Acidity of Carboxylic Acids
The acidity of a carboxylic acid is due to the resonance stabilization of its anion.
Because of the resonance, both the carbon oxygen bond in the carboxylate anion have identical bond length. In the carboxylic acid, these bond lengths are no longer identical.
The acidity of carboxylic acid depends very much on the substituent attached to – COOH group. Since acidity is due to the resonance stabilization of anion, substituent causing stabilization of anion increases acidity whereas substituent causing destabilization of anion decrease acidity. For example, electron withdrawing group disperses the negative charge of the anion and hence makes it more stable causing increase in the acidity of the corresponding acid, on the other hand, electron-releasing group increases the negative charge on the anion and hence makes it less stable causing the decrease in the acidity. In the light of this, the following are the orders of a few substituted carboxylic acids.
(a) Increase in the number of Halogen atoms on a-position increases the acidity, eg.
Cl3CCOOH > Cl2CHCOOH > ClCH2COOH > CH3COOH
(b) Increase in the distance of Halogen from COOH decreases the acidity e.g.
This is due to the fact that inductive effect decreases with increasing distance.
(c) Increase in the electronegativity of halogen increases the acidity.
FCH2COOH > BrCH2COOH > ICH2COOH
Illustration 1. Which one of the following would be expected to be most highly ionised in water?
(A) CH2Cl–CH2–CH2COOH (B) CH3CHCl –CH2–COOH
(C) CH3–CH2–CHCl–COOH (D) CH3CH2–CCl2 –COOH
Carboxylic acids are weak acids and their carboxylate anions are strong conjugate bases and are slightly alkaline due to the hydrolysis of carboxylate anion compared to other species, the order of acidity and basicity of corresponding conjugate bases are as follows:
Acidity RCOOH > HOH > ROH > HC º CH > NH3 > RH
Basicity RCOO– < HO– < RO– < HC º C– < < R–
Illustration 1. The carboxylic acids react with metals to liberate hydrogen and are soluble in both NaOH and NaHCO3 solutions. For example.
2CH3COOH + 2Na -> 2CH3COO–Na+ + H2
CH3COOH + NaOH -> CH3COO–Na+ + H2O
CH3COOH + NaHCO3 -> CH3COO–Na+ + H2O + CO2
Illustration 2. Arrange the following illustration in order of increasing acidity
(i) HCOOH, ClCH2COOH, CH3COOH
(ii) CH3COOH, (CH3)2CHCOOH, (CH3)3CCOOH
(iii) ClCH2COOH, Cl2CHCOOH, Cl3CCOOH
(iv) ClCH2COOH, CH3CH2COOH, ClCH2CH2COOH, (CH3)2CHCOOH,CH3COOH
(v) CH3COOH, Cl2CHCOOH, CH3CH2COOH, Cl3CCOOH, ClCH2COOH
Solution: (i) CH3COOH < HCOOH < ClCH2COOH
(ii) (CH3)3CCOOH < (CH3)2CHCOOH < CH3COOH
(iii) ClCH2COOH < Cl2CHCOOH < Cl3CCOOH
(v) CH3CH2COOH < CH3COOH < ClCH2COOH < Cl2CHCOOH < Cl3CCOOH
Illustration 3. Acetic acid reacts with chlorine in the presence of catalyst, anhydrous FeCl3 to give
(A) acetyl chloride (B) methyl chloride
(C) tri-chloro acetic acid (D) chloral hydrate
4. Substitution in alkyl or aryl group
Halogenation of Aliphatic Acids (Hell-Volhard-Zelinsky Reaction)
In the presence of phosphorus, aliphatic carboxylic acids react smoothly with chlorine or bromine to yield a compound in which a-hydrogen has been replaced by halogen.
The function of the phosphorus is ultimately to convert a little of the acid into acid halide so that it is the acid halide, not the acid itself, that undergoes this reaction.
P + X2 -> PX3
R – CH2 – COOH + PX3 -> RCH2 – COX
The halogen of these halogenated acid undergoes nucleophilic displacement and elimination same as it does in the simple alkyl halides. Halogenation is therefore the first step in the conversion of a carboxylic acid into many important substituted carboxylic acid.
(b) Ring substitution in aromatic acids:
¾COOH deactivates and directs incoming electrophilic to meta position.
Illustration 1. Hydrolysis of a compound A (C7H3Cl5) gives an acid B of the formula C7H4Cl2O2. Decarboxylation of acid yields a neutral substance (C), the nitration of which forms only one mono derivative (D). Identify A, B, C
Illustration 2. Which of the following will not undergo HVZ reaction?
(A) 2, 2-dimethyl propanoic acid (B) propanoic acid
(C) acetic acid (D) 2-methyl propanoic acid
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