MHT-CET Chemistry · Halogen Derivatives of Alkanes

Elimination and Aromatic Nucleophilic Substitution

A strong base in alcohol pulls HX off an alkyl halide to give the more substituted alkene (Saytzeff), fastest for tertiary halides; on a haloarene the C–X bond breaks only when nitro groups ortho or para to it stabilise the intermediate, while electrophiles still substitute the ring at ortho and para.

All Halogen Derivatives of Alkanes notes MHT-CET Chemistry strategy

Why this matters

24 PYQs, 1 HARD. Of the fourteen set from 2021 to 2025, seven are nitro-activated substitution on chloroarenes — which has the greatest difficulty breaking C–Cl (the meta-nitro isomer), which is most reactive (2,4,6-trinitro), what substrate gives picric acid. Four are elimination — the 3° > 2° > 1° order, the Saytzeff alkene, which reagent or base eliminates. Three are haloarene reactions: Fittig and the o/p nitration of chlorobenzene.

Concept 1 of 3: Dehydrohalogenation and the Saytzeff Rule

A base takes a β-hydrogen while the halide leaves, making a C=C. The more substituted the halide, the easier (3° > 2° > 1°) and the more substituted the alkene, the more stable — so the major product carries the most alkyl groups on the double bond (Saytzeff). Alcoholic KOH, alcoholic NH₃ and sodium ethoxide are the eliminating bases; aqueous OH⁻ substitutes instead.

Definition

Ease: $3^\circ > 2^\circ > 1^\circ$ . tert-Butyl bromide + alc. NH₃ (or alc. KOH) → isobutylene (2-methylpropene).
Saytzeff: the alkene with more alkyl groups on C=C forms most easily — $\text{R}_2\text{C=CR}_2 > \text{R}_2\text{C=CHR} > \text{RCH=CHR} > \text{RCH=CH}_2$ . 2-Bromobutane → but-2-ene (major), but-1-ene (minor).
Bulky base (sodium ethoxide) on a secondary halide: elimination wins — isopropyl chloride + $\text{C}_2\text{H}_5\text{ONa}$ → propene + ethanol + NaCl.
Aqueous KOH → alcohol (substitution); alcoholic KOH → alkene (elimination). The solvent is the whole question.

β-Elimination

\text{R}_2\text{CH-CR}_2\text{X} \xrightarrow{\text{alc. KOH}} \text{R}_2\text{C=CR}_2 + \text{HX} \quad (\text{Saytzeff: more substituted alkene})

Worked example

Give the major alkene from 2-bromo-2-methylbutane with alcoholic KOH, and the product of the same halide with aqueous KOH.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 19 April Shift I · Q100Moderate

Example 1 · Halogen Derivatives of Alkanes · Elimination and Aromatic Nucleophilic Substitution

Which of the following alkenes is most easily formed by dehydrohalogenation of alkyl halides?

Expecting the ether from ethoxide and a 2° halide

Ethoxide is a strong base as well as a nucleophile; on a secondary or tertiary halide it eliminates. 2-Ethoxypropane is the offered wrong answer; propene is the product.

Drill 3 more on dehydrohalogenation and the saytzeff rule

Concept 2 of 3: Nitro Groups Activate the C–X Bond of a Haloarene

Chlorobenzene resists nucleophiles — the ring's electrons repel them and the C–Cl bond has partial double-bond character. A nitro group ORTHO or PARA to the chlorine can take the negative charge of the attacking intermediate by resonance, so each such nitro group speeds substitution enormously; a META nitro group cannot and does nothing. With three nitro groups (picryl chloride) even warm water substitutes, giving picric acid.

Definition

Reactivity in C–X cleavage: 2,4,6-trinitrochlorobenzene > 2,4-dinitrochlorobenzene > p- (or o-) nitrochlorobenzene > chlorobenzene.
Greatest DIFFICULTY among o-, m-, p-nitro and trinitro: m-nitrochlorobenzene — the meta nitro cannot delocalise the charge onto the C–Cl carbon.
Conditions: chlorobenzene needs NaOH at 623 K and 300 atm (Dow); p-nitrochlorobenzene 15% NaOH at 433 K; 2,4-dinitro warm NaOH; 2,4,6-trinitro warm water.
Picric acid (2,4,6-trinitrophenol) from picryl chloride and water — the substrate 'S' question.

