Derivatives of Carboxylic Acids: A substituent or moiety in a molecule that is responsible for the molecule’s characteristic chemical reactions is known as a functional group. Whatever be the composition of the rest of the molecule, the same functional group in the molecules will undergo similar chemical reactions. In organic chemistry, there are various functional groups. Among them, the carboxylic acid is a functional group-containing carboxyl group represented as \({\rm{RCOOH,}}\) where \({\rm{‘R’}}\) represents the alkyl or aryl group.

The carbon in the carboxyl group has a double bond with an oxygen atom and a single bond with a hydroxyl group. It gets deprotonated to release a proton \(\left( {{{\rm{H}}^{\rm{ + }}}} \right)\) and the carboxylate anion \(\left( {{\rm{RCO}}{{\rm{O}}^{\rm{ – }}}} \right){\rm{.}}\) Carboxylic acids from several derivatives are described as compounds that can be converted to carboxylic acids by simple acidic or basic hydrolysis. Some of the most important carboxylic acid derivatives are esters, amides and nitriles, acid halides and anhydrides.

What are the Derivatives of Carboxylic Acids?

We know some of the important classes of organic compounds such as alcohols, phenols, ethers, amines, and halides. These functional groups consist of alkyl or aryl groups bonded to hydroxyl, alkoxyl, amino and halo substituents, respectively. On attachment of these same functional groups to an acyl group \({\rm{(RCO – )}}\) change their properties to a great extent, and they are known as carboxylic acid derivatives.

In carboxylic acids, a hydroxyl group is bonded to an acyl group, and their derivatives are prepared by replacement of the hydroxyl group with the respective substituents, such as halo, alkoxyl, amino and acyloxy.

Esters as Derivatives of Carboxylic Acids

Esters are one of the derivatives of carboxylic acids where the hydroxyl group is replaced by an alkoxy group. Esters are produced by the reaction between alcohol and a carboxylic acid, and this process is known as the ‘Esterification reaction’. 

Esters are derived from alcohol and carboxylic acid. They are commonly named as “Alkoxy alkanoate” where the first word of the name comes from the alkyl group of the alcohol, and the second part comes from the carboxylate group of the acid used. For example, \({\rm{Methanol}}({\rm{Alcohol}}){\rm{ + Ethanoic}}\,{\mkern 1mu} {\rm{acid}}({\rm{Carboxylic}}\,{\rm{acid}}) \to {\rm{Methyl}}{\mkern 1mu} \,{\rm{ethanoate}}({\rm{Ester}}){\rm{ + Water}}\)

Acid Halides as Derivatives of Carboxylic Acids

Acid halides or acyl are one of the most activated derivatives of carboxylic acids. Acid chlorides are formed by the reaction of thionyl chloride with a carboxylic acid. They are often used to prepare the other derivatives of carboxylic acid. In such a compound, the halogen atom inductively withdraws electron density away from the electrophilic carbon of the carbonyl group. Thus, acid halides are very reactive towards nucleophilic attack, and nucleophilic acyl substitution reaction takes place since the chloride is a good leaving group.

Anhydrides as Derivatives of Carboxylic Acids

As the word anhydride literally means without water. Thus, an acid anhydride can be defined as the combination of two molecules of carboxylic acid followed by the elimination of one molecule of water. Anhydrides are even considered as activated forms of carboxylic acids, but they are not as reactive as acid halides. The anhydride group also withdraws electron density from the carbonyl carbon by inductive effect, and the carboxylate anion behaves like a good leaving group.

Amides as Derivatives of Carboxylic Acids

An amide is a combination of a carboxylic acid and an amine (or ammonia). The salt formed when an amine and carboxylic acid react together along with the removal of the water molecule is known as an Amide. They are much less basic than their parent amines due to the delocalisation of lone pair of electrons on Nitrogen on the carbonyl oxygen. In fact, in strong acid, it is the oxygen that firstly gets protonated. Amides are of different types: primary amides, secondary amides, and tertiary amides.

Nitriles as Derivatives of Carboxylic Acids

Unlike the other carboxylic acid derivatives, though Nitriles contain the Cyano group, and lacks the carbonyl group, they are still classified as one of the derivatives of carboxylic acid because they are hydrolysed to carboxylic acids and can be prepared by dehydration of primary amides. 

If we look at the structure, both the carbon and nitrogen atoms of a nitrile are sp hybridised, having a bond angle of \({180^ \circ }\) The structure of a nitrile is like an alkyne, except for the presence of lone pair of electrons on Nitrogen instead of a bond to hydrogen.

Reactivity of Derivatives of Carboxylic Acids

Carboxylic acid derivatives undergo nucleophilic acyl substitution reaction where the acyl unit, \({\rm{R – C = O,}}\) undergoes substitution. 

A carboxylic acid derivative is an acyl group, \({\rm{R – C = O,}}\) with different substituents attached to it. 
The reactivity of Carboxylic acid derivatives is governed by this substituent as:

• Electron donating substituents reduces the electrophilicity and makes the Carboxylic acid derivative less reactive.
• Electron withdrawing substituents increases the electrophilicity and makes the Carboxylic acid derivative more reactive.
• The substituent’s ability to function as a leaving group. 

