Chemical reactions in aldehydes, ketones and carboxylic acids (nucleophilic addition, oxidation and reduction)

Aldehydes, ketones, and carboxylic acids are important organic compounds that play a vital role in chemistry and biochemistry due to their functional groups. Understanding their chemical behavior is essential to understanding how they interact in various reactions. This lesson will explore the reactions of these compounds, focusing on nucleophilic addition, oxidation, and reduction.

Introduction to aldehydes, ketones and carboxylic acids

Aldehydes, ketones, and carboxylic acids contain a carbonyl group (C=O) but differ in structure and reactivity:

• Aldehyde: The carbonyl group is bonded to at least one hydrogen atom. General formula: RCHO.
• Ketone: The carbonyl group is bonded to two carbon atoms. General formula: RCOR'.
• Carboxylic acid: The carbonyl group is attached to the hydroxyl group, making it more reactive. General formula: RCOOH.

Nucleophilic addition reaction

Nucleophilic addition is a common reaction in aldehydes and ketones due to the presence of the polar carbonyl group.

Mechanism of nucleophilic addition

In a nucleophilic addition, a nucleophile donates an electron pair to the electrophilic carbonyl carbon. Here is a general mechanism:

Nucleophile attack: Protonation (if necessary):
R-CO + Nu⁻ → RC(O⁻)(Nu) RC(O⁻)(Nu) + H⁺ → RC(OH)(Nu)
,

Here is a simple visual representation of nucleophilic addition:

Feedback often includes:

• Grignard reagent: RMgX attaches to the carbonyl carbon to form an alcohol after protonation.
• Hydride ions: NaBH4 or LiAlH4 reduce the carbonyl group to alcohol.

Oxidation reactions

Oxidation involves an increase in oxidation state and often the addition of oxygen or the removal of hydrogen. Oxidation to aldehydes and carboxylic acids is an important reaction.

Oxidation of aldehyde

Aldehydes are easily oxidized to carboxylic acids in the presence of KMnO4 or K2Cr2O7 agents. For example:

CH3CHO + [O] → CH3COOH

This converts the ethereal carbonyl group into a more reactive carboxylic acid.

Oxidation of ketones

Ketones resist oxidation because they do not have a hydrogen atom on the carbonyl carbon. However, under strong conditions or with strong oxidizing agents, they can be decomposed to form carboxylic acids or smaller ketones.

Oxidation of carboxylic acids

Carboxylic acids are generally resistant to oxidation due to their stable resonance structures, but under extreme conditions, further oxidation can occur, resulting in the breakdown of the carbon skeleton into carbon dioxide.

Reduction reactions

Reduction is the opposite of oxidation, in which there is a decrease in the oxidation state, usually by adding hydrogen or removing oxygen.

Reduction of aldehydes and ketones

Alcohols are formed by the reduction of aldehydes and ketones. Common reducing agents include sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4):

Aldehyde reduction:
R-CHO + H2 → R-CH2OH
(using NaBH4 or LiAlH4)

For ketones, the mechanism is similar, but results in a secondary alcohol:

Ketone Reduction:
R-COR' + H2 → R-CH(OH)R'
(using NaBH4 or LiAlH4)

Reduction of carboxylic acids

To reduce carboxylic acids to primary alcohols a strong reducing agent like LiAlH4 is required as NaBH4 is insufficient.

RCOOH + 4[H] → RCH2OH + H2O
(using LiAlH4)

Conclusion

Understanding the chemical behavior of aldehydes, ketones, and carboxylic acids through nucleophilic addition, oxidation, and reduction provides information about their reactivity and transformation. Each reaction serves a specific purpose and allows the formation of various other functional groups necessary for the construction of more complex molecules in organic synthesis.

These fundamental reactions not only help in understanding the concepts of organic chemistry but are essential tools in the synthesis of medicinal compounds, polymers and many other chemical products.

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