Many organic molecules contain more than one functional group. During synthesis, chemists often need to change one part of a molecule without disturbing another. If a reagent reacts mainly with one functional group while leaving another mostly unchanged, the reaction is chemoselective. Chemoselectivity is the preference of a chemical reaction to occur at one functional group or reactive site in the presence of another. ^[1,2]

Why Does Chemoselectivity Occur

Chemoselectivity occurs because different functional groups do not react at the same rate under the same conditions. A reagent may react quickly with one functional group but slowly, or not at all, with another. It depends on the reactivity of the functional groups, the strength of the reagent, steric and electronic effects, and reaction conditions such as solvent, temperature, catalyst, reagent amount, and reaction time. ^[2]

Therefore, chemoselectivity depends on both the molecule and the conditions used.

Examples

1. Chemoselective Reduction with NaBH₄ ^[3]

Sodium borohydride (NaBH₄) is a mild reducing agent. It usually reduces aldehydes and ketones under mild conditions, but not esters or carboxylic acids under the same conditions.

Aldehyde-containing ester —[NaBH₄]→ Alcohol-containing ester

For example, methyl 4-formylbenzoate (4-(CHO)C₆H₄COOCH₃) contains both an aldehyde group (–CHO) and an ester group (–COOCH₃). When treated with NaBH₄, it yields methyl 4-(hydroxymethyl)benzoate (4-(CH₂OH)C₆H₄COOCH₃):

4-(CHO)C₆H₄COOCH₃ + 2[H] —[NaBH₄]→ 4-(CH₂OH)C₆H₄COOCH₃ 

Here, the aldehyde group is reduced to a primary alcohol, while the ester group remains unchanged. It happens because the aldehyde is more reactive toward NaBH₄ than the ester. The mild reducing ability of NaBH₄ helps maintain selectivity in the reaction.

2. Reduction with LiAlH₄ ^[3]

Lithium aluminum hydride (LiAlH₄) is a much stronger reducing agent than sodium borohydride. It can reduce many carbonyl-containing functional groups, including aldehydes, ketones, esters, and carboxylic acids.

Aldehyde-containing ester —[LiAlH₄]→ Diol + alcohol by-product

Consider methyl 4-formylbenzoate again. When treated with LiAlH₄ and followed by an acidic workup, both functional groups are reduced, yielding 1,4-benzenedimethanol (4-(CH₂OH)C₆H₄CH₂OH):

4-(CHO)C₆H₄COOCH₃ —[LiAlH₄, then H₃O⁺]→ 4-(CH₂OH)C₆H₄CH₂OH + CH₃OH

Methanol (CH₃OH) is formed from the methoxy group of the ester.  

These two reactions show that, for a molecule containing both an aldehyde and an ester, LiAlH₄ is less selective than NaBH₄ because it reduces both functional groups under these conditions.

3. Selective Hydrogenation ^[4]

Under suitable catalytic hydrogenation conditions, a carbon-carbon double bond can be reduced to a single bond while some other functional groups remain unchanged.

Alkene-containing compound —[H₂, catalyst]→ Alkane-containing compound

For example, methyl cinnamate (C₆H₅–CH=CH–COOCH₃) contains a carbon-carbon double bond (C=C) along with an ester group. Under mild catalytic hydrogenation conditions, the carbon-carbon double bond is reduced to a single bond (C–C), while the ester group remains unchanged, forming methyl 3-phenylpropanoate (C₆H₅–CH₂–CH₂–COOCH₃):

C₆H₅–CH=CH–COOCH₃ —[H₂, Pd/C]→ C₆H₅–CH₂–CH₂–COOCH₃

However, this selectivity depends strongly on the catalyst, temperature, pressure, solvent, and reaction time. Under stronger hydrogenation conditions, other functional groups may also react. 

4. Chemoselective Oxidation ^[5]

Chemoselective oxidation occurs when one functional group is oxidized while another part of the molecule remains unchanged.

For example, methyl 4-(hydroxymethyl)benzoate (4-(CH₂OH)C₆H₄COOCH₃) contains a hydroxymethyl group (–CH₂OH) and an ester group. When treated with a mild oxidizing agent such as PCC, the primary alcohol can be oxidized to an aldehyde (–CHO), while the ester group remains unchanged. The final product is methyl 4-formylbenzoate (4-(CHO)C₆H₄COOCH₃):

4-(CH₂OH)C₆H₄COOCH₃ + [O] —[PCC]→ 4-(CHO)C₆H₄COOCH₃  + H₂O

The oxidizing agent and reaction conditions must be carefully chosen. A stronger oxidizing agent may oxidize a primary alcohol further to a carboxylic acid or affect other parts of the molecule.