Nucleophilic Addition to Carbonyls

• Nucleophilic Addition: A reaction where a nucleophile attacks an electrophilic carbon atom, breaking a π bond (typically in  ) and forming a new bond.

• Carbonyl Compounds: Molecules containing a polar bond, where the more electronegative oxygen atom leaves the carbon atom slightly electron deficient and leaving it susceptible to nucleophilic attack. e.g. aldehydes and ketones.

  

• The carbon in the bond is electrophilic , attracting nucleophiles.

• A nucleophile (electron pair donor) attacks the electron deficient carbon atom resulting the the π bond breaking.

• A lone pair on the now negatively charged O atom is protonated (accepts an ion)

Formation of Hydroxynitriles

Hydroxynitrile general formula:

• Reagent: Potassium cyanide followed by dilute acid hydrogen cyanide

• The cyanide ion  acts as the nucleophile due to a lone pair of electrons on the .

  

• This reaction is often used to extend the length of the carbon chain which is useful for synthesis.

Mechanism

• Cyanide ion attacks the carbon atom of the

• Formation of tetrahedral intermediate with

• Protonation by dilute acid to form the hydroxynitrile.

  e.g Propanone to 2-hydroxy-2-methylpropanenitrile.

Equation Example:

• For an aldehyde:

• For a ketone:

Choice of Reagent

Potassium cyanide followed by acid is generally preferred over direct reaction with hydrogen cyanide because:

• is highly toxic.

• is a weak acid and so only partially dissociates into and ions. This results in a low concentration of the cyanide ions required for this reaction to take place.

• Optical isomerism arises when a compound has a carbon atom with four different groups attached to it (a chiral carbon centre).

• Optical isomers (enantiomers) are non-superimposable mirror images (see `Optical Isomerism` note)

  

• Nucleophilic addition reactions with carbonyl compounds can often lead to the formation of a racemic mixture where a 50/50 mixture of both optical isomers (enantiomers) are formed.

• These mixtures do not have an effect on plane polarised light as the enantiomers rotate it in equal and opposite directions (cancelling the effects out).

Forming racemic mixtures

Racemic mixtures are formed as:

• The carbonyl group is planar.

• Nucleophilic attack can occur from either above or below the plane.

• There is equal probability of attack from both sides which leads to the formation of a racemic mixture (50:50 mixture of enantiomers).