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Carboxylic acids

Organic compounds containing carboxyl group (–COOH) as functional group are called carboxylic acids. Examples:

Classification of Carboxylic acids

On the basis of number of –COOH groups in their molecules carboxylic acids are classified as:

1. Monocarboxylic acids: The carboxylic acids containing one –COOH group in their molecule.

2. Dicarboxylic acids: The carboxylic acids containing two –COOH groups in their molecule.

3. Tricarboxylic acids: The carboxylic acids containing three –COOH groups in their molecule.

Isomerism in carboxylic acids

1. Chain isomerism: eg.

2. Functional isomerism:

Monocarboxylic acids show functional isomerism with ester. Eg.

General methods of preparation of monocarboxylic acids

1. By the oxidation of primary alcohols and aldehydes:

Primary alcohols are easily oxidized first to aldehyde and then to carboxylic acids. Eg.

2. By hydrolysis of alkyl cyanides [i.e. alkane nitriles]:

Complete hydrolysis of alkane nitriles give carboxylic acids. Eg.

3. By hydrolysis of 1,1,1-trihalides:

Carboxylic acids are obtained when 1,1,1-trihalides are hydrolysed in presence of strong alkalies like KOH. The unstable intermediate formed undergoes dehydration to give carboxylic acid. Eg.

4. From Grignard reagent:

When carbon dioxide gas is bubbled into the ethereal solution of Grignard reagent followed by subsequent hydrolysis with dilute acid then carboxylic acid is obtained.

5. From sodium alkoxides and carbonmonoxide:

When sodium alkoxide is heated with CO under pressure it gives sodium salt of carboxylic acid which upon subsequent acidification gives carboxylic acid. Eg.

6. From dicarboxylic acid:

A dicarboxylic acid having two –COOH groups on same carbon atom on heating undergoes decarboxylation giving a monocarboxylic acid. Eg.

Formic acid is prepared in the laboratory by the decarboxylation of oxalic acid with glycerol at 1100C.

7. Preparation of benzoic acid from the oxidation of alkyl benzene:

Benzoic acid can be obtained by oxidation of alkyl benzene with acidic KMnO4 and K2Cr2O7. During oxidation, side chain is oxidized to –COOH group irrespective of the length of the carbon chain. Eg.

Physical properties of carboxylic acids

1. Solubility: The first four carboxylic acids are soluble in water, the next two acids are slightly soluble. Acids having seven or more carbon atoms are insoluble in water. This is due to the fact that lower acids can form H-bond with water but with increased number of carbon atom the polarity of molecules decreases and it cannot form H-bond.

However, all carboxylic acids are soluble in less polar organic solvents such as ether, alcohol, etc.

2. Boiling point:

(Q) The boiling point of methanoic acid is higher than ethanol though they have same molecular mass, explain.

The boiling point of carboxylic acids is much higher than those of alcohols of comparable molecular mass. This is due to the fact that acids form stronger intermolecular H-bond than alcohol as the O-H bond in acids is more polarized due to presence of adjacent electron withdrawing C=O group. It is also due to the fact that carboxylic acid can form a cyclic dimer by forming hydrogen bond between –COOH group.

Acidity of carboxylic acids

Due to presence of polar O-H group, carboxylic acids ionize to give proton and hence behave as acids.

Both carboxylic acid as well as carboxylate anion are stabilized by resonance.

The resonating structures of carboxylic acid are:

The resonating structures of carboxylate anion are:

However, carboxylate anion is more stabilized by resonance as both the resonating structures of carboxylate anion are equivalent. Due to the more stabilized carboxylate ion the equilibrium lies very much in forward direction. Hence, carboxylic acids behave as acids.

Effect of substituents on acidic strength of carboxylic acids:

1. Effect of electron donating (releasing) substituent :

Q) Why is methanoic acid stronger than ethanoic acid?

Positive inductive effect (+I effect) of electron donating(releasing) groups like alkyl groups( CH3 – , CH3CH2 -, etc.) increases the electron density on O – H bond(group). It makes the release of H+ ion difficult. Therefore, the acidic nature decreases with the increase in + I effect. Hence, methanoic acid (formic acid) is stronger acid than ethanoic acid (acetic acid).

2. Effect of electron withdrawing substituent :

Q) Why is chloroacetic acid stronger than acetic acid?

Negative inductive effect (- I effect) of electron withdrawing groups like halogens, – NO2, -CN, etc. decreases the electron density on O – H bond (group). It makes the release of H+ ion easier. Therefore, the acidic nature increases with the increase in – I effect. Hence, chloroacetic acid is stronger acid than acetic acid.

Chemical properties of carboxylic acids

1. Acidic nature:

a. Reaction with metals:

Carboxylic acids react with active metals like Na, K, Ca, Zn, Mg, etc. forming their respective salt and hydrogen gas. Eg.

b. Reaction with alkalies:

Carboxylic acids can neutralize alkalies like NaOH or KOH to form salt and water. Eg.

c. Reaction with carbonates and bicarbonates:

Carboxylic acids decomposes metal carbonates and bicarbonates which produces effervesce due to liberation of CO2 gas. Eg.

4. Reaction with metal oxides:

Carboxylic acids react with basic metal oxides to form salt and water. Eg.

2. Reactions involving cleavage of –OH group:

a. Reaction with alcohols ( Formation of ester) :

Carboxylic acids react with alcohols in the presence of conc. H2SO4 or dry HCl to form esters. This reaction is called esterification reaction. Eg.

b. Reaction with ammonia(Formation of amide) :

Carboxylic acids react with ammonia to form ammonium salt which on heating give amides. Eg.

c. Reaction with PCl5, PCl3 or SOCl2 ( formation of acid chloride) :

Carboxylic acids react with phosphorus pentachloride(PCl5), phosphorus trichloride (PCl3) or thionyl chloride (SOCl2) to form acid chloride. Eg.

d. Formation of acid anhydrides (Dehydration):

Carboxylic acids on heating in the presence of dehydrating agent like P2O5 form acid anhydrides. Eg.

3. Reduction:

If carboxylic acids are reduced with Lithium aluminium hydride (LiAlH4), only the CO group of carboxylic acid is reduced to CH2 to yield alcohols. Eg.

4. Reactions involving alkyl group:

Halogenation : Hell-Volhard Zelinsky [HVZ] reaction:

Carboxylic acids (except formic acid) reacts with chlorine or bromine in presence of red phosphorous to give α-chloro or α-bromo acids. The reaction does not stop at monosubstituted product but continues till all α-hydrogen atoms are replaced.

Abnormal behaviour of formic acid

In formic acid molecule, the carboxylic acid group is attached to hydrogen atom (not to an alkyl group as in case of other higher members). As a result, it possesses dual functional groups i.e. carboxylic group as well as aldehyde group.

Hence, it behaves as an acid and an aldehyde.

1. Action with Tollen’s reagent: When formic acid is boiled with Tollen’s reagent, a silver mirror is deposited.

Acetic acid does not give this test.

2. Action with Fehling’s solution: When formic acid is warmed with Fehling’s solution, a brick red ppt. of cuprous oxide is obtained.

Acetic acid does not give this test.

Reactions of aromatic carboxylic acid (Benzoic acid)

1. Reactions due to carboxyl group:

Reaction due to benzene ring:

-COOH group present in benzoic acid is electron withdrawing group. Thus it deactivates benzene ring by decreasing electron density at ortho- and para- position. Electron density at meta position is comparatively high and hence electrophile attacks the benzene ring at meta position to give meta substituted product.