JEE Mains Chemistry · Haloalkanes and Haloarenes

Haloarenes and Reactions with Metals

A haloarene resists nucleophiles until nitro groups ortho or para to the halogen stabilise the intermediate anion, while its halogen slows electrophilic substitution yet directs it ortho and para; with magnesium an organic halide gives a Grignard reagent, and with sodium it couples.

All Haloalkanes and Haloarenes notes JEE Mains Chemistry strategy

Why this matters

Ten PYQs, all multiple choice, none from 2026. Seven are about the ring: how nitro groups speed up nucleophilic substitution, which halogen is replaced, and where an electrophile goes on chlorobenzene; three put a halide through magnesium or sodium.

Concept 1 of 2: Substitution on the haloarene ring: nucleophilic and electrophilic

A nucleophile cannot push out the halogen of chlorobenzene by SN1 or SN2. It can, however, add to the ring carbon if the negative charge that builds up has somewhere to go. A nitro group ortho or para to the halogen takes that charge onto its oxygens (the Meisenheimer intermediate), and then the halide leaves. Each extra ortho or para nitro group makes this easier. Electrophiles are a different story: the halogen withdraws electrons and slows them, but its lone pair still steers them ortho and para.

Definition

Aryl halides are unreactive to ordinary substitution because the C–X bond has partial double-bond character, the carbon is sp², a phenyl cation is very unstable, and the electron-rich ring repels an incoming nucleophile.
Chlorobenzene with NaOH needs 623 K and 300 atm (Dow process). One nitro para to Cl lowers this to 443 K; nitro groups at 2 and 4 to 368 K; at 2, 4 and 6 warm water is enough.
A meta nitro group cannot take the charge by resonance and helps only a little, through its −I effect. Donors such as $\mathrm{OCH_3}$ and $\mathrm{CH_3}$ slow the reaction further.
When a ring carries two halogens, the one ortho or para to the nitro group is the one replaced.
Electrophilic substitution: halogens deactivate the ring (−I) but direct ortho and para (+R), with para usually major. Chlorobenzene gives mainly 1-chloro-4-nitrobenzene on nitration, 4-chlorobenzenesulphonic acid on sulphonation, 1-chloro-4-methylbenzene with $\mathrm{CH_3Cl/AlCl_3}$ , and 4-chloroacetophenone with $\mathrm{CH_3COCl/AlCl_3}$ .

Chlorobenzene to phenol (Dow process)

\mathrm{C_6H_5Cl \xrightarrow[\text{(ii) } H^+]{\text{(i) NaOH, 623 K, 300 atm}} C_6H_5OH}

Worked example

1,4-Dichloro-2-nitrobenzene is heated with sodium methoxide. Which chlorine is replaced? Name the product.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2021 · Paper 2 · Q31Moderate

Example 1 · Haloalkanes and Haloarenes · Haloarenes and Reactions with Metals

The correct order of the following compounds showing increasing tendency towards nucleophilic substitution reaction is: (i) chlorobenzene (ii) 1-chloro-4-nitrobenzene (iii) 1-chloro-2,4-dinitrobenzene (iv) 2-chloro-1,3,5-trinitrobenzene

Halogens deactivate yet direct ortho and para

A halogen on benzene makes electrophilic substitution slower than on benzene itself (−I), but its lone pair places the new group ortho or para (+R). Deactivating does not mean meta-directing here.

Aryl substitution is neither SN1 nor SN2

The nucleophile adds to the ring to give an anionic intermediate and the halide then leaves. There is no backside attack and no aryl cation.

Drill 6 more on substitution on the haloarene ring: nucleophilic and electrophilic

Concept 2 of 2: Reactions of organic halides with magnesium and sodium

Magnesium slips into the C–X bond in dry ether and turns the carbon into a strong nucleophile and base: a Grignard reagent. Any acidic hydrogen, even from water, then replaces the magnesium, so the ether must be dry. Sodium does something else: it joins two carbon groups together, which is the Wurtz family of couplings.

