Introduction:

The carboxylic group of the acids consists of a carbonyl group and a hydroxyl group. The hydroxyl group can be substituted by -Cl, -COCR, -NH2 or -OR' groups giving the compounds termed as acyl chloride (acid chloride), acid anhydrides, amides and esters correspondingly. They can be symbolized by the general formula:

Fig: Carboxylic group and acyl group

Here 'Z' stands for Cl, OCOR', NH2 or OR'

R and R' might be alkyl or aryl.

The group illustrated in the above figure, common to all the derivatives is termed as the acyl group.

General Comparative Properties of Carboxylic Acids:

Chlorides, anhydrides and esters encompass normal boiling points and are similar in value to those of aldehydes and ketones of the comparative relative molecular mass. Amides encompass a higher than expected boiling points and at normal temperatures, they exist as solid due to a fairly high degree of intermolecular hydrogen bonding. The capability of amides to form hydrogen bonds by water molecules allows them to be much more soluble than the other derivatives. Simple chlorides and anhydrides experience spontaneous hydrolysis in the water.

Reactivity:

The derivatives of carboxylic acids experience nucleophilic substitution reactions. This is characterized through nucleophilic substitutions of the halogen, carboxylate, alkoxy or amino groups and the reaction is identical to the condensation reactions of aldehydes and ketones. All the substituent groups have lone pairs of electrons that are conjugated by the carbonyl group. As the electronegativity of the group increases, the degree of conjugation decreases and the electron availability concerning the carbonyl oxygen is reduced.

Fig: Reactivity of carboxylic group

The order of electronegativity of the substituent group is as shown below:

- Cl > OOCR' > OR' > -NH₂

The overall polarity of the carbonyl group is improved by the more electronegative substituent because of the simultaneous withdrawal of electron away from the carbon atom, making it more susceptible to the nucleophilic attack. The decreasing order of reactivity of carboxylic acid itself and its derivatives are as shown:

Fig: Order of reactivity of carboxylic acid

Acyl Chlorides (Acid Chlorides):

Common Methods of Formation:

a) From acids:

Acyl chlorides can be prepared via heating carboxylic acids with phosphorus trichloride (PCl₃) or phosphorus pentachloride, PCl₅) or sulphur dichloride oxide (thionyl chloride), SOCl₂.

RCOOH + PCl₃ → RCOCl + H₃PO₃

RCOOH + PCl₅ → RCOCl + POCl₃ + HCl

RCOOH + SOCl₂ → RCOCl + HCl + SO₃

b) From the salt of the acids:

Acyl chloride can be made through distilling the salts of the acid by phosphorus trichloride, PCl₃ or phosphorus trioxychloride, POCl₃ or sulphuryl chloride, SO₂Cl₂.

CH₃COONa + PCl₃ → CH₃COCl + Na₃PO₃

                              Ethanoyl chloride

2CH₃COONa + PCl₅ → 2CH₃COCl + NaCl + NaPO₃

(CH3COO)Ca + SO₂Cl₂ → 2CH₃COCl + CaSO₄

Physical Properties:

1) The lower members are colourless, volatile liquids by irritating smell.

2) Their boiling point is much lower than those of acids from which they are obtained. This is because the absence of intermolecular hydrogen bonding.

3) They mainly fume in moist air producing vapors of the hydrogen chloride.

4) They are usually insoluble. Though they hydrolyze slowly to go to solution.

Reactions of Acyl chlorides:

The reactions of acyl chlorides are basically identical to those of carboxylic acids. The halogen readily experiences nucleophilic substitution by -OH, -OR', NH₂ and so forth. The method is identical to those of condensation reactions of the aldehydes and ketones.

Benzoyl chloride is much less reactive as compare to any aliphatic acyl chloride because of the reduction in the positive nature of the carbonyl carbon caused via delocalization of electrons between it and the benzene ring (that is, resonance effect), leading to the reduction in the strength of electron deficient nucleophilic site.

Fig: Reactions of Acyl chlorides

a) Reactions with water (Hydrolysis):

Acyl chlorides hydrolyze in water to form the parent carboxylic acids and hydrogen chloride.

RCOCl + H₂O → RCOOH + HCl

For illustration: 

CH₃COCl + H₂O → CH₃COOH + HCl

b) Ester Formation:

Acyl chlorides react by alcohols and phenols to form the repective ester and hydrogen chloride.

RCOCl + R'OH → RCOOR' + HCl

Esterification by phenol needs an alkaline medium.

Fig: Esterification with phenol

c) Amide Formation:

Acyl chlorides react by ammonia to give the respective amides and ammonium chloride.

