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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 notesMHT-CET Chemistry strategy

Why this matters

14 PYQs, 1 HARD. 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

Intuition

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∘>2∘>1∘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 — R2C=CR2>R2C=CHR>RCH=CHR>RCH=CH2\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 + C2H5ONa\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

R2CH-CR2X→alc. KOHR2C=CR2+HX(Saytzeff: more substituted alkene)\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.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Halogen Derivatives of AlkanesMODERATE
Which of the following alkenes is most easily formed by dehydrohalogenation of alkyl halides?

[Q100 · 19 April Shift I · 2025]

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

Intuition

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

rate↑ with each −NO2 at o/p;m-NO2 has no effect\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.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Halogen Derivatives of AlkanesMODERATE
Which of the following compounds has difficulty in breaking the C-Cl bond?

[Q77 · 3rd May 2nd Shift · 2023]

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

Intuition

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: 2Ar-X+2Na→dry etherAr-Ar2\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

C6H5Cl→HNO3/H2SO4o- and p-ClC6H4NO2;2ArX+2Na→Ar-Ar\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?
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Halogen Derivatives of AlkanesMODERATE
Identify the product formed from chlorobenzene on heating with conc. HNO3HNO_{3} in presence of conc. H2SO4H_{2}SO_{4}.

[Q77 · 19 April Shift I · 2025]

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

    R2CH-CR2X→alc. KOHR2C=CR2+HX(Saytzeff: more substituted alkene)\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

    rate↑ with each −NO2 at o/p;m-NO2 has no effect\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

    C6H5Cl→HNO3/H2SO4o- and p-ClC6H4NO2;2ArX+2Na→Ar-Ar\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)

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