Haloarenes and Polyhalogen Compounds: Quick Revision

A brief note on Haloarenes and Polyhalogen Compounds. Read it before a test. Try each question at the end before you open its answer.

Preparation of haloarenes

• Ring halogenation: $\text{X}_{2}$ with $\text{Fe}$, $\text{FeCl}_{3}$ or $\text{AlCl}_{3}$, in the dark. Light chlorinates the side chain of toluene instead.
• Diazonium route: $\text{NaNO}_{2}$ and $\text{HCl}$ at $273$ to $278\ \text{K}$. Then $\text{CuCl}$ or $\text{CuBr}$ (Sandmeyer), Cu powder (Gattermann), $\text{KI}$, or $\text{HBF}_{4}$ and heat (Balz-Schiemann).
• Properties: boiling point rises from F to I. The para isomer has the highest melting point. 1,4-Dichlorobenzene has no dipole moment.

Reactions

• Inert to nucleophiles: resonance, an sp2 carbon, an unstable phenyl cation, and repulsion. Dow process: NaOH at $623\ \text{K}$ and $300\ \text{atm}$ gives phenol.
• Nitro groups at ortho and para positions help: $443\ \text{K}$ for one, $368\ \text{K}$ for two, warm water for three.
• Electrophilic substitution: halogens deactivate, and direct to ortho and para. The para product usually dominates.
• Metals: Wurtz (R-R), Wurtz-Fittig (Ar-R) and Fittig (Ar-Ar) with Na in dry ether. Mg in dry ether gives $\text{RMgX}$, which water destroys.

Polyhalogen compounds

• $\text{CH}_{2}\text{Cl}_{2}$: solvent and paint remover. $\text{CHCl}_{3}$: solvent and R-22. It forms phosgene in light and air, so it is kept in full dark bottles. $\text{CHI}_{3}$: antiseptic, from the iodoform test.
• $\text{CCl}_{4}$ from $\text{CS}_{2}$ and $\text{Cl}_{2}$. Freons from $\text{CCl}_{4}$ and HF.
• A chlorine atom from a CFC destroys about $10^5$ ozone molecules. The Montreal Protocol phased out CFCs.
• DDT is made from chlorobenzene and chloral. It is persistent and builds up along food chains.

A worked example

A mixture has $0.100\ \text{mol}$ of bromobenzene and $0.100\ \text{mol}$ of bromoethane. Excess sodium in dry ether couples them at random. How much of the mixed product, ethylbenzene, forms?

1. Couplings. $\dfrac{0.200}{2} = 0.100\ \text{mol}$ of couplings.
2. Share. Half of the pairs are mixed, so $0.0500\ \text{mol}$.
3. Mass. Ethylbenzene has a molar mass of $106\ \text{g/mol}$, so the mass is $5.30\ \text{g}$.

Practice with new numbers

A program to try

The program lists the routes to five haloarenes and finds the chlorine in chlorobenzene.

Mistakes to avoid

• Chlorinating the ring with light. Light goes to the side chain.
• Calling a halogen an activating group because it directs to ortho and para.
• Storing chloroform in a clear bottle. It forms phosgene.
• Saying the ozone-destroying chlorine atom is used up. It is regenerated.

Check yourself

1. How is chlorobenzene prepared from aniline?

2. Which dichlorobenzene has the highest melting point?

3. Which compound is used to make the refrigerant R-22?

Sources

• NCERT, Chemistry Part II for Class XII, the chapter Haloalkanes and Haloarenes.
• OpenStax, Organic Chemistry, Chapters 10 and 16.