The carbonyl group here has a dipole with a delta positive carbon and a delta negative oxygen

General Mechanism with an aldehyde:General Mechanism with a ketone:?

In both reactions, the nucleophile (Nu) attacks the carbonyl carbon to form a negatively charged intermediate which quickly reacts with a proton

Addition of HCN to carbonyl compounds

• The nucleophilic addition of hydrogen cyanide to carbonyl compounds is a two-step process, as shown below
• In?step 1,?the cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate
• In?step 2,?the negatively charged oxygen atom in the reactive intermediate quickly reacts with aqueous H+?(either from HCN, water or dilute acid) to form 2-hydroxynitrile compounds,
  • e.g. 2-hydroxypropanenitrile

   

• This reaction is important in organic synthesis, because it adds a carbon atom to the chain, increasing the chain length
• The products of the reaction are hydroxynitriles
  • The nitrile group is the priority functional group so it is attached to carbon 1 and results in the suffix -nitrile
  • The hydroxyl group is not the priority functional group so the hydroxyl group is named using the hydroxy- prefix, rather than the -ol suffix

   

The attack from the :CN-?has a 50:50 chance of taking place on either side of the C=O bond

A racemic mixture, or racemate, of each enantiomer is formed

• The enantiomers in a racemic mixture both rotate plane polarised light, but they rotate it in opposite directions
• Because there is a 50:50 mixture of both enantiomers, each rotating light in equal amounts but opposite directions, the effects on plane polarised light are cancelled out
• Therefore, there will be no effect on plane polarised light with a racemic mixture
  • The optical rotation of the racemic mixture is zero

   

• This can be used as a test to determine whether a mixture is racemic
  • If you know that a sample contains enantiomers of chiral compounds, and when tested there is no effect on plane polarised light, then the reaction mixture must be racemic
  • If there is an effect on plane polarised light, then the sample is not racemic