Carboxylic acid s are organic compound s containing carboxy1 group These compounds possess sufficient acidic character and are called carboxylic acids. Many common chemicals such as citric acid (lemon juice), ethanoic acid (vinegar) are carboxylic acids. The carboxyl group is made up carbonyl, and hydroxyl, -OH group, hence, its name is carboxyl group (carb from carbonyl and oxyl from hydroxyl). Carboxylic acids may be aliphatic (R-COOH) or aromatic (Ar-COOH) depending upon whether -COOH group is attached to aliphatic alkyl chain or aryl groups respectively.
Aliphatic monocarboxylic acids (containing <?ne carboxyl group) are known as fatty acids because .some of their higher members (C12-C18) like palmitic acid (C15H31COOH) and stearic acid (C17H35COOH) exist in natural fats as esters and are obtained by their hydrolysis.
Table 46.1. Sources of Alkanoic Acids
GENERAL METHODS OF PREPARATION
1. By Oxidation Reactions
(i) By Oxidation of Primary Alcohols. Carboxylic acids can be easily prepared by oxidation of primary alcohols with acidified potassium permanganate or acidified potassium dichromate.
(ii) By Oxidation of Aldehydes. Aldehydes on oxidation with usual oxidizing agents give carboxylic acids with same number of carbon atoms as in the aldehyde.
R – CHO + [O] à R – COOH
CH3 CHO + [O] à CH3 COOH + H2 O
Ethy1 alcohol Acetic acid
(iii) By Oxidation of Methyl Ketones. Methyl ketones can also be easily oxidized to carboxylate ion by hypohalite solutions. The salts of carboxylic acid so formed, can be hydrolysed to corresponding carboxylic acid. In this method, the acid formed has one carbon atom less than the parent ketone.
2. From Alkyl Cyanides
Alkyl cyanides on hydrolysis with dilute acids or alkalies give carboxylic acids.
This method serves as a very good synthetic method for preparation of carboxylic acids.
3. By Carbonation of Grignard Reagents
Carboxylic acids can be obtained By carbonation of Grignard reagents. The reaction is carried out by bubbling CO2 (dry ice is employed as source of carbon dioxide) through ethereal solution of Grignard reagent and subsequent hydrolysis with dil. acids.
This method gives carboxylic acid with one carbon more than the starting compound.
4. From Sodium Alkoxide and Carbon Monoxide
Heating of sodium alkoxide with CO under pressure yields sodium salt of fatty acids which on subsequent acidification gives acid.
5. From Olefins
Higher fatty acids can be obtained on large scale by heating an olefin with CO and steam under pressure at 570 K-675 K in the presence of phosphoric acid as catalyst.
Benzoic acid can be prepared by oxidation of alkyl benzenes with alkaline potassium permanganate, chromic anhydride or cone. nitric acid. The alkyl side chain gets oxidised to -COOH group irrespective of the size of the chain. Many aromatic acids are obtained industrially by this method.
PHYSICAL PROPERTIES OF ALKANOIC ACIDS
1. Physical State. The first three aliphatic acids are colourless liquids with pungent smell. The next six are oily liquids with an odour of rancid butter while the higher members are colourless, odourless waxy solids. Benzoic acid is a crystalline solid.
2. Solubility. The first four aliphatic members are soluble in water due to intermolecular hydrogen bonding with water molecules.
With increasing size of the alkyl group, the non-polar part of the molecule predominates thereby reducing the solubility in water. The higher members are practically insoluble in water.
3. Boiling Points. Carboxylic acids have quite high boiling points due to the presence of intermolecular hydrogen bonding which results in the formation of dimeric structures.
Due to dimeric structure, the effective molecular mass of the acid becomes double the actual mass. Hence, carboxylic acids have higher boiling points than alcohols and alkanes of comparable molecular masses. Moreover, O – H bond in carboxylic acids is more polar than O-H bond in alcohols. This is due to electron withdrawing effect of carbon group on O-H. Hence, H-bonds in carboxylic acids are relatively stronger than those in alcohols.
4. Melting Points. In first ten members of the homologous series, the alternation effect is observed. The alternation effect implies that the melting point of an acid with even number of carbon atoms is higher than the acid with odd number of carbon atoms above and below it. However, no such effect is observed in homologues with more than ten carbons. The alternation effect can be explained on the basis of the fact that in the carboxylic acids with even number of carbon atoms, the terminal methyl group and carboxylic group are on the opposite sides of zig-zag carbon chain. Hence, they fit better in the crystal lattice and it results in stronger intermolecular forces. On the other hand, acids with odd number of C atoms have carboxyl and terminal methyl group & on the same side of zig-zag carbon chain. Therefore, such molecules being relatively unsymmetrical, fit poorly in the crystal lattice. This causes weaker intermolecular forces and accounts for the relatively lower melting points.
The melting and boiling points of aromatic acids are usually higher than those of aliphatic acids of comparable molecular masses. This is presumably due to the fact that planar benzene ring in these acids can pack closely in the crystal lattice than zig-zag aliphatic acids.
