Carbonyl compounds can be transformed into alcohols through reduction reactions. This process involves adding hydrogen atoms to the carbonyl group, changing its oxidation state. The choice of reducing agent, like NaBH4 or LiAlH4, affects the reaction's strength and selectivity.

Different carbonyl compounds yield specific alcohol types when reduced. Aldehydes become primary alcohols, while ketones form secondary alcohols. Carboxylic acids and esters can also be reduced to primary alcohols, but require stronger reducing agents like LiAlH4.

Reduction of Carbonyl Compounds to Alcohols

Reduction of aldehydes and ketones

• Aldehydes and ketones reduced to alcohols using hydride reducing agents ($NaBH_4$ or $LiAlH_4$)
  • Aldehydes reduced to primary alcohols (carbonyl carbon has one alkyl or aryl group attached)
  • Ketones reduced to secondary alcohols (carbonyl carbon has two alkyl or aryl groups attached)
• Reduction process involves addition of hydride ion ($H^-$) to carbonyl carbon and proton ($H^+$) to carbonyl oxygen
  • Forms new carbon-hydrogen bond and hydroxyl group ($-OH$) at former carbonyl carbon
  • Example: Reduction of acetone (ketone) to 2-propanol (secondary alcohol) using $NaBH_4$
• Reduction involves nucleophilic addition of hydride to the carbonyl group

NaBH4 vs LiAlH4 as reducing agents

• $NaBH_4$ and $LiAlH_4$ commonly used hydride reducing agents for reduction of carbonyl compounds to alcohols
• $NaBH_4$ milder reducing agent compared to $LiAlH_4$
  • Selectively reduces aldehydes and ketones to alcohols
  • Does not reduce carboxylic acids, esters, or amides
  • Used in protic solvents (methanol or ethanol)
• $LiAlH_4$ stronger reducing agent compared to $NaBH_4$
  • Reduces aldehydes, ketones, carboxylic acids, esters, and amides to alcohols
  • Must be used in aprotic solvents (diethyl ether or THF) due to reactivity with protic solvents
  • Example: Reduction of ethyl acetate (ester) to ethanol (primary alcohol) using $LiAlH_4$
• Reduction potential affects the strength and selectivity of reducing agents

Reduction of carboxylic acids and esters

• Carboxylic acids and esters reduced to primary alcohols using $LiAlH_4$
  • $LiAlH_4$ preferred reducing agent due to higher reactivity compared to $NaBH_4$
• Reduction of carboxylic acids to primary alcohols:
  1. Addition of four hydrogen atoms (two $H_2$ molecules)
  2. Proceeds through aldehyde intermediate, further reduced to primary alcohol
  3. Overall reaction: $RCOOH + 4[H] \rightarrow RCH_2OH + H_2O$
  • Example: Reduction of acetic acid to ethanol using $LiAlH_4$
• Reduction of esters to primary alcohols:
  1. Addition of four hydrogen atoms (two $H_2$ molecules)
  2. Proceeds through aldehyde intermediate, further reduced to primary alcohol
  3. Overall reaction: $RCOOR' + 4[H] \rightarrow RCH_2OH + R'OH$
  4. Also produces alcohol ($R'OH$) derived from alkoxy group of ester
  • Example: Reduction of methyl benzoate to benzyl alcohol and methanol using $LiAlH_4$

Stereochemistry and Oxidation State in Carbonyl Reductions

• Reduction of carbonyl compounds decreases the oxidation state of the carbon atom
• Stereochemistry of the product depends on the structure of the starting material and the reducing agent
• The transition state during reduction influences the stereochemical outcome of the reaction