NCERT · Class 12 · Chemistry · Alcohols, Phenols and EthersDiscuss the important name reactions associated with the preparation and chemical properties of phenols. Include the Kolbe's reaction, Reimer-Tiemann reaction, and the mechanism of electrophilic aromatic substitution in phenol (specifically bromination and nitration).

Important Name Reactions and Chemical Properties of Phenols

\nPhenols exhibit unique chemical reactivity due to the presence of the hydroxyl group attached directly to the benzene ring, which strongly activates the aromatic ring towards electrophilic substitution and undergoes characteristic name reactions.

• 1. Kolbe’s Reaction (Kolbe-Schmitt Reaction):

  • Description: Phenoxide ion, generated by treating phenol with sodium hydroxide, is more reactive than phenol towards electrophilic aromatic substitution. Thus, it undergoes electrophilic substitution with carbon dioxide ($CO_2$), a weak electrophile, to form salicylic acid (2-hydroxybenzoic acid) as the primary product.
  • Reaction Equation: $C_6H_5OH + NaOH \rightarrow C_6H_5ONa + H_2O$ $C_6H_5ONa + CO_2 \xrightarrow{398K, 4-7 atm} C_6H_4(OH)(COONa) \xrightarrow{H^+} C_6H_4(OH)(COOH)$ (Salicylic acid).

• 2. Reimer-Tiemann Reaction:

  • Description: When phenol is treated with chloroform ($CHCl_3$) in the presence of sodium hydroxide ($NaOH$) at 340 K, an aldehyde group ($-\text{CHO}$) is introduced at the ortho-position of the benzene ring, resulting in the formation of salicylaldehyde (2-hydroxybenzaldehyde). The reaction proceeds via the generation of an electrophilic carbene intermediate, dichlorocarbene (: $CCl_2$).
  • Reaction Equation: $C_6H_5OH + CHCl_3 + 3NaOH \rightarrow C_6H_4(OH)(CHO) + 3NaCl + 2H_2O$.

• 3. Electrophilic Aromatic Substitution in Phenol: The $-\text{OH}$ group attached to the benzene ring is strongly activating and ortho-, para-directing due to the $+R$ (resonance) effect, which increases electron density at ortho and para positions.

  • Nitration:
    • With dilute nitric acid at low temperature (298 K), phenol yields a mixture of ortho- and para-nitrophenols.
    • With concentrated nitric acid ($HNO_3$) in the presence of concentrated $H_2SO_4$, phenol is converted into 2,4,6-trinitrophenol, commonly known as picric acid.
  • Bromination:
    • When phenol is treated with bromine in a low polarity solvent such as chloroform ($CHCl_3$) or $CS_2$ at low temperature, a mixture of ortho- and para-bromophenol is formed, with the para-isomer being the major product.
    • When phenol is treated with bromine water (aqueous $Br_2$), a white precipitate of 2,4,6-tribromophenol is formed instantly due to high activation of the ring.