Haloalkanes & Haloarenes

1. Nucleophilic Substitution

• $S_N1$: Two steps, carbocation intermediate, racemization. Rate $\propto [Substrate]$. Stability: $3^\circ > 2^\circ > 1^\circ$.
• $S_N2$: One step, transition state, inversion of configuration (Walden Inversion). Rate $\propto [Substrate][Nu^-]$. Reactivity: $CH_3X > 1^\circ > 2^\circ > 3^\circ$.

Interactive diagram: open this page in a browser to use it.

2. Elimination vs Substitution

With a strong, bulky base (alcoholic KOH, $t$-BuOK) and heat, haloalkanes undergo β-elimination. Saytzeff's rule: the more substituted alkene is the major product. Aqueous KOH favours substitution (alcohol); alcoholic KOH favours elimination (alkene).

3. Reactivity of the C–X Bond

For the same alkyl group: $R{-}I > R{-}Br > R{-}Cl > R{-}F$, because the C–I bond is longest and weakest. KCN gives alkyl cyanides (attack via C), while AgCN gives isocyanides (attack via N), because the Ag–C bond is covalent.

4. Haloarenes

Aryl halides are much less reactive towards nucleophilic substitution than alkyl halides:

• Resonance gives the C–X bond partial double-bond character.
• The carbon is $sp^2$, so the C–X bond is shorter and stronger.
• A phenyl cation is unstable, and back-side attack is blocked by the ring.

Electron-withdrawing groups at ortho/para positions (like $-NO_2$) make substitution much easier: 2,4,6-trinitrochlorobenzene hydrolyses with warm water.

5. Named Reactions

• Finkelstein: $RCl + NaI \xrightarrow{acetone} RI + NaCl$ (NaCl precipitates, driving the reaction).
• Swarts: $RCl + AgF \to RF$ (also $Hg_2F_2$, $CoF_2$, $SbF_3$).
• Wurtz–Fittig: aryl halide + alkyl halide + Na gives alkylbenzene; Fittig: two aryl halides give biphenyl.
• Sandmeyer: diazonium salt + $CuX$ gives aryl halide.

6. Polyhalogen Compounds

Chloroform is stored in dark bottles filled to the brim, because light and air oxidise it to poisonous phosgene ($COCl_2$). DDT is a persistent insecticide; freons (CFCs) deplete the ozone layer.

Practice questions

1. Which reacts fastest by the S_N2 mechanism?

   1. CH₃Br
   2. (CH₃)₂CHBr
   3. (CH₃)₃CBr
   4. CH₃CH₂Br
   Answer

   A. S_N2 requires back-side attack; methyl halides are the least hindered.

2. Treatment of 2-bromobutane with alcoholic KOH gives mainly:

   1. But-1-ene
   2. But-2-ene
   3. Butan-2-ol
   4. Butane
   Answer

   B. By Saytzeff's rule, the more substituted alkene (but-2-ene) is the major product.

3. Alkyl halide + AgCN gives mainly:

   1. Alkyl cyanide
   2. Alkyl isocyanide
   3. Alcohol
   4. Amine
   Answer

   B. AgCN is mainly covalent, so the nitrogen lone pair attacks, giving R–NC.

4. The Finkelstein reaction is used to prepare:

   1. Alkyl fluorides
   2. Alkyl iodides
   3. Aryl chlorides
   4. Alkenes
   Answer

   B. RCl/RBr + NaI in dry acetone gives RI; insoluble NaCl/NaBr drives the equilibrium.

5. Chlorobenzene is less reactive than chloroethane towards nucleophilic substitution because:

   1. It is non-polar
   2. The C–Cl bond has partial double-bond character
   3. It has a higher molar mass
   4. Cl is less electronegative in it
   Answer

   B. Resonance with the ring and sp² hybridisation strengthen and shorten the C–Cl bond.