The physical properties of some homologous of alkanoic acids are tabulated below:
CHEMICAL CHARACTERISTICS OF ALKANOICACIDS
In carboxylic acids, the functional group is carboxyl group Carboxylic acids are resonance hybrid of the following structures
From these structures, it is clear that the carbonyl part of the carboxyl group does not have a double bond character but a reduced double bond character. Thus, it does not give the reactions of the carbonyl group. Also it is evident that the two contributing structures of carboxylic acid are not equivalent, therefore, resonance stabilization in them is not much. Moreover, structure II involves charge separation, so this structure has higher energy and hence makes less contribution. It may be noted here that oxygen atom of –OH group has positive charge in structure n, this indicates its electron deficient nature. Hence, the shared pair of electrons of O-H bond will be strongly pulled towards oxygen and this makes the O -H bond quite polar. Thus, the reactions of carpoxylic acids are characteristic of the carboxyl group and alkyl group. Now, let us study their chemical characteristics.
1. ACIDIC NATURE
Carboxylic acids are quite strong acids because of the presence of polar O-H group. They ionize to give hydrogen ions and hence, behave as acids.
Carboxylic acids behave as fairly strong acids. This can be explained as follows:
Carboxylic acids as well as carboxylate ion both are stabilized by resonance. However, carboxylate ion is more stabilized by resonance because its contributing structures are exactly identical. On the other hand, the contributing structures of carboxylic acid involve charge separation. Since carboxylate ion is more stabilized by resonance than carboxy lie acid, therefore, equilibrium in Eqn. ( 46.1) lies very much in forward direction, i.e., in favour of ionized form. Hence, carboxylic acids behave as fairly strong acids. They tum blue litmus red. Some chemical reactions showing the acidic nature of carboxylic acids are:
(a) Reaction with Metals. Carboxylic acids react with metals such as Na, K, Zn, etc., and liberate hydrogen gas.
2R – COOH + ZN à (RCOO)2 ZN + H2
2CH3 COOH + ZN à (CH3COO)2 ZN + H2
(b) Reaction with Alkalies. Carboxylic acids react with alkalies (NaOH, KOH) to form salt and water.
RCOOH + NaOH à RCOONa + H2O
CH3COOH + NaOH à CH3COONa + H2O
Acetic acid Sodjum acetate
(c) Reaction with Bicarbonates and Carbonates. Carboxylic acids react with bicarbonates and carbonates and produce brisk effervescence due to liberation of CO2.
RCOOH + NaHCO3 à RCOONa + C02 + Hp
CH3COOH + NaHCO3 à CH3COONa + CO2 + H2O
Acetic acid Sodium Sodium acetate
2CHFOOH + NaFO3 à 2CH3COONa + CO2 + H2O
(d) Reaction with Ammonia. Carboxylic acid react with ammonia to form ammonium salts. Ammonium salts on heating give amides.
Comparison of acidic strengths of carboxylic acids and alcohols.
In order to understand, the relative acidic strengths of carboxylic acids and alcohols, Jet us consider their dissociation to give H+ Ions.
In Eqn. (46.2) though both carboxylic acid and carboxylate anion are resonance stabilised but resonance stabilization is larger in carboxylate anion than in carboxylic acid. It is because the contributing structures of carboxylate anion are identical but in carboxylic acid, the contributing structures are nonequivalent. Therefore, the equilibrium lies more towards right. On the other hand, in case of alcohols (Eqn. 46.3) neither alcohol nor alkoxide ion are stabilised by resonance and hence alcohols are weakly dissociated and behave as weaker acids than carboxylic acids.
Effect of substituents on acidic strength of carboxylic acids. The factors which increase the stability of carboxylate ion more than the carboxylic acids, increase the acidic strength of acid and the factors that decrease the stability of carboxylate ion decrease the acid strength.
The electron withdrawing groups stabilize the carboxylate anion by dispersal of the negative charge and increase the strength of the acid. On the other hand, the electron releasing groups cause concentration of negative charge, destabilize the carboxylate anion and hence, decrease the strength of the acid.
Electron withdrawing groups such as halo group,-NO2, -CN, etc., increase the acidity of carboxylic acids whereas electron donating groups like alkyl groups decrease the acidity of carboxylic acids.
The effect of various substituents on the strength of acids has been further illustrated with the help of following examples.
1. The effect of number of the substituents is illustrated by the chloro substituted acetic acids. The acid strength increases from chloroacetic acid to trichloroacetic acid.
CICH2 COOH < Cl2CHCOOH < Cl3CCOOH
It is due to the reason that the increase ‘n the number of chloro substituents on a-carbon atom of acetic acid makes the electron withdrawing effect more pronounced and increases the stability of corresponding conjugate base,. i.e., carboxylate ion.
This causes the increase in the strength of their corresponding acids.
2. The effect of nature of the substituent is illustrated by the various halo acetic acids. Their strength increases as
The above order is due to the fact that the electron withdrawing effect of halo-groups increase from
I< Br< Cl<F.
Consequently, the strength of acids also increases.
3. The effect of the position of the substituent is illustrated by the acidity of a-chloro and β chloro propionic acids.
The effect of the substituent decreases as its distance from the – COOH group increases. Thus, electron withdrawing effect in β-chloropropionic acid is less pronounced because -Cl group is relatively away from -COOH group. Thus, α-chloropropionic acid is stronger acid than β-chloropropionic acid.
Measurement of acidity of carboxylic acids. The acid strength of carboxylic acids is measured from their dissociation constant. For example,
where K a is the dissociation constant of acid. Higher the value of K a stronger the acid will be. As the numerical values of Ka vary by large magnitude, therefore, it is more convenient to express the acidic strength in terms of its p K a value(= -log K). Now, smaller is the value of p KO , stronger is the acid and vice versa. The K a and p K a values of some acids are given in Table 46.3.
Table 46.3. K a and Pk a Values of Some Acids