Cyanohydrin Formation

• Cyanide adds to aldehydes and ketones to give a cyanohydrin.
• The reaction is usually carried out using NaCN or KCN with HCl.
• HCN is a fairly weak acid, but very toxic.
• The reaction is useful since the cyano group can be converted into other useful functional groups (-CO₂H or -CH₂NH₂)

NUCLEOPHILIC ADDITION OF CYANIDE TO AN ALDEHYDE
Step 1: The nucleophilic C in the cyanide adds to the electrophilic C in the polar carbonyl group, electrons from the C=O move to the electronegative O creating an intermediate alkoxide.
Step 2: An acid/base reaction. Protonation of the alkoxide oxygen creates the cyanohydrin product.

Reactions of RLi and RMgX with Aldehydes and Ketones
(review of Chapter 14)

Reactions usually in Et₂O or THF followed by H₃O⁺ work-ups

Reaction type:  Nucleophilic Addition

Summary

• Organolithium or Grignard reagents react with the carbonyl group, C=O, in aldehydes or ketones to give alcohols.
• The substituents on the carbonyl dictate the nature of the product alcohol.
• Addition to methanal (formaldehyde) gives primary alcohols.
• Addition to other aldehydes gives secondary alcohols.
• Addition to ketones gives tertiary alcohols.
• The acidic work-up converts an intermediate metal alkoxide salt into the desired alcohol via a simple acid base reaction.

• Reaction of RLi and RMgX with esters
• Reaction of RLi and RMgX with epoxides
• Reaction of RLi and RMgX with carbon dioxide
• Hydride Reductions of Aldehydes and Ketones

NUCLEOPHILIC ADDITION OF RLi or RMgX TO AN ALDEHYDE
Step 1: The nucleophilic C in the organometallic reagent adds to the electrophilic C in the polar carbonyl group, electrons from the C=O move to the electronegative O creating an intermediate metal alkoxide complex.
Step 2: This is the  work-up step, a simple acid/base reaction. Protonation of the alkoxide oxygen creates the alcohol product from the intermediate complex.

The Wittig Reaction
 

ylide           aldehyde or ketone                  alkene

Reaction type:  Nucleophilic Addition then Elimination

Summary

• The Wittig reaction is an important method for the formation of alkenes.
• The double bond forms specifically at the location of the original aldehyde or ketone.
• Ylides are neutral molecules but have +ve and -ve centers on adjacent atoms that are connected by a s bond.
• The ylide is prepared via a two step process:

An SN2 reaction between triphenyl phosphine and an alkyl halide followed by treatment with a strong base such as an organolithium reagent.

Related Reactions

• Dehydration of Alcohols
• Dehydrohalogenation of Alkyl Halides

THE WITTIG REACTION
Step 1: The nucleophilic C of the ylide Wittig reagent adds to the electrophilic C in the polar carbonyl group, electrons from the C=O p bond are used to form a s bond to the +ve P atom. This creates a cyclic intermediate called an oxaphosphetane.
Step 2: Decomposition of the intermediate by breaking the C-P and C-O s bonds leads to the formation of the C=C p bond of the alkene and triphenyl phosphine oxide.

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