19.6 Nucleophilic Addition of HCN: Cyanohydrin Formation

Nucleophilic Addition of HCN: Cyanohydrin Formation

Cyanohydrins form when HCN adds across a carbonyl group, creating a new C–C bond in the process. This reaction is one of the most important examples of nucleophilic addition to aldehydes and ketones because it extends the carbon skeleton, giving you a versatile intermediate that can be transformed into amines, carboxylic acids, and other functional groups used in pharmaceutical and natural product synthesis.

Mechanism of Cyanohydrin Formation

The carbonyl carbon is electrophilic because of the polarization of the $\ce{C=O}$ bond (oxygen pulls electron density toward itself). The cyanide anion ($\ce{CN-}$) serves as the nucleophile, attacking that electron-poor carbon. This works with both aldehydes (like benzaldehyde) and ketones (like acetophenone).

Generating the nucleophile: HCN on its own is a weak acid and a poor source of free cyanide. A base catalyst is used to deprotonate HCN and generate the much more reactive $\ce{CN-}$:

$\ce{HCN + OH- -> CN- + H2O}$

Common reagent combinations include $\ce{NaCN}$ or $\ce{KCN}$ in a slightly acidic aqueous solution, or HCN with a catalytic amount of $\ce{NaOH}$.

Step-by-step mechanism:

1. The cyanide anion attacks the electrophilic carbonyl carbon. The $\pi$ bond of $\ce{C=O}$ breaks, and both electrons shift onto oxygen, forming a tetrahedral alkoxide intermediate.
2. The alkoxide is protonated by water (or another protic source) to give the cyanohydrin product. This protonation step also regenerates $\ce{OH-}$, so the base is catalytic.

For example, benzaldehyde gives mandelonitrile, and acetophenone gives 2-hydroxy-2-methylphenylpropanenitrile.

This reaction is reversible. The equilibrium generally favors cyanohydrin formation for aldehydes and unhindered ketones, but bulky ketones shift the equilibrium back toward starting materials because steric strain destabilizes the tetrahedral product.

Favorability of Cyanohydrin Reactions

Two factors drive this reaction forward:

• Cyanide is a good nucleophile. The carbon of $\ce{CN-}$ bears a lone pair and attacks the carbonyl carbon effectively. The linear geometry of cyanide also means minimal steric demand during the addition.
• A strong C–C bond forms. The new $\ce{C-C}$ bond is thermodynamically stable, which helps pull the equilibrium toward product.

Aldehydes vs. ketones: Aldehydes react more readily than ketones. Ketones have two carbon substituents flanking the carbonyl, which introduces steric hindrance and also donates electron density into the carbonyl carbon (making it less electrophilic). Aldehydes, with only one substituent and a hydrogen, are both less hindered and more electrophilic.

Lewis Acid–Base Perspective

You can also frame this reaction using Lewis acid–base theory:

• The carbonyl carbon acts as a Lewis acid (electron-pair acceptor). It has a partial positive charge and an empty $\pi^*$ orbital that accepts electron density from the nucleophile.
• The cyanide anion acts as a Lewis base (electron-pair donor). It donates its lone pair into the carbonyl carbon.

Note that the new tetrahedral carbon in the cyanohydrin product is bonded to four different groups when the starting material is an aldehyde (other than formaldehyde) or an unsymmetrical ketone. That makes it a chiral center, which is why cyanohydrin formation is relevant to asymmetric synthesis. Enzymes called hydroxynitrile lyases can catalyze this reaction enantioselectively.

Synthetic Applications of Cyanohydrins

Cyanohydrins are valuable because the nitrile ($\ce{-CN}$) group can be converted into other functional groups while the hydroxyl group is retained.

Conversion to primary amines:

1. Treat the cyanohydrin with a reducing agent such as $\ce{LiAlH4}$ (followed by aqueous workup) or use catalytic hydrogenation ($\ce{H2}$/metal catalyst).
2. The nitrile group is reduced to a primary amine ($\ce{-CH2NH2}$).

This produces β-hydroxy amines, a motif found in drugs like ephedrine and pseudoephedrine.

Conversion to carboxylic acids:

1. Hydrolyze the nitrile group by heating under either acidic or basic conditions:

   • Acidic: reflux with $\ce{HCl}$ or $\ce{H2SO4}$ in water.
   • Basic: reflux with $\ce{NaOH}$ in water, then acidify.

2. The nitrile is converted to a carboxylic acid ($\ce{-COOH}$).

This yields α-hydroxy acids. Lactic acid and mandelic acid are both α-hydroxy acids that can be prepared through cyanohydrin intermediates. These compounds are important in biochemistry and in the synthesis of natural products.