CBSE 2025 · Region 6 · Set 1 · Q29 · 4 marks
Alcohols undergo a number of reactions involving the cleavage of C - OH bond. However, phenols do not undergo reactions involving the cleavage of C - OH bond. Alcohols are weaker acids than water. Alcohols react with halogen acids to form the corresponding haloalkanes. Phenols are stronger acids than alcohols. A characteristic feature of phenols is that they undergo electrophilic substitution reactions such as halogenation, nitration, etc. Since - OH group is a strong activating group, phenol gives trisubstituted products during halogenation, nitration, etc.(a)What happens when phenol is treated with the following ?(i)$\displaystyle \mathrm{Br}_{2}$ water(ii)Conc. $\displaystyle \mathrm{HNO}_{3}$(b)Write the mechanism of alcohol reacting as nucleophile in a reaction with $\displaystyle \mathrm{CH}_{3}^{\oplus}$.(ii)Why do phenols not undergo reactions involving cleavage of $\displaystyle \mathrm{C}-\mathrm{OH}$ bond ?(c)How can you distinguish between Butan-$\displaystyle 1$-ol and $\displaystyle 2$-Methylpropan-$\displaystyle 2$-ol by using HCl in the presence of anhydrous $\displaystyle \mathrm{ZnCl}_{2}$ ?
Alcohols undergo a number of reactions involving the cleavage of C - OH bond. However, phenols do not undergo reactions involving the cleavage of C - OH bond. Alcohols are weaker acids than water. Alcohols react with halogen acids to form the corresponding haloalkanes. Phenols are stronger acids than alcohols. A characteristic feature of phenols is that they undergo electrophilic substitution reactions such as halogenation, nitration, etc. Since - OH group is a strong activating group, phenol gives trisubstituted products during halogenation, nitration, etc.
(a)
What happens when phenol is treated with the following ?
(i)
$\displaystyle \mathrm{Br}_{2}$ water
(ii)
Conc. $\displaystyle \mathrm{HNO}_{3}$
(b)
Write the mechanism of alcohol reacting as nucleophile in a reaction with $\displaystyle \mathrm{CH}_{3}^{\oplus}$.
(ii)
Why do phenols not undergo reactions involving cleavage of $\displaystyle \mathrm{C}-\mathrm{OH}$ bond ?
(c)
How can you distinguish between Butan-$\displaystyle 1$-ol and $\displaystyle 2$-Methylpropan-$\displaystyle 2$-ol by using HCl in the presence of anhydrous $\displaystyle \mathrm{ZnCl}_{2}$ ?
Marking-scheme solution
(a)
i)
/ $\displaystyle 2,4,6$-Tribromophenol is formed
(ii)
/ $\displaystyle 2,4,6$-Trinitrophenol / Picric acid is formed.
(b)
(i)(b)
(ii) due to sp2 hybridisation leading to shorter bond length / Due to resonance leading to partial
double bond character of C-OH bond
(c)
$\displaystyle 2$-Methylpropan-$\displaystyle 2$-ol gives turbidity immediately whereas butan-$\displaystyle 1$-ol does not react.
Alcohols, Phenols and EthersAlcohols and PhenolsApplycase_studymedium
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CBSE Class 12 Chemistry past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.