The Nierenstein reaction is an organic reaction

Organic reaction

Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In organic synthesis,...

 describing the conversion of an acid chloride into an haloketone

Haloketone

A haloketone in organic chemistry is a functional group consisting of a ketone group or more general a carbonyl group with a α-halogen substituent. The general structure is RR'CCR where R is an alkyl or aryl residue and X any one of the halogens...

 with diazomethane

Diazomethane

Diazomethane is the chemical compound CH2N2. It is the simplest of diazo compounds. In the pure form at room temperature, it is a extremely sensitive explosive yellow gas, thus it is almost universally used as a solution in diethyl ether...

. It is an insertion reaction

Insertion reaction

An insertion reaction is a chemical reaction where one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:...

 in that the methylene

Methylene

Methylene is a chemical species in which a carbon atom is bonded to two hydrogen atoms. Three different possibilities present themselves:* the -CH2- substituent group: e.g., dichloromethane ....

 from the diazomethane is inserted into the carbon-chlorine bond of the acid chloride.

Reaction mechanism

Like the related Arndt-Eistert reaction, this reactions proceeds through a diazoketone  intermediate (5). The loss of nitrogen to gives the desired haloketone (2).

The synthesis of benzyl chloromethyl ketone from phenylacetyl chloride in fact requires the addition of HCl gas to the diazoketone intermediate for it to succeed. The unassisted reaction failed.

Scope

One original 1924 Nierenstein reaction:

and a reaction starting from benzoyl bromide

Benzoyl

In organic chemistry, benzoyl is the acyl of benzoic acid, with structure C6H5CO-. It should not be confused with benzyl, which is the radical or ion formed from the removal of one of the methyl hydrogens of toluene...

 going haywire with formation of the dioxane dimer: