bonds in addition to everything else.

Note: If you aren't happy about intermolecular forces including van der Waals dispersion forces and hydrogen bonds then you really ought to follow this link before you go on. Acyl chlorides are extremely reactive, and in their reactions the chlorine atom is replaced by other things.

Nu is the bit of the molecule which contains the nucleophilic oxygen or nitrogen atom. The attached hydrogen turns out to be essential to the reaction. The general equation for the reaction is:. In each case, the net effect is that you replace the -Cl by -Nu, and hydrogen chloride is formed as well. Since the initial attack is by a nucleophile, and the overall result is substitution, it would seem reasonable to describe the reaction as nucleophilic substitution.

However, the reaction happens in two distinct stages. The first involves an addition reaction, which is followed by an elimination reaction where HCl is produced.

Steven Farmer Sonoma State University. Jim Clark Chemguide. Why are acyl chlorides attacked by nucleophiles? The Overall Reaction We are going to generalize this for the moment by writing the reacting molecule as "Nu-H".