Activation by nitro groups

\text{rate} \uparrow \text{ with each } -\text{NO}_2 \text{ at o/p};\qquad m\text{-NO}_2 \text{ has no effect}

Worked example

Arrange chlorobenzene (I), 2,4-dinitrochlorobenzene (II) and 2,4,6-trinitrochlorobenzene (III) by reactivity towards OH⁻, and say which of o-, m-, p-nitrochlorobenzene reacts slowest.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 3rd May Shift 1 · Q77Moderate

Example 2 · Halogen Derivatives of Alkanes · Elimination and Aromatic Nucleophilic Substitution

Which of the following compounds has difficulty in breaking the C-Cl bond?

Counting a meta nitro group as activating

Only ortho and para positions put the ring's negative charge next to the C–Cl carbon. m-Nitrochlorobenzene behaves almost like chlorobenzene — it is the 'most difficult' answer, not p-nitro.

Drill 6 more on nitro groups activate the c–x bond of a haloarene

Concept 3 of 3: Reactions of Haloarenes: Fittig and o/p Electrophilic Substitution

The halogen on a benzene ring is a deactivating but ortho/para-directing group — its lone pair feeds the ring by resonance even as it withdraws by induction. So nitration, halogenation, sulphonation and Friedel–Crafts all give the ortho and para products, with para major. Sodium in dry ether couples two aryl halides to a biaryl (Fittig).

Definition

Chlorobenzene + conc. HNO₃/H₂SO₄ → a MIXTURE of 1-chloro-2-nitrobenzene and 1-chloro-4-nitrobenzene (para major); not the trinitro compound under ordinary conditions.
Chlorobenzene + Cl₂/FeCl₃ → o- and p-dichlorobenzene; + CH₃Cl/AlCl₃ → o- and p-chlorotoluene.
Fittig: $2\text{Ar-X} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{Ar-Ar}$ — biphenyl from bromobenzene. Wurtz–Fittig (Ar-X + R-X) gives an alkylarene.
Haloarenes do NOT undergo SN1/SN2 (sp² carbon, partial double bond); nucleophilic substitution needs o/p nitro activation.

Halogen directs o/p

\text{C}_6\text{H}_5\text{Cl} \xrightarrow{\text{HNO}_3/\text{H}_2\text{SO}_4} o\text{- and } p\text{-ClC}_6\text{H}_4\text{NO}_2;\qquad 2\text{ArX} + 2\text{Na} \to \text{Ar-Ar}

Worked example

What does bromobenzene give with (i) Br₂/FeBr₃ and (ii) sodium in dry ether?

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 19 April Shift I · Q77Moderate

Example 3 · Halogen Derivatives of Alkanes · Elimination and Aromatic Nucleophilic Substitution

Identify the product formed from chlorobenzene on heating with conc.

HNO_{3}

in presence of conc.

H_{2}SO_{4}

Picking 'only para'

Para is the MAJOR product, but the ortho isomer forms too and the paper asks for the mixture. 'Only 1-chloro-4-nitrobenzene' is the planted option.

Drill 2 more on reactions of haloarenes: fittig and o/p electrophilic substitution

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 (3)

Dehydrohalogenation and the Saytzeff Rule
β-Elimination
$\text{R}_2\text{CH-CR}_2\text{X} \xrightarrow{\text{alc. KOH}} \text{R}_2\text{C=CR}_2 + \text{HX} \quad (\text{Saytzeff: more substituted alkene})$
Nitro Groups Activate the C–X Bond of a Haloarene
Activation by nitro groups
$\text{rate} \uparrow \text{ with each } -\text{NO}_2 \text{ at o/p};\qquad m\text{-NO}_2 \text{ has no effect}$
Reactions of Haloarenes: Fittig and o/p Electrophilic Substitution
Halogen directs o/p
$\text{C}_6\text{H}_5\text{Cl} \xrightarrow{\text{HNO}_3/\text{H}_2\text{SO}_4} o\text{- and } p\text{-ClC}_6\text{H}_4\text{NO}_2;\qquad 2\text{ArX} + 2\text{Na} \to \text{Ar-Ar}$

Watch out for (3)

Expecting the ether from ethoxide and a 2° halide
→ Dehydrohalogenation and the Saytzeff Rule
Counting a meta nitro group as activating
→ Nitro Groups Activate the C–X Bond of a Haloarene
Picking 'only para'
→ Reactions of Haloarenes: Fittig and o/p Electrophilic Substitution

Test yourself on Halogen Derivatives of Alkanes

20 past MHT-CET questions from this chapter, timed at 18 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Halogen Derivatives of Alkanes chapter test Drill all 24 questions

Related notes

Preparation — Fittig and Wurtz–Fittig coupling →