Chapter 11: Alcohols, Phenols and Ethers

The –OH group in phenol is directly attached to the sp2hybridized carbon atom of the benzene ring. The carbon-oxygen bond length in phenol is smaller as compared to the carbon-oxygen bond length in alkyl alcohol, and this is due to the partial double bond character or due to the resonance and charge distribution in phenol.

Due to the resonance, the ortho- and para-positions in the benzene ring become electron-rich and, therefore, activates it towards the electrophilic substitution reaction. Thus, nucleophilic substitution reactions are not very common in phenols

Step (1) Protonation of alkene and formation of a carbocation

Step (2) Nucleophilic attack of water

Step (3) Deprotonation occurs, and alcohol is formed. H30+ is released.

O=C=O nonpolar because the dipole moment of the two C=O bonds is exactly equal and opposite of each other. Therefore, they cancel each other, and so, the net dipole moment of O=C=O is zero.

This is because of the steric hindrance of the alkyl group and the stability of carbocation.

Primary alcohol does not show any reaction at room temperature because the 1° carbocation is the least stable.

Secondary alcohol does not show any turbidity at room temperature, but on heating, turbidity appears.

Tertiary alcohol immediately shows turbidity after the addition of Lucas reagent as they can form halides easily due to the higher stability of the carbocation.

Phenol is treated with NaOH to produce phenoxide ions. Phenoxide ion then undergoes electrophilic substitution with CO2 to yield salicylic acid as the major product. This is called Kolbe’s reaction.

Phenol is more nitrated because of the presence of the hydroxyl group in phenol. Due to the resonance effect caused by –OH group, the ortho- and para-positions in the benzene ring becomes electron-rich and, therefore, activates it towards electrophilic substitution reaction. Thus, Nitration, an aromatic electrophilic substitution, occurs at a position where the electron density is high.

In Kolbe’s reaction, instead of phenol, phenoxide ion is treated with carbon dioxide (a weak electrophile) because phenoxide ion is more reactive towards electrophilic aromatic substitution.

The dipole moment of phenol is smaller than that of methanol due to the electron-withdrawing effect of the phenyl ring. Due to the resonance, the polarity of the C-O bond in phenol decreases.

Ethers can be prepared by Williamson synthesis, in which an alkyl halide is reacted with sodium alkoxide. Di-tert-butyl ether can’t be prepared by this method because in this case, elimination is more favoured over substitution.