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UndergraduateOrganic chemistryFunctional Group


Carboxylic acids and derivatives


Carboxylic acids and their derivatives are an important class of functional groups in organic chemistry. These compounds are characterized by their specific structural groups and functional properties that make them essential in both biological and industrial contexts.

Structure and nomenclature of carboxylic acids

The carboxylic acid functional group consists of a carbon atom that is double-bonded to an oxygen atom and single-bonded to a hydroxyl group. This combination of atoms forms a -COOH functional group.

R−C(=O)OH

In IUPAC nomenclature, carboxylic acids are named by replacing the ending -e in the name of the parent alkane with -oic acid. For example, the carboxylic acid derived from methane is called methanoic acid.

Examples of carboxylic acids:

  • formic acid (HCOOH), also known as methanoic acid
  • acetic acid (CH3COOH), also known as ethanoic acid
  • propanoic acid (CH3CH2COOH)
  • butanoic acid (CH3(CH2)2COOH)
  • benzoic acid (C6H5COOH)

Visualization of carboxylic acids

The structure of a carboxylic acid can be understood from a simple formula:

R Oh Hey

This structure represents a carboxyl group attached to a general organic group, denoted as R

Properties of carboxylic acids

Carboxylic acids are known for their acidic properties, which is due to their ability to donate hydrogen ions (H+). The acidity arises because the carboxylate ion formed after the hydrogen ion is donated is stabilized by resonance.

  • They generally have higher boiling points than aldehydes, ketones, and alcohols of comparable molecular weight because they form hydrogen bonds.
  • These are generally weak acids, with pKa values ranging from about 4 to 5.
  • They can affect their solubility in water by forming hydrogen bonds.

Carboxylic acid derivatives

Carboxylic acids can form many derivatives by replacing the hydroxyl part of -COOH group with another group. These derivatives include:

Aster

Esters are obtained from carboxylic acids by replacing the hydrogen in the hydroxyl group with an alkyl group. This changes -COOH to -COOR' where R' is an alkyl group.

R−C(=O)OR'

Esters are named based on the alcohol and acid from which they are derived. For example, ethyl acetate is derived from acetic acid and ethanol.

Amides

When the hydroxyl group in a carboxylic acid is replaced by an amino group, it forms an amide. The general structure of an amide is -CONHR', where R' can be either a hydrogen or an alkyl group.

R−C(=O)NHR'

Amides are named by replacing -ic acid or -oic acid suffix of the carboxylic acid with -amide.

Anhydrons

Anhydrides are formed by the condensation of two carboxylic acid molecules, releasing a water molecule. Their general structure is (RCO)₂O.

R−C(=O)O−C(=O)R'

Anhydrides are named after the carboxylic acids from which they are derived, such as acetic anhydride, from acetic acid.

Reactions of carboxylic acids and their derivatives

Carboxylic acids and derivatives are involved in a wide variety of chemical reactions due to their functional group versatility.

Shortage

Carboxylic acids can be reduced to primary alcohols using reducing agents such as lithium aluminum hydride (LiAlH4). The reduction proceeds as follows:

R−C(=O)OH + 4 [H] ⟶ R−CH2OH + H2O

Esterification

Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters, a process called esterification.

R−C(=O)OH + R'OH ⇌ R−C(=O)OR' + H2O

This equilibrium reaction can be moved to the right by removing water from it during the formation of water.

Nucleophilic acyl substitution

Carboxylic acid derivatives undergo nucleophilic acyl substitution where a nucleophile replaces the leaving group. This reaction mechanism is important for converting one type of carboxylic acid derivative into another.

Importance and applications

Carboxylic acids and their derivatives have great importance in various fields. They are used extensively in the pharmaceutical industry, as preservatives in food and as intermediates in the synthesis of various chemical compounds.

  • Acetic acid is also used as a preservative and as a reagent in laboratories and industry.
  • Esters are widely used as fragrances, flavors, and solvents.
  • Amides are found in proteins and are important in the development of drugs.

Summary

Carboxylic acids and their derivatives are a fundamental topic in organic chemistry, providing information about the behavior and transformation of organic molecules. Their diverse structures and reactivity make them important in both natural and industrial chemical processes.


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