Is Halo Alkanes and Halo Arenes part of the EAPCET Chemistry syllabus?

Yes. Halo Alkanes and Halo Arenes is taught in Intermediate second year Chemistry, and the EAPCET Engineering syllabus is based on the Intermediate first- and second-year syllabus. Check the official notification each year for any changes.

How many Chemistry questions are there in EAPCET?

40 of the 160 questions in the Engineering paper are Chemistry, spread across 24 chapters.

What mistakes do students make in Halo Alkanes and Halo Arenes?

Expecting 3° halides to react fastest by SN2 — steric hindrance makes them the slowest; they react by SN1 instead. Mixing up the stereochemistry — SN2 gives inversion of configuration, while SN1 gives racemisation. Using aqueous and alcoholic KOH interchangeably — aqueous KOH gives substitution (alcohol), alcoholic KOH gives elimination (alkene). Writing alkyl cyanide as the product with AgCN — AgCN attacks through nitrogen and gives an isocyanide; KCN gives the cyanide.

EAPCET Chemistry · Intermediate second year

Halo Alkanes and Halo Arenes

This chapter covers the preparation, properties and reactions of haloalkanes and haloarenes, with a focus on substitution and elimination mechanisms.

Subject: Chemistry
Taught in: Intermediate second year
Chemistry in the paper: 40 of 160 questions

Key formulas and results

SN1 proceeds through a carbocation and favours 3° substrates, giving racemisation
SN2 is a one-step backside attack favouring 1° substrates, giving inversion of configuration
SN2 reactivity: CH₃X > 1° > 2° > 3°
Elimination gives the more substituted alkene (Saytzeff’s rule)
Haloarenes are less reactive towards substitution because of resonance
Named reactions: Wurtz, Wurtz–Fittig, Sandmeyer
Rate laws: SN1 rate $= k[\text{RX}]$ (first order); SN2 rate $= k[\text{RX}][\ce{Nu-}]$ (second order)
Alcohol to alkyl chloride: $\ce{ROH + SOCl2 -> RCl + SO2 ^ + HCl ^}$ (best method, both by-products are gases); $\ce{3ROH + PCl3 -> 3RCl + H3PO3}$ ; $\ce{ROH + PCl5 -> RCl + POCl3 + HCl}$
Finkelstein reaction: $\ce{RCl}$ or $\ce{RBr} + \ce{NaI}$ (dry acetone) $\to \ce{RI} + \ce{NaCl}$ or $\ce{NaBr}$ ; Swarts reaction: $\ce{CH3Br + AgF -> CH3F + AgBr}$
Allylic halogenation: $\ce{CH2=CH-CH3 + Cl2 ->[\text{773 K}] CH2=CH-CH2Cl + HCl}$ ; NBS with light gives the allylic bromide
Grignard reagent: $\ce{R-X + Mg ->[\text{dry ether}] RMgX}$ ; $\ce{RMgX + H2O -> R-H + Mg(OH)X}$ (any active hydrogen from $\ce{H2O}$ , ROH, $\ce{NH3}$ , $\ce{RNH2}$ or $\ce{HC#CH}$ gives the alkane)
Grignard reagent with carbonyls (then H₃O⁺): HCHO → 1° alcohol, other aldehydes → 2° alcohol, ketones → 3° alcohol, CO₂ → RCOOH
Iodoform reaction: $\ce{R-CO-CH3 + 3I2 + 4NaOH -> RCOONa + CHI3 v}$ (yellow) $\ce{+ 3NaI + 3H2O}$
Carbylamine test for 1° amines: $\ce{RNH2 + CHCl3 + 3KOH}$ (alc.) $\ce{-> RNC}$ (foul-smelling isocyanide) $\ce{+ 3KCl + 3H2O}$

What this chapter covers

Classification and nomenclature
Preparation
SN1 and SN2 mechanisms
Elimination reactions
Reactions of haloarenes
Polyhalogen compounds

How it is usually asked

Mechanism and product of a substitution
Reactivity order
Named reactions
Substitution versus elimination

Understanding Halo Alkanes and Halo Arenes

Haloalkanes have a halogen on an sp³ carbon, and the polar C–X bond makes that carbon open to attack by nucleophiles. They are prepared from alcohols (with HX, PCl₅ or SOCl₂, the last giving gaseous by-products), from alkenes by adding HX, and by halogen exchange: the Finkelstein reaction uses NaI in acetone and the Swarts reaction uses metal fluorides such as AgF. Reactivity follows bond strength: R–I > R–Br > R–Cl > R–F.

