Is it correct to say: Writing SN1 as a one-step mechanism with rate = k[R-X][Nu⁻].?

SN1 is TWO-step, first-order, with rate depending ONLY on [R-X]. Only SN2 is single-step and bimolecular.

Is it correct to say: Confusing tertiary halide reactivity — thinking 3° halides are fast in SN2.?

3° halides are fast in SN1 (stable carbocation) but slowest in SN2 (steric block). Reactivity orders are OPPOSITE.

Is it correct to say: Claiming haloarenes undergo SN1 easily because C–Cl is polar.?

Haloarenes are unusually UNREACTIVE — resonance strengthens C–X and destabilises aryl carbocation. SN needs harsh conditions (Dow process 623 K + 300 atm).

Is it correct to say: Predicting Saytzeff (more substituted alkene) product with a bulky base like t-butoxide.?

Bulky bases give the HOFMANN (less substituted, terminal alkene) product because they can only access the less-hindered β-H.

Is it correct to say: IUPAC nomenclature — putting halogen suffix instead of prefix, or wrongly numbering the chain.?

Halogen is always a PREFIX (bromo-, chloro-, iodo-), and chain numbered to give the FIRST point of difference lowest locant (halogen and substituent counted together).

Is it correct to say: Saying chiral molecule always rotates plane-polarised light.?

A racemic mixture (50:50 enantiomers) is optically INACTIVE even though individual molecules are chiral — rotations cancel.

Formula sheet (PDF) →

Chemistry · Chapter 6

Haloalkanes and Haloarenes

Nomenclature, SN1/SN2 substitution, E1/E2 elimination, optical activity, and name reactions of C-X compounds — NCERT Class 12 Chemistry Ch 6

11 formulas · 3 solved examples · 6 practice questions · Board weight: 6-8 marks · CBSE Class 12 · 2025–26 syllabus

Board Exam Tips

→SN1 vs SN2 comparison table (rate law, mechanism, stereochemistry, solvent, substrate) is a repeat 3-mark question.
→Reactivity order: for SN1 use carbocation stability (3° > 2° > 1° > CH3); for SN2 use steric access (CH3 > 1° > 2° > 3°) — memorise both.
→Haloarenes (aryl halides) are UNREACTIVE towards SN1 & SN2 — explain via resonance stabilisation and partial double-bond character of C–X.
→Learn Wurtz, Fittig, Wurtz-Fittig, Finkelstein, Swarts, Sandmeyer, and Gattermann reactions by heart with reagents and products.
→Optical isomerism: chiral centre = 4 different groups on one carbon. Racemisation (SN1) vs inversion (SN2) is examinable.

Formulas(11)

SN2 Rate Law (Bimolecular Substitution)Board favourite

Rate = k [R-X] [Nu^{-}]

Symbol	Meaning
$k$	Second-order rate constant (L mol⁻¹ s⁻¹)
$[R - X]$	Substrate concentration
$[Nu^{-}]$	Nucleophile concentration

SN1 Rate Law (Unimolecular Substitution)Board favourite

Rate = k [R-X]

E2 Rate Law (Bimolecular Elimination)Board favourite

Rate = k [R-X] [Base]

E1 Rate Law

Rate = k [R-X]

SN2 Reactivity Order

CH_{3} X > 1^{\circ} > 2^{\circ} > 3^{\circ} (primary allylic/benzylic > primary)

SN1 Reactivity Order

3^{\circ} > 2^{\circ} > 1^{\circ} > CH_{3} X

Finkelstein ReactionBoard favourite

R-Cl (or R-Br) + NaI acetone R-I + NaCl (or NaBr)

Swarts Reaction

R-Br + AgF \to R-F + AgBr

Wurtz Reaction

2 R-X + 2 Na dry ether R-R + 2 NaX

Wurtz-Fittig Reaction

Ar-X + R-X + 2 Na dry ether Ar-R + 2 NaX

Sandmeyer ReactionBoard favourite

Ar-N_{2}^{+} X^{-} CuCl / CuBr Ar-Cl (or Ar-Br) + N_{2}

Solved Examples

1Solved Exampleeasy3 steps

Give the IUPAC name of (CH₃)₂CHCH₂Br.

Draw the structure and number the longest chain to give the halogen the LOWEST locant

Structure: CH_{3} -CH(CH_{3})-CH_{2} -Br

2Solved Exampleboard4 steps

Compare SN1 vs SN2 reactions of (CH₃)₃C-Br and CH₃-Br with hydroxide ion. Predict the mechanism and stereochemistry of the product.

(CH₃)₃C-Br is a tertiary halide — no free access for back-side attack; carbocation is stable

(CH_{3})_{3} C-Br slow (CH_{3})_{3} C^{+} + Br^{-}

3Solved ExampleHOTS5 steps

Explain why chlorobenzene is much less reactive towards nucleophilic substitution than chloroethane, even though both have a C-Cl bond.

