4 Addition of hydrogen cyanide

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 – :-