Making acid chlorides from carboxylic acids

The use of this should be obvious - Cl^- is a much better leaving group than OH^-.

1. Thionyl chloride

This is a fun volatile liquid with a choking smell. Industrial production is controlled under the Chemical Weapons Convention, something nearly every country has agreed to. This reagent is probably the first compound of this calibre which you will encounter in an undergrad lab.

It is generally the preferred reagent for converting COOH to COOCl since both its byproducts - HCl and SO₂ - are gaseous.

Mechanism:

The reaction is driven by loss of SO₂ and HCl gas from the reaction mixture.

2. Phosphorus pentachloride

This is a white solid, though many samples are light yellow from HCl contamination.

The mechanism is extremely similar to thionyl chloride. I suggest trying to guess it yourself and see how it compares to the one below:

3. Oxalyl chloride plus dimethylformamide (DMF)

Oxalyl chloride is the diacid chloride of oxalic acid. It is a colorless liquid with a sharp smell.

Dimethylformamide is also colorless. It has no odor unless some of it has degraded to dimethylamine, which has a fishy smell. It is often used as a solvent - one with a low evaporation rate.

Oxalyl chloride also has gaseous by-products. But it also has a minor byproduct which is a potent carcinogenic, and it is a lot more expensive.

Mechanism:

Notice how this creates a highly electrophillic intermediate which attacks the carboxylic acid. It is then kicked off by a chloride ion, producing an acid chloride while regenerating the catalyst.