Nucleophilic Addition (Oxford AQA International A Level Chemistry)

Nucleophilic Addition

• Many of the reactions which carbonyl compounds undergo are nucleophilic addition reactions

• The carbonyl group -C=O, in aldehydes and ketones is polarised

• The oxygen atom is more electronegative than carbon drawing electron density towards itself

• This leaves the carbon atom slightly positively charged and the oxygen atom slightly negatively charged

• The carbonyl carbon is therefore susceptible to attack by a nucleophile, such as the cyanide ion

The dipole in the carbonyl group

 General Mechanism with an aldehyde:

General Mechanism with a ketone: 

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

Forming Enantiomers

• Even if a starting material does not display optical isomerism, it can still form a product which does display optical isomerism

• This is the case when aldehydes and ketones undergo nucleophilic addition with hydrogen cyanide, HCN

• Due to the shape of the aldehyde or ketone, the :CN^- can attack on either side of the carbonyl

  • When it attacks on one side, it will produce one enantiomer and when it attacks on the other side, it will produce the other enantiomer

    

• The reaction mixture which is produced will be a racemic mixture

• There will be a 50:50 mixture of both enantiomers, because there is a 50:50 chance of attack happening on each side

• Racemic mixtures are formed when addition reactions are done with a planar starting material, because the reaction takes place with equal probability from either side of the plane

Nucleophilic attack

• 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