Explain why nucleophilic substitution reactions are not very common in phenols?
Resonance is an important factor in phenols. During resonance -OH group in phenol gives its electrons to the benzene ring. As a result of this, the electron density on benzene ring is very high. This increased electron density repels nucleophiles.
Therefore, nucleophiles cannot attack the benzene ring and phenols usually do not give nucleophilic substitution reaction.
Preparation of alcohols from alkenes involves the electrophilic attack on alkene carbon atom. Explain its mechanism.
Preparation of alcohols from alkene by the hydration of alkenes in presence of sulphuric acid.
$$\mathrm{CH}_2=\mathrm{CH}_2+\mathrm{H}_2 \mathrm{O} \xrightarrow{\mathrm{H}^{+}} \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{OH}$$
This addition reaction takes place in accordance with Markownikoff's rule.
Mechanism The mechanism of the reaction involves the following three steps
Step 1 Protonation of alkene to form carbocation by electrophilic attack of $\mathrm{H}_3^{+} \mathrm{O}$.
Step 2 Attack of water molecule to the secondary carbocation.
Step 3 Loss of the hydrogen from the protonated alcohol.
Explain why is $O=\mathrm{C}=O$ non-polar while $\mathrm{R}-O-\mathrm{R}$ is polar?
$\mathrm{CO}_2$ is a linear molecule. The dipole moment of two $\mathrm{C}=\mathrm{O}$ bonds are equal and opposite and they cancel each other and hence the dipole moment of $\mathrm{CO}_2$ is zero and it is a non-polar molecule.
While for ethers, two dipoles are pointing in the same direction. These two dipoles do not cancel the effect of each other. Therefore, there is a finite resultant dipoles and hence $R-O-R$ is a polar molecule.
Why is the reactivity of all the three classes of alcohols with conc. HCl and $\mathrm{ZnCl}_2$ (Lucas reagent) different?
The reaction of alcohols with Lucas reagent (conc. HCl and $\mathrm{ZnCl}_2$ ) follow $\mathrm{S}_{\mathrm{N}}{ }^1$ mechanism. $\mathrm{S}_{\mathrm{N}}{ }^1$ mechanism depends upon the stability of carbocations (intermediate). More stable the intermediate carbocation, more reactive is the alcohol.
Tertiary carbocations are most stable among the three classes of carbocations and the order of the stability of carbocation is $3^{\circ}>2^{\circ}>1^{\circ}$. This order, inturn, reflects the order of reactivity of three classes of alcohols i.e., $3^{\circ}>2^{\circ}>1^{\circ}$.
Thus, as the stability of carbocations are different so the reactivity of all the three classes of alcohols with Lucas reagent is different.
Write steps to carry out the conversion of phenol to aspirin.
Aspirin can be prepared by the reaction of salicylic acid with acetic anhydride. Salicylic acid is prepared by the reaction of phenol with $\mathrm{CO}_2$ and NaOH.
This process is known as Kolbe's reaction. The product salicylic acid is used in the preparation of aspirin. After wards, when salicylic acid is treated with acetic anhydride then acetyl group replaces the hydrogen of - OH group i.e., acetylation occurs at $-$OH group of salicylic acid. Reaction is as follows
