Question 1
Rank the following compounds in order of increasing acidity, putting the least acidic first.
A. CH3COOHCH3COOH.
b. ClCH2COOHClCH2COOH.
c. CH3CH2OHCH3CH2OH.
d. ClCH2CH2OHClCH2CH2OH.
Question 2
Rank the following compounds in order of increasing acidity.
(a) Benzoic acid, p-methylbenzoic acid, p-chlorobenzoic acid.
(b) p-Nitrobenzoic acid, acetic acid, benzoic acid.
Question 3
Rank the following compounds in order of increasing acidity, putting the least acidic first.
Answer to question 1
Both CH3COOHandClCH2COOHCH3COOHandClCH2COOH are carboxylic acids. In contrast, both CH3CH2OHandClCH2CH2OHCH3CH2OHandClCH2CH2OH are alcohols.
It is known that carboxylic acids are more acidic than alcohols. Hence, both CH3COOHandClCH2COOHCH3COOHandClCH2COOH are more acidic than both CH3CH2OHandClCH2CH2OHCH3CH2OHandClCH2CH2OH.
Among CH3COOHandClCH2COOHCH3COOHandClCH2COOH, ClCH2COOHClCH2COOH is more acidic due to the presence of an electron-withdrawing −Cl−Cl group.
Similarly, among CH3CH2OHandClCH2CH2OHCH3CH2OHandClCH2CH2OH, ClCH2CH2OHClCH2CH2OH is more acidic due to the presence of an electron-withdrawing −Cl−Cl group.
Hence, the given compounds are arranged in order of increasing acidity as shown below:
CH3CH2OH<ClCH2CH2OH<CH3COOH<ClCH2COOH
Answer to question 2
We are told to arrange the following compounds in the increasing order of acidity:
- Electron donating group decreases the acidic strength of the compound because of the increase of the electron density on the conjugate base formed by the acid due to which the stability of the conjugate base decreases and hence, acidic strength decreases.
- Electron withdrawing group increases the acidic strength of the compound because it decreases the electron density on the conjugate base formed by the acid due to which the stability of the conjugate base increases and hence, acidic strength increases.
(a) Benzoic acid, p-methylbenzoic acid, p-chlorobenzoic acid.
Methyl is electron donating group. It has +I and +R effect.
Cl is electron donating group. It has -I and +R effect.
The order of acidity of the compounds is:
p−methylbenzoicacid<p−chlorobenzoicacid<benzoicacidp−methylbenzoicacid<p−chlorobenzoicacid<benzoicacid
(b) p-Nitrobenzoic acid, acetic acid, benzoic acid.
Nitro is electron withdrawing group. It has -I and -R effect.
Acetic acid is an aliphatic carboxylic acid.
Benzoic acid is an aromatic carboxylic acid. It has more resonating structures than acetic acid, so, the stability of conjugate base formed by benzoic acid is more than that formed by acetic acid.
The order of acidity of the compounds is as follows:
aceticacid<benzoicacid<p−Nitrobenzoicacid
Answer to question 3
FCH2COOH>ClCH2COOH>BrCH2COOH>CH3COOHFCH2COOH>ClCH2COOH>BrCH2COOH>CH3COOH
Since F, Cl, and Br are electron withdrawing groups, the above order can be explained on the basis of the extent of electron withdrawing group ability and electronegativity. F has the greatest electron withdrawing nature and is the most electronegative element, hence it has a maximum -I effect. This means FCH2COOHFCH2COOH has the maximum acidic character of the group. The order of electronegativity and electron withdrawing tendency decrease as:
F>Cl>Br>CH3F>Cl>Br>CH3.
As a result, the order of acidic character becomes: FCH2COOH>ClCH2COOH>BrCH2COOH>CH3COOHFCH2COOH>ClCH2COOH>BrCH2COOH>CH3COOH with CH3COOHCH3COOH being least acidic in nature of the group of compounds given.