In SN2, the nucleophile attacks from the back in one step, so the rate depends on both reactants, crowded substrates react slowest (CH₃X > 1° > 2° > 3°) and configuration is inverted. In SN1, the halide first leaves to form a carbocation, so the rate depends only on the substrate, 3° halides react fastest and a chiral substrate gives a racemic mixture. Aqueous KOH favours substitution to alcohols, while alcoholic KOH favours elimination to the more substituted alkene (Saytzeff’s rule).

Haloarenes are much less reactive towards nucleophiles because the C–X bond gains partial double-bond character through resonance and the carbon is sp². They react only under harsh conditions, or when electron-withdrawing groups such as –NO₂ sit at ortho or para positions. In electrophilic substitution, the halogen deactivates the ring but directs ortho and para. Ambident nucleophiles matter too: KCN gives nitriles but AgCN gives isocyanides.

Worked example

EAPCET-style question

Which of the following reacts fastest with $\ce{OH-}$ by the SN2 mechanism?

(1) $\ce{CH3Cl}$
(2) $\ce{CH3I}$
(3) $\ce{(CH3)2CHCl}$
(4) $\ce{(CH3)3CI}$

Solution

SN2 is a one-step backside attack, so it is fastest for the least hindered carbon: methyl > 1° > 2° > 3°.
This rules out the 2° isopropyl chloride and, even more, the 3° tert-butyl iodide.
Between $\ce{CH3Cl}$ and $\ce{CH3I}$ , the leaving group decides: $\ce{I-}$ is a better leaving group because the C–I bond is weakest.
So $\ce{CH3I}$ reacts fastest by SN2.

Answer: $\ce{CH3I}$

Shortcuts and results to remember

KCN gives alkyl cyanide R–CN but AgCN gives isocyanide R–NC; KNO₂ gives alkyl nitrite R–O–N=O but AgNO₂ gives nitroalkane R–NO₂.
Polar protic solvents (H₂O, ROH, CH₃COOH) favour SN1, while polar aprotic solvents (DMSO, DMF, acetone) favour SN2.
A better nucleophile favours substitution and a stronger or bulkier base favours elimination, so 3° halides with alkoxide give alkenes and Williamson ether synthesis must use a 1° halide.
Only compounds with CH₃CO– or CH₃CH(OH)– give the iodoform test: ethanol, acetaldehyde, acetone, acetophenone and propan-2-ol do, but methanol, pentan-3-one and acids, esters or amides do not.
Electron-withdrawing –NO₂ groups at ortho or para positions make aryl halides much more reactive towards nucleophilic substitution; with none present, chlorobenzene needs NaNH₂/liquid NH₃ (benzyne mechanism).

Common mistakes to avoid

Expecting 3° halides to react fastest by SN2 — steric hindrance makes them the slowest; they react by SN1 instead.
Mixing up the stereochemistry — SN2 gives inversion of configuration, while SN1 gives racemisation.
Using aqueous and alcoholic KOH interchangeably — aqueous KOH gives substitution (alcohol), alcoholic KOH gives elimination (alkene).
Writing alkyl cyanide as the product with AgCN — AgCN attacks through nitrogen and gives an isocyanide; KCN gives the cyanide.

How to prepare it

Identify whether the substrate is 1°, 2° or 3° first — it largely decides the mechanism.

Once the formulas feel familiar, test them the way the exam does: practise Halo Alkanes and Halo Arenes questions chapter-wise, then attempt a full previous year paper to see whether it holds up under a 3-hour timer.

Related Chemistry chapters

Chemistry in Everyday LifeIntermediate second year

See every chapter in the EAPCET Chemistry syllabus.

Practise Halo Alkanes and Halo Arenes chapter-wise

Chapter-wise quizzes show exactly which topics in this chapter you still get wrong — before a full paper does.

Frequently asked questions

Is Halo Alkanes and Halo Arenes part of the EAPCET Chemistry syllabus?: Yes. Halo Alkanes and Halo Arenes is taught in Intermediate second year Chemistry, and the EAPCET Engineering syllabus is based on the Intermediate first- and second-year syllabus. Check the official notification each year for any changes.
How should I prepare Halo Alkanes and Halo Arenes for EAPCET?: Identify whether the substrate is 1°, 2° or 3° first — it largely decides the mechanism.
How many Chemistry questions are there in EAPCET?: 40 of the 160 questions in the Engineering paper are Chemistry, spread across 24 chapters.
What mistakes do students make in Halo Alkanes and Halo Arenes?: Expecting 3° halides to react fastest by SN2 — steric hindrance makes them the slowest; they react by SN1 instead. Mixing up the stereochemistry — SN2 gives inversion of configuration, while SN1 gives racemisation. Using aqueous and alcoholic KOH interchangeably — aqueous KOH gives substitution (alcohol), alcoholic KOH gives elimination (alkene). Writing alkyl cyanide as the product with AgCN — AgCN attacks through nitrogen and gives an isocyanide; KCN gives the cyanide.