In chlorobenzene, the Cl lone pair conjugates with the benzene π-system

Cl ⟷ resonance Cl^{+} =C ring (partial C=Cl double bond)

Traps & Common Mistakes

Common Mistakes6

1
Writing SN1 as a one-step mechanism with rate = k[R-X][Nu⁻].
✓SN1 is TWO-step, first-order, with rate depending ONLY on [R-X]. Only SN2 is single-step and bimolecular.
2
Confusing tertiary halide reactivity — thinking 3° halides are fast in SN2.
✓3° halides are fast in SN1 (stable carbocation) but slowest in SN2 (steric block). Reactivity orders are OPPOSITE.
3
Claiming haloarenes undergo SN1 easily because C–Cl is polar.
✓Haloarenes are unusually UNREACTIVE — resonance strengthens C–X and destabilises aryl carbocation. SN needs harsh conditions (Dow process 623 K + 300 atm).
4
Predicting Saytzeff (more substituted alkene) product with a bulky base like t-butoxide.
✓Bulky bases give the HOFMANN (less substituted, terminal alkene) product because they can only access the less-hindered β-H.
5
IUPAC nomenclature — putting halogen suffix instead of prefix, or wrongly numbering the chain.
✓Halogen is always a PREFIX (bromo-, chloro-, iodo-), and chain numbered to give the FIRST point of difference lowest locant (halogen and substituent counted together).
6
Saying chiral molecule always rotates plane-polarised light.
✓A racemic mixture (50:50 enantiomers) is optically INACTIVE even though individual molecules are chiral — rotations cancel.

Practice Yourself

Practice Yourself6 questions

Q1
Give the IUPAC name of CH₃-CHBr-CH₂-CH(CH₃)₂.
Q2
Which of the following will undergo SN1 fastest: (a) CH₃Br (b) CH₃CH₂Br (c) (CH₃)₂CHBr (d) (CH₃)₃CBr?
Q3
Complete the reaction: CH₃-CH₂-Cl + NaI → (in dry acetone).
Q4
Why is (±)-2-butanol optically inactive?
Q5
How is chlorobenzene converted to benzene? Give reagents.
Q6
State two conditions that favour E2 over SN2.

Notes

Haloalkanes and Haloarenes

Organic compounds bearing a halogen (F, Cl, Br, I) bonded to sp³ (haloalkane) or sp² aromatic (haloarene) carbon. Their polar C–X bond makes them the workhorses of nucleophilic substitution and elimination chemistry — the two mechanisms that dominate organic synthesis.

Classification

Mono-, di-, poly-halo compounds by number of X.
1°, 2°, 3° haloalkanes by nature of C bearing X.
Aryl (haloarene), benzyl, allyl, vinyl halides by attachment.

Substitution vs elimination — which dominates?

Factor	SN2	SN1	E2	E1
Substrate	1° > 2°	3° > 2°	3° > 2° > 1°	3° > 2°
Nucleophile / base	Strong Nu	Weak Nu	Strong bulky base	Weak base
Solvent	Polar aprotic (DMSO, acetone)	Polar protic (H₂O, ROH)	Polar aprotic	Polar protic
Rate law	k[R-X][Nu]	k[R-X]	k[R-X][B]	k[R-X]
Stereochemistry	Inversion (Walden)	Racemisation	Anti-periplanar	Mixture

Optical activity — the SN reveal

A molecule with a chiral centre (4 different groups on one C) is optically active. Passing plane-polarised light through the solution rotates its plane. SN1 goes via a planar carbocation which loses chirality ⇒ product is racemic. SN2 attacks from the opposite face ⇒ product has inverted configuration but retains chirality.

Aryl halides — the unusual ones

Chlorobenzene, bromobenzene, and iodobenzene do not react with dilute NaOH, KCN, NH₃ or AgNO₃ at ordinary temperature because:

The Cl lone pair conjugates into the ring, giving the C–Cl bond partial double-bond character.
The C bearing X is sp²-hybridised, hence more electronegative and forms a stronger C–X.
SN1 would need an aryl cation — unstable because the empty orbital is in the ring plane, unable to enjoy resonance.
SN2 back-side attack is blocked by the ring.

Under forcing conditions (Dow process: 623 K, 300 atm NaOH) or with strongly electron-withdrawing groups at the ortho/para position (as in 2,4-dinitrochlorobenzene) SN does proceed.

Environmental relevance

CFCs (Freons, Cl-F alkanes), DDT, and BHC are polyhalo compounds with historical industrial use but severe environmental impact — ozone depletion, bioaccumulation. Modern refrigerants replace them with less harmful HFCs.