Fig: Amide formation

d) Aldehyde Formation (Rosenmund Reduction):

Acyl chlorides are reduced to aldehydes whenever heated by palladium poisoned with barium tetraoxosulphate (VI). The barium tetraoxosulphate (VI) poison the catalytic activity of the palladium restricting the reduction to only aldehyde. Without the poisonous effect of the barium tetraoxosulphate (VI) the aldehyde may be reduced to alcohol too.

RCOCl + (H₂, heat)(Poisoned Pd/BaSO₄ cat) → RCHO + HCl

e) Ketone Formation (Friedel-Crafts acylation):

Acyl Chlorides reacts by aromatic hydrocarbon in the presence of an anhydrous aluminium chloride- Lewis acid, to provide a good outcome of the aromatic alkanone.

For illustration the reaction with benzene, the reaction mixture is refluxed on the water bath at 50^oC.

Fig: Ketone Formation

f) Anhydride formation:

Acyl chlorides react by the sodium salts of carboxylic acids at an elevated temperature to give acid anhydrides. For illustration:

CH₃COONa + CH₃COCl + (distil) → (CH₃CO)₂O + NaCl 

Acid Anhydrides:

Acid anhydride might be regarded as being derived from acid(s) by the elimination of one molecule of water from two molecules of acids.

Fig: Acid Anhydrides

General methods of Formation of Acid Anhydrides:

a) By heating the acyl chloride with anhydrous sodium salt of the acid or the acid

CH₃COCl + NaOCOCH₃ → CH₃COOCOCH₃ + NaCl

CH₃COOH + CH₃COCl + (pyridine) → CH₃COOCOCH₃ + HCl

b) By treating surplus of anhydrous sodium salt of the acid by phosphorus oxy-chloride or thionyl chloride

3CH₃COONa + POCl₃ → 3CH₃COCl + Na₃PO₄

CH₃COCl + NaOCOCH₃ → CH₃COOCOCH₃ + NaCl

2CH₃COONa + SOCl₂ → CH₃COOCOCH₃ + SO₂ + NaCl 

c) Via the dehydration of anhydrous acids in the presence of appropriate dehydrating agent such as P₂O₅

2CH₃COOH + (heat, P₂O₅) → CH₃COOCOCH₃ + H₂O

d) Anhydrides of higher acids are obtained via heating their sodium salt of acids by ethanoic anhydride.

2RCOONa + (CH₃CO)₂O → RCOOCOR + 2CH₃COONa

Physical Properties of Acid Anhydrides:

1) They are colourless liquids or solids having a sharp irritating purgent smell.

2) They are insoluble in water.

3) Their boiling points are higher as compare to those of the acid from which they are derived as their big size and greater Van der Wall's interaction.

Chemical Properties of Acid Anhydrides:

Acid anhydrides readily experience nucleophilic substitution reaction similar to other carboxylic acid derivatives. The carbonyl carbon is the leaving group.

a) Hydrolysis:

Acid anhydrides hydrolyse in water to provide the parent carboxylic acids.

(RCO)₂O + H₂O → 2RCOOH

For illustration:

(CH₃CO)₂O + H₂O → 2CH₃COOH

Ethanoic                    Ethanoic acid

anhydride

b) Ester formation:

The acid anhydrides react by alcohols and phenols to provide esters and carboxylic acids. The reaction is irreversible.

(RCO)₂O + R'OH → RCOOR' + RCOOH

                               Ester    Carboxylic acid

For illustration:

(CH₃CO)₂O + CH₃OH + (reflux) → CH₃-COOCH₃ + CH₃COOH

Ethanoic acid  methanol               methyl ethanoate  ethanoic acid

anhydride

Fig: Example of Ester formation

c) Amide formation:

Amides are made up whenever acid anhydride reacts with ammonia, NH₃ and primary amine, R'NH2.

(RCO)₂O + NH₃ → RCONH₂ + RCOOH

              Ammonia    Amide

(RCO)₂O + R'NH₂ → RCONHR + R'COOH

                  Amine   N-substituted

                                     amide

For illustration:

(CH₃CO)₂O + NH₃ → CH₃CONH₂  + CH₃COOH

                                  Ethanamide    Ethanoic acid

(CH₃CO)₂O + CH₃CH₂NH₂ → CH₃CONHCH₂CH₃ + CH₃COOH

Ethanoic acid  Ethylamine       N-ethyl ethanamide  Ethanoic acid

d) Ketone formation: Friedel - Craft acylation

Acid anhydrides react by benzene in the presence of anhydrous aluminium chloride catalyst to form aromatic ketone and carboxylic acids.

