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Reduction of Aldehydes and Ketones. 19.8 Reduction of Aldehydes and Ketones to Alcohols. Reduction with LiAlH 4 and NaBH 4. 19.8 Reduction of Aldehydes and Ketones to Alcohols. Reduction with LiAlH 4. LiAlH 4 serves as a source of hydride ion (H: - ) - PowerPoint PPT Presentation
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Reduction of Aldehydes and Ketones
119.8 Reduction of Aldehydes and Ketones to Alcohols
Reduction with LiAlH4 and NaBH4
219.8 Reduction of Aldehydes and Ketones to Alcohols
Reduction with LiAlH4
• LiAlH4 serves as a source of hydride ion (H:-)
• LiAlH4 is very basic and reacts violently with water; anhydrous solvents are required
319.8 Reduction of Aldehydes and Ketones to Alcohols
Reduction with LiAlH4
• Like other strong bases, LiAlH4 is also a good nucleophile
• Additionally, the Li+ ion is a built-in Lewis-acid
419.8 Reduction of Aldehydes and Ketones to Alcohols
Reduction with LiAlH4
• Each of the remaining hydrides become activated during the reaction
519.8 Reduction of Aldehydes and Ketones to Alcohols
Reduction with NaBH4
• Na+ is a weaker Lewis acid than Li+ requiring the use of protic solvents
• Hydrogen bonding then serves to activate the carbonyl group
619.8 Reduction of Aldehydes and Ketones to Alcohols
Reduction with LiAlH4 and NaBH4
• Reactions by these and related reagents are referred to as hydride reductions
• These reactions are further examples of nucleophilic addition
719.8 Reduction of Aldehydes and Ketones to Alcohols
Selectivity with LiAlH4 and NaBH4
• LiAlH4 reduces all carbonyl groups
• NaBH4 is less reactive– Reduces
• Aldehydes• Ketones• Esters (slowly)
– Does not reduce• Carboxylic acids• Amides• Nitro groups• Alkyl halides• Alkyl tosylates
819.8 Reduction of Aldehydes and Ketones to Alcohols
Problems
• Which reducing agent, LiAlH4 or NaBH4 is more appropriate for reduction of the following reagents? Draw the products.
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Reduction by Catalytic Hydrogenation
• Hydride reagents are more commonly used• However, catalytic hydrogenation is useful for
selective reduction of alkenes
1019.8 Reduction of Aldehydes and Ketones to Alcohols
Grignard Addition
• Grignard reagents with carbonyl groups is the most important application of the Grignard reagent in organic chemistry
1119.9 Reactions of Aldehydes and Ketones with Grignard and Related Reagents
Grignard Addition
1219.9 Reactions of Aldehydes and Ketones with Grignard and Related Reagents
Importance of the Grignard Addition
• This reaction results in C-C bond formation
• The synthetic possibilities are almost endless
1319.9 Reactions of Aldehydes and Ketones with Grignard and Related Reagents
Importance of the Grignard Addition
1419.9 Reactions of Aldehydes and Ketones with Grignard and Related Reagents
Grignard Addition
• R-MgX reacts as a nucleophile; this group is also strongly basic behaving like a carbanion
• The addition is irreversible due to this basicity
1519.9 Reactions of Aldehydes and Ketones with Grignard and Related Reagents
Problems
• Give the products for the following Grignard reactions:
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Organolithium and Acetylide Reagents
• These reagents react with aldehydes and ketones analogous to Grignard reagents
1719.9 Reactions of Aldehydes and Ketones with Grignard and Related Reagents
Problems
• Give the products for the following rxns:
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Reactions with Amines
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Secondary Amine
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Reactions with Primary Amines
• Dehydration is typically the rate-limiting step
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Reactions with Secondary Amines
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Problems
1) Draw the products for the following reactions:
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2) Draw the mechanism for the formation of the imine in the previous question
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