3.8 nucleophilic addition mechanisms

reduction of aldehydes and ketones by nucleophilic addition using warm aqueous NaBH4:

overall

  • 2 H atoms are added onto the C=O bond, one onto the C and one onto the O 

mechanism 

  • NaBH4 generates hydride ion, H-

  • lone pair on H- is donated to δ+ C, forming a dative covalent bond 

  • at the same time, the pi bond in the C=O bond breaks by heterolytic fission,  and both electrons in the pi bond move onto the O atom

  • lone pair on O- in intermediate is donated to H+ from a water molecule

nucleophilic addition reaction between aldehydes or ketones and KCN with dilute acid:

overall

  • CN adds onto the C of the C=O, H adds onto the O of the C=O

mechanism

  • KCN generates cyanide ion, CN-

  • lone pair on CN- is donated to δ+ C, forming a dative covalent bond

  • at the same time, the pi bond in the C=O bond breaks by heterolytic fission, and both electrons in the pi bond move onto the O atom

  • lone pair on O- in intermediate is donated to H+ from dilute acid, e.g. H2SO4