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Question
Give the order of acidic character of the following:
i. a. p-Nitrobenzoic acid
b. 2, 4-Dinitrobenzoic acid
c. 2, 4, 6-Trimethylbenzoic acid
d. 2, 3, 4-Trinitrobenzoic acid
ii. a. Ph3CH
b. Ph2CH2
c. PhCH3
iii. 2, 6-Di-tert-butyl phenol is a much weaker acid than phenol.
iv. a. Phenol
b. Cyclohexanol
c. p-Bromophenol
d. p-Methoxy phenol
vi. a. Benzoic acid
b. p-tert-Butylbenzoic acid
c. p-Nitrobenzoic acid
i. a. p-Nitrobenzoic acid
b. 2, 4-Dinitrobenzoic acid
c. 2, 4, 6-Trimethylbenzoic acid
d. 2, 3, 4-Trinitrobenzoic acid
ii. a. Ph3CH
b. Ph2CH2
c. PhCH3
iii. 2, 6-Di-tert-butyl phenol is a much weaker acid than phenol.
iv. a. Phenol
b. Cyclohexanol
c. p-Bromophenol
d. p-Methoxy phenol
vi. a. Benzoic acid
b. p-tert-Butylbenzoic acid
c. p-Nitrobenzoic acid
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Solution
(i) 2,4-dinitro benzoic acid is most acidic due to -I and -R effect of two nitro groups.
2,4,6-trimethyl benzoic acid is least acidic due to +I and hyprconjugative effects of three methyl groups.
Hence, the correct order of acidic character is b > d > a > c.
(ii) The correct order of acidic strength is a>b>c.
As the number of phenyl groups increases, the delocalization of the charge of the carbanion (obtained by deprotonation) increases which increases the stabilization of the carbanion.
(iii) Due to steric inhibition of resonance of bulky ortho-t-butyl groups, it prevents the solvation of corresponding phenoxide ion, making it less stable than phenoxide ion.
Hence,2, 6-Di-tert-butyl phenol is a much weaker acid than phenol.
(iv) The correct order of acidic strength is c(p−bromophenol)>a(phenol)>d(p−methoxyphenol)>b(cyclohexanol)
p-bromophenol is most acidic due to electron withdrawing effect of bromine which stablizes the carbanion obtained by deprotonation.
p-methoxy phenol is less acidic than phenol due to electron releasing effect of methoxy group which destabilizes the carbanion obtained by deprotonation.
Phenol is more acidic than cyclohexanol because in phenol, the carbanion obtained by deprotonation is stabilized by resonance which is not possible in case of cyclohexanol.
(vi) nitro group is electron withdrawing group. Hence, it stablizes the carbanion obtained by the deprotonation of p-nitro benzoic acid.
Hence, p -nitro benzoic acid is more acidic than benzoic acid.
p-tert butyl benzoic acid on the other hand is less acidic than benzoic acid because, due to electron releasing effect of tert butyl group, the carbanion formed by the deprotonation of p-tert butyl benzoic acid is destabilized.
Hence, the correct order of the acid strength is c (p-nitro benzoic acid) > a (benzoic acid) > b (p-tert butyl benzoic acid)
2,4,6-trimethyl benzoic acid is least acidic due to +I and hyprconjugative effects of three methyl groups.
Hence, the correct order of acidic character is b > d > a > c.
(ii) The correct order of acidic strength is a>b>c.
As the number of phenyl groups increases, the delocalization of the charge of the carbanion (obtained by deprotonation) increases which increases the stabilization of the carbanion.
(iii) Due to steric inhibition of resonance of bulky ortho-t-butyl groups, it prevents the solvation of corresponding phenoxide ion, making it less stable than phenoxide ion.
Hence,2, 6-Di-tert-butyl phenol is a much weaker acid than phenol.
(iv) The correct order of acidic strength is c(p−bromophenol)>a(phenol)>d(p−methoxyphenol)>b(cyclohexanol)
p-bromophenol is most acidic due to electron withdrawing effect of bromine which stablizes the carbanion obtained by deprotonation.
p-methoxy phenol is less acidic than phenol due to electron releasing effect of methoxy group which destabilizes the carbanion obtained by deprotonation.
Phenol is more acidic than cyclohexanol because in phenol, the carbanion obtained by deprotonation is stabilized by resonance which is not possible in case of cyclohexanol.
(vi) nitro group is electron withdrawing group. Hence, it stablizes the carbanion obtained by the deprotonation of p-nitro benzoic acid.
Hence, p -nitro benzoic acid is more acidic than benzoic acid.
p-tert butyl benzoic acid on the other hand is less acidic than benzoic acid because, due to electron releasing effect of tert butyl group, the carbanion formed by the deprotonation of p-tert butyl benzoic acid is destabilized.
Hence, the correct order of the acid strength is c (p-nitro benzoic acid) > a (benzoic acid) > b (p-tert butyl benzoic acid)
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