19.6 Nucleophilic Addition of HCN: Cyanohydrin Formation
- Page ID
- 90969
Objectives
After completing this section, you should be able to
- write an equation to describe the formation of a cyanohydrin from an aldehyde or ketone.
- identify the cyanohydrin formed from the reaction of a given aldehyde or ketone with hydrogen cyanide.
- identify the aldehyde or ketone, the reagents, or both, needed to prepare a given cyanohydrin.
- write the detailed mechanism for the addition of hydrogen cyanide to an aldehyde or ketone.
Make certain that you can define, and use in context, the key term below.
- cyanohydrin
For successful cyanohydrin formation it is important to have free cyanide ions available to react with the ketone or aldehyde. This can be achieved by using a salt (e.g. KCN or NaCN) or a silylated (e.g. Me3SiCN) form of cyanide under acidic conditions or by using HCN with some base added to produce the needed CN− nucleophile.
Cyanohydrins have the structural formula of R2C(OH)CN. The “R” on the formula represents an alkyl, aryl, or hydrogen. To form a cyanohydrin, a hydrogen cyanide adds reversibly to the carbonyl group of an organic compound thus forming a hydroxyalkanenitrile adducts (commonly known and called as cyanohydrins).
Figure 19.6.1: General structure of a cyanohydrin
Mechanism of Cyanohydrin Formation
Acid-catalyzed hydrolysis of silylated cyanohydrins has recently been shown to give cyanohydrins instead of ketones; thus an efficient synthesis of cyanohydrins has been found which works with even highly hindered ketones.
Acetone Cyanohydrins
Acetone cyanohydrins (ACH) have the structural formula of (CH3)2C(OH)CN. It is an organic compound serves in the production of methyl methacrylate (also known as acrylic).
Figure 19.6.2: Acetone cyanohydrins
It is classified as an extremely hazardous substance, since it rapidly decomposes when it's in contact with water. In ACH, sulfuric acid is treated to give the sulfate ester of the methacrylamid. Preparations of other cyanohydrins are also used from ACH: for HACN to Michael acceptors and for the formylation of arenas. The treatment with lithium hydride affords anhydrous lithium cyanide.
Figure 19.6.3: Reduction of Acetone cyanohydrins
Other Cyanohydrins
Other cyanohydrins, excluding acetone cyanohydrins, are: mandelonitrile and glycolonitrile.
Figure 19.6.4: Structures of Madelonitrile (left) and glycolonitrile (right)
Mandelonitrile have a structural formula of C6H5CH(OH)CN and occur in pits of some fruits. Glycolonitrile is an organic compound with the structural formula of HOCH2CN, which is the simplest cyanohydrin that is derived by formaldehydes.
Exercise
What product is formed in this reaction?
- Answer
Contributors and Attributions
Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University)
Prof. Steven Farmer (Sonoma State University)
William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry
Jim Clark (Chemguide.co.uk)