MP Board · Class 12 · Chemistry · Haloalkanes and HaloarenesA haloalkane 'A' with molecular formula $C3H7Cl$ on treatment with aqueous KOH gives compound 'B' of formula $C3H8O$. Compound 'A' on treatment with alcoholic KOH gives compound 'C' of formula $C3H6$. Identify 'A', 'B', and 'C' and write all the chemical equations involved. Also, explain why aryl halides are extremely less reactive towards nucleophilic substitution reactions compared to alkyl halides.

Identification of Compounds 'A', 'B', and 'C'

1. Given molecular formula of 'A': $C_3H_7Cl$
2. Reaction with aqueous KOH: Haloalkane 'A' reacts with aqueous KOH via nucleophilic substitution to form an alcohol 'B' ($C_3H_8O$).
3. Reaction with alcoholic KOH: Haloalkane 'A' undergoes dehydrohalogenation with alcoholic KOH to form an alkene 'C' ($C_3H_6$).
4. Therefore, compound 'A' is 1-chloropropane ($CH_3-CH_2-CH_2Cl$), compound 'B' is propan-1-ol ($CH_3-CH_2-CH_2OH$), and compound 'C' is propene ($CH_3-CH=CH_2$).

Chemical Equations:

• Formation of 'B': $$CH_3-CH_2-CH_2Cl + KOH (aq) \rightarrow CH_3-CH_2-CH_2OH + KCl$$
• Formation of 'C': $$CH_3-CH_2-CH_2Cl + KOH (alc) \rightarrow CH_3-CH=CH_2 + KCl + H_2O$$

Low Reactivity of Aryl Halides towards Nucleophilic Substitution:\nAryl halides (such as chlorobenzene) are extremely less reactive towards nucleophilic substitution compared to alkyl halides due to the following factors:

1. Resonance Effect: In haloarenes, the lone pairs of electrons on the halogen atom are in conjugation with the $\pi$-electrons of the benzene ring. This gives the carbon-chlorine bond partial double bond character, making it stronger and harder to cleave than a single bond.
2. Difference in Hybridization: In haloalkanes, the carbon atom attached to halogen is $sp^3$ hybridized, whereas in haloarenes, the carbon atom is $sp^2$ hybridized. An $sp^2$ hybridized carbon is more electronegative and holds the electron pair tightly, resulting in a shorter and stronger bond.
3. Instability of Phenyl Cation: If an $S_N1$ mechanism were to occur, it would form an unstable phenyl cation, which is energetically unfavorable.
4. Repulsion: Due to the electron-rich nature of the benzene ring, electron-rich nucleophiles experience electrostatic repulsion.