Addition of hydrogen cyanide to carbonyls to form hydroxynitriles Reaction: carbonyl hydroxynitrile Reagent: sodium cyanide (NaCN) and dilute sulphuric acid. Conditions: Room temperature and pressure Mechanism: nucleophilic addition NC C R H OH hydroxynitrile The NaCN supplies the nucleophilic CNions. The H2SO4 acid supplies H+ ions needed in second step of the mechanism CH3COCH3+ HCN CH3C(OH)(CN)CH3 CH3CHO + HCN CH3CH(OH)CN When naming hydroxy nitriles the CN becomes part of the main chain 2-hydroxy-2-methylpropanenitrile 2-hydroxypropanenitrile Nucleophilic Addition Mechanism H+ from sulphuric acid NC C CH3 CH3 OH NC C CH3 H OH We could use HCN for this reaction but it is a toxic gas that is difficult to contain. The KCN/NaCN are still, however, toxic, because of the cyanide ion. H3C C CH3 O :H- δ + δ – : H+ C H H3C CH3 O H – H3C C CH3 O :CN- δ + δ – :- H+ C CN H3C CH3 O C CN H3C CH3 O H N Goalby chemrevise.org H3C C H O 3 :CN NC: C CH3 O H C NC CH3 OH H C H3C CN OH H There is an equal chance of either enantiomer forming so a racemate forms. No optical activity is seen Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical) when the trigonal planar carbonyl is approached from both sides by the HCN attacking species: results in the formation of a racemate Mechanism for the reaction (drawn the same for both enantiomers) :CN- δ + δ – :-
3.3.8 Aldehydes and ketones (A-level only)
The nucleophilic addition reactions of carbonyl compounds with KCN, followed by dilute acid, to produce hydroxynitriles.
Aldehydes and unsymmetrical ketones form mixtures of enantiomers when they react with KCN followed by dilute acid.
The hazards of using KCN.
Students should be able to:
• write overall equations for the formation of hydroxynitriles using HCN
• outline the nucleophilic addition mechanism for the reaction with KCN followed by dilute acid
• explain why nucleophilic addition reactions of KCN, followed by dilute acid, can produce a mixture of enantiomers.