* The Arndt-Eistert reaction is employed in converting a carboxylic acid  to a higher homologue with one additional carbon atom.

SOCl2 CH2N2 Ag+
R-COOH  ------------->   R-COCl    -------------->   R-COCHN  ---------------------->   R-CH2CONu 
ether, Et3N Nucleophile (Nu)

* The following steps are involved in the Arndt-Eistert reaction.

1) Conversion of carboxylic acid to an active compound like acid chloride or an anhydride.

R-COOH + SOCl2 --------> R-COCl + SO2 + HCl

2) Conversion of acid chloride to a diazoketone. A base like Et3N is employed to neutralize HCl liberated in this step.

R-COCl + CH2N2 --------> R-COCHN2 + HCl

3) The Wolff rearrangement of diazoketone into a ketene and subsequent conversion of it to a higher carboxylic acid or its derivative by using a nucleophile.

Ag2O, Δ or hν, -N2 Nucleophile (Nu)
R-COCHN2  ------------------------------------->   R-CH=C=O  -------------------------------->  R-CH2CONu
diazoketone Wolff rearrangement ketene higher homologue

* Nucleophile (Nu) = water or alcohol or amines


1) Initially the diazomethane is acylated by the acid chloride to give a diazoketone.

mechanism of arndt-eistert reaction

* The HCl liberated in the first step must be neutralized by a suitable base to avoid the formation of chloromethyl ketone. Usually two moles of diazomethane are used to avoid this side reaction.

2) Thus formed diazoketone is rearranged to a ketene. This is called Wolff-rearrangement.

wolff rearrangement

* Silver salts like PhCO2Ag, Ag2O along with heat or light catalyze the Wolff rearrangement.

* The configuration of 'R' group during Wolff rearrangement is retained.

3) The ketene is immediately attacked by an appropriate nucleophile in the solution.

arndt-eistert reaction 1-3



1) Arndt-Eistert reaction is used in the conversion of 4-nitrobenzoic acid to (4-nitrophenyl)acetic acid as shown below.

arndt-eistert reaction 1-4

* The nitro group is not affected in the above reaction.



2)arndt-eistert reaction 1-5

* The carboxylic group is first converted to an anhydride using ethyl chloroformate, ClCO2Et.

* Also note that the configuration at stereogenic carbon is retained.


Author: Aditya vardhan Vutturi