(RCO)₂O + C₆H₆ + (AlCl₃ Catalyst) → C₆H₅COR + RCOOH

For illustration:

(CH₃CO)₂O + C₆H₆ + (AlCl₃ Catalyst) → C₆H₅COCH₃ + CH₃COOH

Esters:

Esters encompass the general formula RCOOR'

General Methods of Preparation of esters:

a) By direct esterification:

Esters are made by refluxing the acid with alcohols in the presence of small quantity of concentrated tetraoxosulphate (vi) acid.

For illustration: CH₃COOH + HOC₂H₅ ↔ CH₃COOC₂H₅ + H₂O

b) By the action of alcohols on acyl chloride or anhydrides:

For illustration:

CH₃COCl + HOC₂H₅ → CH₃COOC₂H₅ + HCl

(CH₃CO)₂O + HOC₂H₅ → CH₃COOC₂H₅ + CH₃COOH

c) By the action of alkyl halides on the silver salts of fatty acids

For illustration:

CH₃COOAg + C₂H₅I → CH₃COOC₂H₅ + AgI

d) From diazomethane:

For illustration: CH₃COOH + CH₂N₂ → CH₃COOC₂H₅ + N₂

                                         Diazomethane

                                      (Ethereal solution)

e) From Carboxylic acid and alkene:

An ester is made if an acid is treated by an alkene in the presence of boron trifluoride as the catalyst.

RCOOH + CH₂ = CH₂ + BF₃ → RCOOC₂H₅

f) From ethers and carbon (II) oxide:

Ethers react by carbon (II) oxide at 125-180^oC under 500 atmospheric pressure in the presence of boron trifluoride and little water to make ester.

R-O-R + CO + (Heat/BF₃) → RCOOR

Physical Properties of esters:

1) They are colourless liquids having characteristic sweet odors.

2) The boiling points of esters are normal, increasing as the relative molecular masses increase. Methyl and ethyl ester though have much lower boiling points than their related parent acid despite having higher molecular masses. The boiling points of normal-chain esters are higher than those of the branched chain isomers.

3) Methyl methanoate is very soluble in water; though there is a progressive and fast decrease in solubility of the higher compounds as the relative molecular mass increase. The esters of aromatic carboxylic acids are insoluble.

Reactions of esters:

Esters experience nucleophilic substitution reactions in which the alky group -OR' is replaced by the weak nucleophiles under acid or base catalyzed conditions essential to improve the electron shortage of the carbonyl carbon atom.

a) Hydrolysis:

Esters hydrolyzed under an acid or base catalyzed condition to make carboxylic acids and alcohols or produce the carboxylates and alcohols correspondingly. The acid-catalyzed method is the precise opposite of esterification.

i) Acid catalyzed:

RCOOR' + H₂O ↔ (H+/Reflux) ↔ RCOOH + R'OH

                          Carboxylic alcohol

                                Acid

For illustration:

HCOOCH₂CH₂CH₃ + H₂O ↔ (H⁺) ↔ HCOOH + CH₃CH₂CH₂OH

Propyl methanoate                          Methanoic       Propan-1-ol

                                                            acid

ii) Alkali- catalyzed:

RCOOR' + OH^- ↔ RCOO^- + R'OH

                        Carboxylate  alcohol

For illustration:

CH₃COOCH₂CH₃ + NaOH ↔ CH₃COONa + CH₃CH₂OH

                                                Sodium          Ethanol

                                               Ethanoate

Hydrolysis in alkaline conditions is termed to as saponification as it is the kind of reaction employed in soap making process. 'Soapy detergents' are alkali metal derivatives of carboxylic acids and they hold between 10 and 18 carbon atoms. Fats and oils (that is, triesters) are hydrolyzed to produce the sodium carboxylate (that is, soap).

For illustration:

Fig: Example of Hydrolysis reactions

b) Amide formation:

Esters reacts by ammonia dissolved in an alcoholic medium or otherwise concentrated aqueous ammonia having dissolved ammonium salts to provide amide and alcohol.

RCOOR + NH₃ + (RO^- in alcohol or NH⁺ in concentrated Aq. solution) → RCONH₂ + R'OH

c) Formation of ketones and tertiary alcohols:  

Esters react by Grignard reagent by using ether as solvent, the product is a ketone that can react further by Grignard reagent to make tertiary alcohol through hydrolysis of the intermediate alkyl magnesium halide by using the aqueous ammonium chloride.

Fig: Formation of ketones and tertiary alcohols

d) Ester Exchange (Transesterification):

This is the method whereby one alcohol replaces the other in an ester.  The reaction might be catalyzed through an acid or via the basic alkoxide, RO^-

RCOOR' + R"OH ↔ (H⁺ or RO^-) ↔ RCOOR" + R'OH

e) Reduction:

The reduction of esters can be taken out via reacting it by the reagent lithium tetrahydridoaluminate (LiAIH₄) and then by water to produce the mixture of two alcohols.

