Typical reaction: addition of hydrogen cyanide to an aldehyde or some ketones. Potasium cyanide in aqueous ethanol at pH8 is heated under reflux with the carbonyl compound.

The mechanism:

Points:

• The pH is 8 because if it is lower, there is too little cyanide ion present for the first step; if it is higher, there is too little tendency for the anionic species to pick up a proton and form the -OH group.
• The mechanism shows the proton coming wither from HCN or from water.
• There are some nucleophilic additions to C=O that involve attack of a proton on the carbonyl oxygen to give a carbocation that is then attacked by the nucleophile.
• Few ketones give the reaction - see the Organic Reactions Catalogue pages.

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Dr Rod Beavon   17 Dean's Yard   London    SW1P 3PB

e-mail:  rod.beavon@westminster.org.uk