Why can aryl halides not be prepared by reaction of phenol with HCl in the presence of $\mathrm{ZnCl}_2$ ?
Due to resonance in phenol, $\mathrm{C}-\mathrm{O}$ bond of phenol has some partial double bond character. Partial double bond character strengthen the bond. So, It is difficult to break this $\mathrm{C}-\mathrm{O}$ bond of phenol while the $\mathrm{C}-\mathrm{O}$ bond of alcohol is purely single bond and comparatively weaker bond.
So alkyl halides can be prepared by the reaction of alcohols with HCl in the presence of $\mathrm{ZnCl}_2$ while aryl halides can not be prepared by reaction of phenol with HCl in the presence of $\mathrm{ZnCl}_2$.
Which of the following compounds would undergo $S_N 1$ reaction faster and why?
Compound (B) will give $S_N 1$ reaction faster than compound $(A)$ because $S_N 1$ reaction depends upon the stability of carbocation. Benzyl chloride on ionisation gives $\mathrm{C}_6 \mathrm{H}_5 \stackrel{+}{\mathrm{C}} \mathrm{H}_2$ carbocation which is resonance srabilised while the carbocation obtained from compound $(A)$ is not stabilised by resonance.
Allyl chloride is hydrolysed more readily than n-propyl chloride. Why?
As we know that $\mathrm{S}_{\mathrm{N}} 1$ mechanism depends upon the stability of carbocation. Allyl chloride on hydrolysis gives resonance stabilised carbocation while no resonance is observed in the carbocation formed by $n$-propyl chloride.
Hence, allyl chloride undergoes hydrolysis much faster than $n$-propyl chloride.
Why is it necessary to avoid even traces of moisture during the use of a Grignard reagent?
Grignard reagents are highly reactive and react with water to give corresponding hydrocarbons.
How do polar solvents help in the first step in $\mathrm{S}_{\mathrm{N}}{ }^1$ mechanism?
Polar solvents help in the first step in $\mathrm{S}_{\mathrm{N}} 1$ mechanism because leaving group and carbocation both are stabilised by polar solvent. Polarity of a solvent depends upon the value of dielectric constant. Higher the value of dielectric constant, higher will be the polarity of the solvent, faster will be the rate of $S_N 1$ mechanism. These polar solvents can work as a nucleophile and stabilise the carbocation as follows
