Reactivity 3.2.10 — Reduction of Organic Compounds

Functional groups in organic molecules can undergo reduction, which involves the gain of electrons (or hydrogen), resulting in the conversion of more oxidized groups to less oxidized ones. In the context of organic chemistry, the most common types of reductions include:

• Reduction of carboxylic acids to primary alcohols (via aldehydes)
• Reduction of ketones to secondary alcohols

Role of Hydride Ions (\( \text{H}^- \))

Reduction is commonly achieved using hydride ion donors such as \( \text{H}^- \). These hydride ions act as nucleophiles and add to the carbon of the polar C=O group in aldehydes and ketones, reducing them to alcohols.

Reduction Pathways

1. Reduction of Carboxylic Acids to Primary Alcohols

This occurs in two steps:

Step 1: Carboxylic acid to aldehyde

\( \text{RCOOH} \xrightarrow{[\text{H}]} \text{RCHO} \)

Step 2: Aldehyde to primary alcohol

\( \text{RCHO} \xrightarrow{[\text{H}]} \text{RCH}_2\text{OH} \)

Overall Reaction:

\( \text{RCOOH} \xrightarrow{[\text{H}]} \text{RCH}_2\text{OH} \)

Example:

Reduction of ethanoic acid:

\( \text{CH}_3\text{COOH} \xrightarrow{[\text{H}]} \text{CH}_3\text{CH}_2\text{OH} \)

2. Reduction of Ketones to Secondary Alcohols

Ketones are reduced in a single step:

\( \text{RCOR’} \xrightarrow{[\text{H}]} \text{RCH(OH)R’} \)

Example:

Reduction of propanone:

\( \text{CH}_3\text{COCH}_3 \xrightarrow{[\text{H}]} \text{CH}_3\text{CH(OH)}\text{CH}_3 \)