Definition

$\mathrm{R{-}X + Mg \xrightarrow{\text{dry ether}} R{-}MgX}$ . Vinylic and aryl bromides also form Grignard reagents.
$\mathrm{R{-}MgX + H_2O \to R{-}H + Mg(OH)X}$ ; with $\mathrm{D_2O}$ the product is R–D, with D on the carbon that held X.
Excess magnesium reacts at every C–X bond, so a dibromide gives a di-Grignard reagent in the first step.
Wurtz: $\mathrm{2R{-}X + 2Na \to R{-}R + 2NaX}$ in dry ether. Two different alkyl halides give a mixture of three alkanes.
A 1,3-dihalide with Na or Zn closes a ring and gives a cyclopropane.
Wurtz-Fittig (ArX + RX → Ar–R) and Fittig (2 ArX → Ar–Ar) use sodium in dry ether as well.

Reactants	Conditions	Product	Name
R–X + Mg	Dry ether	R–MgX	Grignard reagent
R–MgX + $\mathrm{H_2O}$	Any trace of water	R–H + Mg(OH)X	Hydrolysis: the reason the ether must be dry
R–MgX + $\mathrm{D_2O}$	Heavy water	R–D	Deuterium labelling at the old C–X carbon
A dibromide + excess Mg	Dry ether	Both C–Br become C–MgBr	Di-Grignard reagent
2 R–X + 2 Na	Dry ether	R–R	Wurtz reaction
$\mathrm{BrCH_2CH_2CH_2Br}$ + Zn or Na	Heat	Cyclopropane	Ring closure (intramolecular Wurtz)
ArX + RX + 2 Na	Dry ether	Ar–R	Wurtz-Fittig reaction
2 ArX + 2 Na	Dry ether	Ar–Ar	Fittig reaction

Magnesium makes a carbon nucleophile; sodium joins two carbon groups.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2021 · Paper 1 · Q35Moderate

Example 2 · Haloalkanes and Haloarenes · Haloarenes and Reactions with Metals

The product formed in the first step of the reaction of

Two different halides give a mixture in the Wurtz reaction

\mathrm{CH_3Br}

and

\mathrm{C_2H_5Br}

with sodium give ethane, propane and butane together. The Wurtz reaction is useful only for symmetric alkanes.

Water is not the only thing that destroys a Grignard reagent

Any compound with an acidic hydrogen, such as an alcohol, an amine or water, converts R–MgX into R–H. A Grignard reagent cannot be made from a halide that also carries an OH or NH₂ group.

Drill 2 more on reactions of organic halides with magnesium and sodium

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Substitution on the haloarene ring: nucleophilic and electrophilic
Chlorobenzene to phenol (Dow process)
$\mathrm{C_6H_5Cl \xrightarrow[\text{(ii) } H^+]{\text{(i) NaOH, 623 K, 300 atm}} C_6H_5OH}$

Reference tables (1)

Reactions of organic halides with magnesium and sodium8 rows

Reactants	Conditions	Product	Name
R–X + Mg	Dry ether	R–MgX	Grignard reagent
R–MgX + $\mathrm{H_2O}$	Any trace of water	R–H + Mg(OH)X	Hydrolysis: the reason the ether must be dry
R–MgX + $\mathrm{D_2O}$	Heavy water	R–D	Deuterium labelling at the old C–X carbon
A dibromide + excess Mg	Dry ether	Both C–Br become C–MgBr	Di-Grignard reagent
2 R–X + 2 Na	Dry ether	R–R	Wurtz reaction
$\mathrm{BrCH_2CH_2CH_2Br}$ + Zn or Na	Heat	Cyclopropane	Ring closure (intramolecular Wurtz)
ArX + RX + 2 Na	Dry ether	Ar–R	Wurtz-Fittig reaction
2 ArX + 2 Na	Dry ether	Ar–Ar	Fittig reaction

Magnesium makes a carbon nucleophile; sodium joins two carbon groups.

Watch out for (4)

Halogens deactivate yet direct ortho and para
→ Substitution on the haloarene ring: nucleophilic and electrophilic
Aryl substitution is neither SN1 nor SN2
→ Substitution on the haloarene ring: nucleophilic and electrophilic
Two different halides give a mixture in the Wurtz reaction
→ Reactions of organic halides with magnesium and sodium
Water is not the only thing that destroys a Grignard reagent
→ Reactions of organic halides with magnesium and sodium

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