RCOOR + 4[H] + [LiAlH₄/(C₂H₅)O]/H₂O → RCH₂OH + R'OH

f) Replacement of the carboxylate group:

If dry hydrogen bromide, concentrated aqueous hydrogen iodide or concentrated tetra-oxosulphate (VI) acid.

Amides:

Amides are the monoacyl derivates of ammonia. They might be categorized as primary, secondary, or tertiary amide based on the number of alkyl groups linked to the nitrogen atom.

General Methods of Preparation of Amides:

a) By heating ammonium salts of carboxylic acids:

RCOONH₄ + (heat) → RCONH₂ + H₂O

b) Via ammonolysis of acyl chloride, anhydrides or ester by concentrated ammonia:

RCOCl + NH₃ → RCONH₂ + HCl

(RCO)₂O + NH₃ → RCONH₂ + CH₃COOH

RCOOR' + NH₃ → RCONH₂ + R'OH

c) By partial hydrolysis of alkyl cyanides through cold concentrated hydrochloric acid, polyphosphoric acid or alkaline hydrogen peroxide.

RC ≡ N + H₂O → RCONH₂

Physical Properties of Amides:

1) Primary amides, RCONH₂ are crystalline solids due to mostly fairly strong intermolecular hydrogen bonding. Though methanamide is a liquid at room temperature (and melting point 3^oC).

2) The melting points of amides are high for the size of the molecules as they can make hydrogen bonds. The hydrogen atoms in the NH₂ group are adequately positive to form a hydrogen bond having a lone pair on the oxygen atom of the other molecule.

Fig: Melting point of amides

Each and every molecule consists of two slightly positive hydrogen atoms therefore there is the potential for lots of hydrogen bonds to be formed. Such hydrogen bonds require a reasonable quantity of energy to break, and therefore the melting points of the amides are pretty high.

3) The small chain amides are soluble in water as they encompass the capability to form hydrogen bond by the water molecules.

Reactions of amides:

Amides are somewhat basic in character, due to the lone pair of electrons on the nitrogen atom. Owing to resonance between the two canonical forms of amide, the lone pair of electrons is not readily available for donation as in amines forming amides much less basic.

a) Hydrolysis of amides:

The hydrolysis of amide is either acid catalyzed or alkaline catalyzed. The acid hydrolysis comprises the use of acids like dilute hydrochloric acid that acts as a catalyst for the reaction among the amide and water.

RCONH₂ + (H₂O, H⁺)/Heat → RCOOH + NH₄⁺

For illustration:

CH₃CONH₂ + H₂O + HCl → CH₃COOH + NH₄⁺Cl^-

The alkaline hydrolysis comprises reaction by hydroxide ions.

RCONH₂ + (OH^-/Heat) → RCOO^- + NH₃

For illustration:

CH₃CONH₂ + NaOH → CH₃COONa + NH₃

The acid catalyzed hydrolysis offers carboxylic acids and ammonium salt, whereas the base catalyzed hydrolysis offers carboxylate and ammonia gas.

b) Hofmann Degradation:

The Hofmann degradation is a reaction between the amide and a mixture of bromine and sodium hydroxide solution in the presence of heat. The general effect of the reaction is a loss of the carboxyl group of the amide to obtain a primary amine the original amide.

RCONH₂ + Br₂ + 4NaOH → RNH₂ + 2NaBr + Na₂CO₃ + 2H₂O

For illustration:

CH₃CONH₂ + Br + 4NaOH → CH₃NH₂ + Na₂CO₃ + 2NaBr + 2H₂O

The Hoffmann degradation is employed as a manner of eliminating carbon atom out of a chain.

c) Dehydration:

Amides are dehydrated via heating a solid mixture of the amide and phosphorus (v) oxides, P₂O₅. Water is eliminated from the amide group to leave a nitrile group, -CN. The liquid nitrile is collected via simple distillation.

RCONH₂ + (_P2O₅/heat) → RCN + H₂O

For illustration:

CH₃CONH₂ + (P₂O₅/heat) → CH₃CN + H₂O

d) Reduction:

Amides can be reduced via the reagent lithium tetrahydridoaluminate (LiAlH₄) to give a primary amine.

Fig: Reduction of Amides

Uses of carboxylic acids and their derivatives:

1) Benzoic acid and its sodium salt are mainly employed as food preservative.

2) Nylon 6.6 and polymers made up from the derivatives of carboxylic acids.

3) Preparation of soap.

4) Liquid esters are broadly employed as solvents for all purpose adhesives, thinners for paints and nail varnish remover.

5) As flavor enhancers in the food processing industry.

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