R3.2.10 Reduction of functional groups

Reduction of Carboxylic Acids to Primary Alcohols and Ketones to Secondary Alcohols

What is Reduction in Organic Chemistry?

This is the opposite of oxidation, which involves the loss of hydrogen or the gain of oxygen.

In the context of functional groups, reduction often involves converting a more oxidized group (like a carboxylic acid or ketone) into a less oxidized one (like an alcohol).

Reduction of Carboxylic Acids to Primary Alcohols (via Aldehydes)

1. Carboxylic acids are highly oxidized functional groups, meaning they contain a significant number of bonds to oxygen.
2. To reduce a carboxylic acid to a primary alcohol, we need to remove oxygen atoms and add hydrogen atoms.
3. This process occurs in two steps: first, the carboxylic acid is reduced to an aldehyde, and then the aldehyde is further reduced to a primary alcohol.

Step 1: Reduction of Carboxylic Acid to Aldehyde

• The reduction of a carboxylic acid to an aldehyde is challenging because carboxylic acids are relatively stable.
• This step typically requires a strong reducing agent, such as lithium aluminum hydride (LiAlH₄). The reaction can be summarized as:

$$RCOOH+[H]\to RCHO+H_{2}O$$

Here, $[H]$ represents the reducing agent, which donates hydrogen atoms to the molecule.

Step 2: Reduction of Aldehyde to Primary Alcohol

• Once the aldehyde is formed, it can be further reduced to a primary alcohol.
• This step is easier because aldehydes are more reactive than carboxylic acids.

The reaction is:

$$RCHO+[H]\to RC{H}_{2}OH$$

• Combining these two steps, the overall reduction of a carboxylic acid to a primary alcohol can be written as:

$$RCOOH+4[H]\to RC{H}_{2}OH+H_{2}O$$

Reduction of Ketones to Secondary Alcohols

1. Ketones are less oxidized than carboxylic acids, so their reduction is simpler.
2. When a ketone is reduced, it gains hydrogen atoms to form a secondary alcohol.
3. This reaction also uses a reducing agent like sodium borohydride (NaBH₄) or lithium aluminum hydride (LiAlH₄).

Reaction Mechanism

The reduction of a ketone to a secondary alcohol can be summarized as:

$$RCO{R}^{\prime }+2[H]\to RCH(OH)R^{\prime }$$

In this reaction:

• The carbonyl group ($C=O$) in the ketone is converted into a hydroxyl group ($-OH$).
• Two hydrogen atoms are added: one to the carbon atom and one to the oxygen atom.

Quick Recap

1. Reduction of Carboxylic Acids to Primary Alcohols:
   • Two-step process: carboxylic acid → aldehyde → primary alcohol.
   • Requires strong reducing agents like $LiAl{H}_4$.
2. Reduction of Ketones to Secondary Alcohols:
   • Single-step process: ketone → secondary alcohol.
   • Can use milder reducing agents like $NaB{H}_4$.