Nucleophilic Addition (AQA A Level Chemistry): Revision Note

Nucleophilic Addition

• Many reactions of carbonyl compounds are nucleophilic addition reactions

• In aldehydes and ketones, the carbonyl group (C=O) is polarised because the oxygen atom is more electronegative than carbon and draws electron density towards itself

• As a result, the carbon atom becomes slightly positively charged, while the oxygen atom becomes slightly negatively charged

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

• 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 carbon chain, increasing the chain length

• The products of the reaction are hydroxynitriles

• In naming these compounds, the nitrile group is the highest-priority functional group, so it is attached to carbon 1 and gives the suffix –nitrile

• The hydroxyl group is not the priority functional group and is therefore named using the hydroxy- prefix rather than the –ol suffix

Forming Enantiomers

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

• This occurs when aldehydes and ketones undergo nucleophilic addition with hydrogen cyanide (HCN)

• Because the carbonyl group in an aldehyde or ketone is planar, the cyanide ion (CN⁻) can attack from either side of the carbonyl carbon

• Attack from one side produces one enantiomer, while attack from the opposite side produces the other enantiomer

  

• The reaction produces a racemic mixture

• This contains a 50:50 mixture of both enantiomers, because there is an equal probability of nucleophilic attack from either side of the planar carbonyl group

• Racemic mixtures are formed when addition reactions occur with planar starting materials, as attack can take place from either side of the plane with equal likelihood

  

• The enantiomers in a racemic mixture both rotate plane-polarised light, but in opposite directions

• Because a racemic mixture contains equal amounts of both enantiomers, the rotations are equal in magnitude and cancel each other out

  • As a result, a racemic mixture has no overall effect on plane-polarised light and its optical rotation is zero.

• This property can be used to test whether a mixture is racemic

  • If a sample is known to contain chiral molecules but shows no rotation of plane-polarised light, the mixture must be racemic

  • If the sample does rotate plane-polarised light, it is not racemic.