Haloalkanes and Haloarenes

Atoms H and Br are added across the double bond. Finkelstein the action is 1 an SN2 (Substitution Nucleophilic Bimolecular) relation mechanism that involves the exchange of one halogen atom for another. Here, $\mathrm{Br}^{-}$is replaced by I.In this reaction, I attacks the carbon atom attached to the halogen Br. General reaction:$ \begin{aligned} & R-X+N a I \rightarrow R-I+N a X \\ & X: C l / b r\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(\mathrm{NaCl}, \mathrm{NaBr}) \end{aligned} $Swart reaction is the reaction used to synthesize alkyl fluorides from alkyl chlorides or bromides. It involves heating the alkyl halide (chloride or bromide) with a metal fluoride (AgF) to replace, the halogen $(\mathrm{Cl}$ or Br$)$ with a fluorine atom.$ \text { Hero, } \mathrm{Br}^{-} \text {is replaced by } \mathrm{F}^{-} $$ \text { General reaction: } $ $ \begin{aligned} & R-X \quad + \quad \text { AgF } \quad \rightarrow R \rightarrow F \quad +\operatorname{Ag} X \\ & (X =Cl, B r)\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(\mathrm{AgCl}, \mathrm{AgBr}) \end{aligned} $ A) $C-X$ bond length in $P, Q$ and $R$ follows the order $Q > R > P $. Bond length order $C-F < C - B r < C-I$ Bond length increases with the size of the halogen atom. Iodine is/ larger than bromine, which is longer than fluorine. Therefore the C-I bond (Q) is longer than $C-B r$ bond $(P)$, which is longer than the C-F bond (R). The correct order is $Q>P>R$. So, the statement is not correct.B) $C-X$, bond enthalpy in $P, Q$ and $R$ flows the order $R>P>Q$ Bond enthalpy is the energy required to break a bond stronger, bonds are shorter in length and require more energy to break the bond. The C-F bond (R) is the strongest and shortest bond, followed by $C -B r$ bond $(P)$, and the $C-I$ bond $(Q)$ is the weakest and longest loons. So, C-F Fond (R) requires higher energy to bond breaking. Its bond enthalpy is higher. C - I bond (Q) is the weakest bond and its bond enthalpy is lower. The bond enthalpy of $C-B r(P)$ is intermediate between $R_1$ and $Q$.$ \text { So, the statement is correct. } $c) Relative reactivity toward $S_{N 2}$ reaction in $P, Q$ and $\ R$ follows the order $P>R>Q$ Reactivity towards. $S_{N^2}$, order is $C-I>C-B r>C-F$ $Q>P>R$$S_N 2$ reactions are favoured by less steric hindrance. The order of reactivity is $I>B r>C l>F$, because larger halogens are better leaving groups. So, the correct order is $Q>P>R$. The statement is not correct.D) PK value of the conjugate acids of the leaving groups In $P, Q$ and $R$ follows the order $\quad R>Q>P$. $ \text { Leaving group } \Rightarrow \text { P }\,\,\,\,\,\,Q \,\,\,\,\,\,\,R $$\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\mathrm{Br}^{-} \,\,\, \mathrm{I}^{-}\quad \mathrm{F}^{-}$$ \text { Conjugate acids } \Rightarrow \mathrm{HBr}\,\,\, HI\,\,\,\,\,HF $$ \text { pka order conjugate acids } \Rightarrow \mathrm{HI}<\mathrm{HBr}<\mathrm{HF} $The acidity of the acids HBr, HI and HF, increases in the order HF < HBr < HI. So, the Orrder of pka is HI< HBr < HF orThe acidity of the acids HBr, HI and HF, increases in the order HF < HBr < HI. So, the Orrder of pka is HI< HBr < HF or$Q < P < R$ or $R > P > Q$ : So, the given statement is not correct. Correct statement is (B). Answer: Option (B)

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

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.

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.