Assertion (A) Ethanol is a weaker acid than phenol.
Reason (R) Sodium ethoxide may be prepared by the reaction of ethanol with aqueous NaOH .
Assertion (A) Phenol forms 2, 4, 6-tribromophenol on treatment with $\mathrm{Br}_2$ in carbon disulphide at 273 K .
Reason (R) Bromine polarises in carbon disulphide.
Assertion (A) Phenols give o -and p -nitrophenol on nitration with conc. $\mathrm{HNO}_3$ and $\mathrm{H}_2 \mathrm{SO}_4$ mixture.
Reason $(\mathrm{R})-\mathrm{OH}$ group in phenol is $\mathrm{o}-\mathrm{p}$-directing.
Write the mechanism of the reaction of HI with methoxybenzene.
In case of alkyl aryl ethers, the products are always phenol and an alkyl halide because due to resonance $\mathrm{C}_6 \mathrm{H}_5-\mathrm{O}$ bond has partial double bond character. The mechanism is given below
Mechanism Protonation of anisole gives methylphenyl oxonium ion.
In this ion, the bond between $\mathrm{O}-\mathrm{CH}_3$ is weaker than the bond between $\mathrm{O}-\mathrm{C}_6 \mathrm{H}_5$ which has partial double bond character. This partial double bond character is due to the resonance between the lone pair of electrons on the O -atom and the $s p^2$ hybridised carbon atom of the phenyl group. Therefore, attack by $I^{-}$ion exclusively breaks the weaker $\mathrm{O}-\mathrm{CH}_3$ bond forming methyl iodide and phenol.
(a) Name the starting material used in the industrial preparation of phenol.
(b) Write complete reaction for the bromination of phenol in aqueous and non-aqueous medium.
(c) Explain why Lewis acid is not required in bromination of phenol?
(a) The starting material used in the industrial preparation of phenol is cumene.
(b) Phenols when treated with bromine water gives polyhalogen derivatives in which all the hydrogen atoms present at ortho and para positions with respect to - OH group are replaced by bromine atoms.
However, in non-aqueous medium such as $\mathrm{CS}_2, \mathrm{CCl}_4, \mathrm{CHCl}_3$, monobromophenols are obtained.
In aqueous solution, phenol ionises to form phenoxide ion. This ion activates the benzene ring to a very large extent and hence the substitution of halogen takes place at all three positions.
On the other hand, in non-aqueous solution ionisation of phenol is greatly suppressed. Therefore, ring is activated slightly and hence monosubstitution occur.
(c) Lewis acid is an electron deficient molecule. In bromination of benzene, Lewis acid is used to polarise $\mathrm{Br}_2$ to form $\mathrm{Br}^{+}$electrophile.
In case of phenol, oxygen atom of phenol itself polarises the bromine molecule to form $\mathrm{Br}^{+}$ion (electrophile). So, Lewis acid is not required in the bromination of phenol.
