Beckmann Rearrangement

The Beckmann rearrangement, named after the German chemist Ernst Otto Beckmann (1853–1923), is an acid-catalyzed rearrangement of an oxime to an amide.  Most commonly used catalysts are Conc.H₂SO₄, HCl, PCl₅,  PCl₃, SOCl₂, ZnO, SiO₂, PPA (Poly phosphoric acid).  Aldoximes are less reactive than ketoximes.  Cyclic oximes yield lactams.

Mechanism of Beckmann rearrangement:

Initially the -OH group of the oxime is protonated. Then 1,2 shift of alkyl group (R¹) onto electron deficient nitrogen and the cleavage of N-O bond occurs simultaneously. 

Always the alkyl group which is 'anti' to the -OH group on nitrogen undergoes 1,2 shift which indicates the concerted nature of the beckmann rearrangement.

Where is it used?

This reagent is useful in ring enlargement of cyclic ketone.  A very good example is the industrial conversion of cyclohexanone to caprolactam, which is used in the manufacture of Nylon-6, involves Beckmann rearrangement

Mechanism of this rearrangement is given below

Also relative migratory aptitude comes in place when there are two different groups as shown below.

Above two isomers for the unsymmetrical oxime are possible.  When these oximes are rearranged mixture of products are formed and the ratio in which they form is same as the isomer ratio in oxime.

Oximes derived from aldehydes are not good for Beckmann rearrangement because of the poor yields for primary amides.  Tosyl chloride forms oxime tosylate which eliminates the stable tosylate anion.  PCl₅ and SOCl₂ induce rearrangement by converting OH to a better leaving group.

Certain ketoximes (oximes of alpha-diketones, alpha-keto acids, alpha-dialkylamino ketones, alpha-hydroxy ketones, beta-keto ethers) can be converted to nitriles by the action of proton or Lewis acids via fragmentation reactions, which are considered side reactions, often these are called as ‘abnormal’ or ‘second order’ Beckmann rearrangements.

Mechanism for Ketone and aldehyde reduction by NaBH4 to alcohol

Sodium borohydride is a salt which is made up of a sodium cation (Na+) and a borohydride anion (BH4-) The sodium ion plays no important role in the reaction, so we will ignore it (ions like sodium and potassium are seldom directly involved in reactions. They are present merely to maintain charge balance so that stable compounds can be added to reaction mixtures. They are often called spectator ions.) The borohydride ion is the important player in this process, and it is the B-H bond that we want to examine.

From periodic it is understood that both boron and aluminum are metals with relatively low electronegativities. Each is less electronegative than carbon, and since hydrogen has about the same electronegativity as carbon we can conclude that the B-H bond is polarized with the boron positive and the hydrogen negative. So B-H bond effectively serves as a source of hydride ion (:H-). This is given below.

 Mechanism of aldehyde to alcohol conversion

 Below attack by a nucleophile at the carbonyl carbon, followed by protonation of the carbonyl oxygen.

The same mechanism applies to the reduction of ketones by sodium borohydride and to reactions of carbonyl groups with lithium aluminum hydride. In case of lithium aluminum hydride, the reagent itself highly reactive with water, so the water is added after the lithium aluminum hydride has reacted.