Haloalkanes and Haloarenes

Presence of $-\mathrm{NO}_2$ group will favour chlorobenzene towards nucleophilic substitution, $\operatorname{ArS}_{\mathrm{N}} 2$ (via addition elimination mechanism) $-R$ effect of $-\mathrm{NO}_2$ at ortho position (I) will favour the reaction most in comparison to $-I$ effect of $-\mathrm{NO}_2$ at meta position (III).

$ \text { So,the reactivity order : I }>\text { III }>\text { II } \text {. } $

Step 1 First of all cyclohexanone react with Et MgBr and $\mathrm{H}_2 \mathrm{O}$.

Step 2 In the presence of $20 \% \mathrm{H}_3 \mathrm{PO}_4$ elimination undergoes and water is removed resulting alkene is formed.

Step 3 HBr react with alkene forms haloderivative.

Friedel Craft’s reaction is an electrophilic substitution reaction. In this reaction, the intermediate carbocation formed acts as an electrophile and attacks the benzene ring. Vinyl chloride does not react with benzene as its intermediate carbocation is too unstable.

The bulky groups cause steric hindrance in the formation of transition state. Therefore, higher homologues (Q) are less reactive than lower homologues (P). In compound (S), the transition state is highly stabilized by Ph-(C=O)- group, so it has highest rate of reaction towards S_N2.

The relative reactivities towards S_N2 reaction S > P > R > Q.

We know that following are the cases when a compound is optically inactive:

1. When it does not have a chiral carbon atom e.g., Methyl bromide

2. When it has a plane or axis of symmetry e.g., Tartaric acid (these compounds are called as Meso compounds)

3. When it forms a racemic mixture e.g.: Equimolar mixture of d-Lactic acid and l-Lactic acid.

The given compound possesses an axis of symmetry

Alkyl halides undergo nucleophilic substitution more readily than aryl halides due to partial double bond character between aryl carbon and halogen. Hence, substitution occurs more readily in the side chain rather than in the ring. PhS$^-$ is a strong nucleophile and dimethylformamide (DMF) is a highly polar aprotic solvent. Hence, nucleophilic substitution (S$_\mathrm{N}2$) takes place at 2$^\circ$ benzylic carbon. Stereochemically, it involves inversion of configuration at benzylic carbon atom.

Alkyl group exhibit neighbouring group participation or anchimeric assistance from aryl group but deactivation by $-$NO$_2$ and $-$F makes S$_\mathrm{N}1$ difficult and retention of configuration at chiral carbon does not takes place.

Bromine orientation is controlled by its mesomeric effect. Its stabilisation of arenium ion by +M activity.

In bromobenzene, the inductive effect of the bromo group is more dominant than the mesomeric effect in directing the incoming electrophile at para position. As a result, only mono substitution of bromobenzene takes place.