Formation of racemic mixture in Nucleophilic Addition.

A nucleophilic addition reaction involves a carbonyl and HCN in the presence of NaCN as the catalyst. Details of the mechanism can be found here.

However, this reaction is capable of forming a racemic mixture. This is possible when the following rules are fulfilled:

1. The carbonyl is asymmetrical.
2. The product formed causes the C on C=O to change from sp² to sp³.
3. The product is an optically active product.

Isn't it slightly ironic that an optically active product is formed and yet we actually obtain a racemic mixture? Before my explanation, it is important to note that a racemic mixture does not mean that an optically active compound is absent. In fact, a racemic mixture implies that the pair of enantiomers exist in the same mixture.

In the reaction between an asymmetrical carbonyl and HCN results in the cyanide ion able to attack the carbonyl above or below the plane of the carbonyl.

If the cyanide ion attacks above the plane, an optically active addition product is formed. If the cyanide attacks below the plane of the carbonyl, the mirror image is formed.

As it is equally probable for the nucleophile to attack above or below the plane of the carbonyl, both enantiomers are formed in equal proportions and hence a racemic mixture is obtained.

This manner of forming racemic mixture, is also applicable the reaction between an alkyl halide and nucleophile, in which, this reaction must occur in the S_N1 mechanism. The carbocation being planar is equally probable for it to be attacked above and below its plane.

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^{Article written by Kwok YL 2010.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Carbonyls - Nucleophilic Addition Mechanism

In the study of the reactions involving carbonyls, one of the most important reactions is the nucleophilic addition reaction (You may like to refer to the definitions of reactions by clicking here.). This is similar to the reactions involving alkylhalides as both requires nucleophiles and electron-deficient carbon atoms. However, carbonyl will show an addition across a double (this time is the C=O), while the alkyl halides shows a replacement (aka substitution.)

The most significant nucleophilic addition reaction which we study in detail, is that between a carbonyl molecule and hydrogen cynanide. This reaction is studied in sufficient detail such that we are able to identify the most suitable conditions for the reaction. In addition, we were able to understand how sometimes NaOH or NaCN were used in trace amounts to boost this reaction.

Racemic mixtures can be formed in the reaction between the carbonyl and HCN. This is because the carbonyl is planar and the nucleophile can attack either side of the plane (of carbonyl) in equal probability. In addition, the carbonyl must first be asymmetrical. If the carbonyl is symmetrical, no racemic mixture is formed.

The following diagrams depicts the illustration of mechanism of the nucleophilic addition reaction when NaOH or NaCN is being used.

(i)When NaCN was used - Regeneration of CN^-:

(ii)When NaOH was used - Regeneration of OH^-

While, the following video helps to demonstrate how the mechanism is drawn.

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Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}