Thus, based on these points, the order of the Carboxylic acid derivatives is given below:

Summary

In short, we can say that the functional group plays a vital role in determining the functions and reactivity of a compound. Though the composition of the rest of the molecule is different but the same functional group in the molecules will undergo similar chemical reactions. For example, though the first four derivatives of carboxylic acids: methanoic acid \(({\rm{HCOOH}}),\) ethanoic acid \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{COOH}}} \right),\) propanoic acid \(\left( {{{\rm{C}}_2}{{\rm{H}}_5}{\rm{COOH}}} \right)\) and butanoic acid \(\left( {{{\rm{C}}_3}{{\rm{H}}_7}{\rm{COOH}}} \right)\) differ in alkyl groups, but all of them will undergo similar chemical reactions. The Carboxyl group is represented as \({\rm{RCOOH,}}\) where \({\rm{‘R’}}\) represents the alkyl or aryl group.

The carbon in the carboxyl group has a double bond with an oxygen atom and a single bond with a hydroxyl group. It gets deprotonated to release a proton \(\left( {{{\rm{H}}^{\rm{ + }}}} \right)\) and the carboxylate anion \(\left( {{\rm{RCO}}{{\rm{O}}^{\rm{ – }}}} \right).\) Carboxylic acids form different derivatives that are described as compounds that can be converted to carboxylic acids by simple acidic or basic hydrolysis. Based on the type of the leaving group, i.e., whether the substituents are electron-withdrawing or electron-donating, the reactivity of these derivatives of carboxylic acid varies.

FAQs on Derivatives of Carboxylic Acids

Q.1. Which derivative of the carboxylic acid is more reactive?
Ans: Acid chloride is the most reactive derivative of carboxylic acid. The halogen atom inductively withdraws electron density away from the electrophilic carbon of the carbonyl group. Thus, acid halides are very reactive towards nucleophilic attack, and nucleophilic acyl substitution reaction takes place since the chloride is a good leaving group.

Q.2. Explain the concept of carboxylic acid derivatives.
Ans: The basic concept about carboxylic acid derivatives is that they are compounds with functional groups that can be converted to carboxylic acids simply by acidic or basic hydrolysis. The carbon in the carboxyl group has a double bond with an oxygen atom and a single bond with a hydroxyl group. It gets deprotonated to release a proton \(\left( {{{\rm{H}}^{\rm{ + }}}} \right)\) and the carboxylate anion \(\left( {{\rm{RCO}}{{\rm{O}}^{\rm{ – }}}} \right).\)

Q.3. How are carboxylic acid derivatives named?
Ans: In the derivatives of carboxylic acid, the hydroxyl group \(\left( {{\rm{ – OH}}} \right)\) of the carboxyl group is replaced by another atom or groups of atoms. Such as in acid halides, the hydroxyl group is replaced by al halogen. For example, in both the common and IUPAC system of nomenclature, Acid halides are named systems by adding the ending “-ic acid” derived from the name of the parent acid and adding the suffix “-yl halide”. For example, \({{\rm{C}}{{\rm{H}}_3}{\rm{COCl}}}\) is named as Ethanoyl chloride. Esters are commonly named as “Alkoxy alkanoate”, where the first word of the name comes from the alkyl group of the alcohol, and the second part comes from the carboxylate group of the acid used.

Q.4. How does leaving group ability affect the reactivity of the carboxylic acid derivative?
Ans: The leaving group of the carboxylic acid derivative is an acyl group, \({\rm{R – C = O,}}\) with different substituents attached to it. The reactivity of carboxylic acid derivatives is governed by this substituent as the electron-donating substituents reduce the electrophilicity and make the Carboxylic acid derivative less reactive. Electron withdrawing substituents increases the electrophilicity and makes the Carboxylic acid derivative more reactive—the substituent’s ability to function as a leaving group.

Q.5. Which is a more stable ester or amide?
Ans: Among ester and amide, the amide is a more stable derivative of carboxylic acid because, when we look upon the \(\left( { – {\rm{CON}}{{\rm{H}}_2}} \right)\) group of amides, Nitrogen has a lone pair of electrons that forms a partial resonating double bond with the carbonyl carbon. Similarly, in ester \(\left( { – {\rm{RCOC}}{{\rm{H}}_3}} \right)\) has lone pair in oxygen that can also form a resonating structure. But when we talk about both amide and ester resonating structures, oxygen in ester is more electronegative than Nitrogen is an amide. Thus the resonating structure of the ester is less stable. Thus, it can be concluded that amide is more stable than an ester.

Q.6. How do carboxylic acids reduce alcohol?
Ans: Carboxylic acid is reduced to alcohol with the help of a reducing agent like \({\rm{LiAl}}{{\rm{H}}_4}.\) It first reduces the carboxylic acid to aldehyde, which rapidly reduces it to alcohol.

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