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MHT-CET Chemistry · Halogen Derivatives of Alkanes

Nucleophilic Substitution: SN1 and SN2

A nucleophile replaces the halide either in two steps through a planar carbocation (SN1 — tertiary fastest, racemisation at a chiral carbon) or in one concerted backside attack (SN2 — methyl fastest, inversion); the nucleophile's attacking atom decides the product, KCN giving a nitrile and AgCN an isocyanide.

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Why this matters

12 PYQs, none HARD. Half ask a mechanism fact — which is NOT a feature of SN2 (a carbocation), which halide is fastest by SN1 or SN2, which substrate racemises; half ask a product — 2-bromobutane with aqueous NaOH, ethyl bromide with silver acetate or silver propanoate, the reagent for an alkyl nitrite or a nitrile. Two cards.

Concept 1 of 2

SN1 Against SN2: Mechanism, Rate Order and Stereochemistry

Intuition

SN1: the C–X bond breaks first, a planar carbocation forms, the nucleophile attacks either face — so the rate depends only on the halide (tertiary fastest, stabilised cation) and a chiral centre is racemised. SN2: the nucleophile pushes in from the back while X leaves in ONE step — no intermediate, the rate depends on both reactants, crowding slows it (methyl fastest), and the configuration inverts.

Definition

  • SN2: single step, bimolecular, backside attack, simultaneous bond making and breaking, inversion (Walden). NO carbocation. Rate: methyl > 1° > 2° > 3°. Fastest among 1-bromobutane, 1-chlorobutane, 2-chlorobutane, 1-iodobutane: the primary IODIDE (best leaving group, unhindered).
  • SN1: two steps, unimolecular, planar carbocation, racemisation at a stereocentre. Rate: 3° > 2° > 1° — tert-butyl iodide fastest.
  • Leaving group: I > Br > Cl > F for both.
  • Racemisation needs a chiral C–X carbon: 2-chlorobutane racemises on SN1 hydrolysis; 2-chloropropane, 3-chloropentane and neopentyl chloride cannot.
  • tert-Butyl bromide + AgF (SN1) keeps its skeleton: 2-fluoro-2-methylpropane. Tertiary alcohols react fastest with HBr for the same reason.

Rate orders

SN1: 3∘>2∘>1∘>CH3X;SN2: CH3X>1∘>2∘>3∘\text{SN1: } 3^\circ > 2^\circ > 1^\circ > \text{CH}_3\text{X};\qquad \text{SN2: } \text{CH}_3\text{X} > 1^\circ > 2^\circ > 3^\circ

Worked example

(R)-2-bromooctane is hydrolysed once with aqueous NaOH (SN2) and once with water in acetone (SN1). Describe the product's stereochemistry in each case.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Halogen Derivatives of AlkanesMODERATE
Which among the following is NOT a feature of SN2S_N2 mechanism?

[Q63 · 15th May Shift 2 · 2023]

Expecting the 3°-fastest order for SN2

Crowding blocks the backside. SN2 is fastest on the LEAST substituted carbon; tertiary halides go SN1 (or eliminate). The two orders are exact opposites.
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Concept 2 of 2

Which Nucleophile Gives Which Product

Intuition

The product is the nucleophile bonded through its attacking atom. Aqueous OH⁻ gives the alcohol; alkoxide gives an ether; a silver carboxylate gives an ester; KCN attacks through carbon to give a nitrile while AgCN attacks through nitrogen to give an isocyanide; KNO₂ gives a nitroalkane while AgNO₂ gives an alkyl nitrite. Silver salts favour the more electronegative atom because Ag⁺ pulls the halide off (SN1-like).

Definition

  • R-X+aq. NaOH→R-OH\text{R-X} + \text{aq. NaOH} \to \text{R-OH}: 2-bromobutane → butan-2-ol (alcoholic KOH would give but-2-ene).
  • R-X+R’ONa→R-O-R’\text{R-X} + \text{R'ONa} \to \text{R-O-R'} (Williamson ether synthesis).
  • R-X+R’COOAg→R’COOR\text{R-X} + \text{R'COOAg} \to \text{R'COOR}: ethyl bromide + silver acetate → ethyl acetate CH3COOC2H5\text{CH}_3\text{COOC}_2\text{H}_5; + silver propanoate → ethyl propanoate.
  • R-X+KCN (alc.)→R-CN\text{R-X} + \text{KCN (alc.)} \to \text{R-CN} (nitrile, C-attack); +AgCN→R-NC+ \text{AgCN} \to \text{R-NC} (isocyanide, N-attack).
  • R-X+KNO2→R-NO2\text{R-X} + \text{KNO}_2 \to \text{R-NO}_2 (nitroalkane); +AgNO2→R-O-N=O+ \text{AgNO}_2 \to \text{R-O-N=O} (alkyl nitrite).
  • R-X+NH3→\text{R-X} + \text{NH}_3 \to amines (Hofmann); +NaSH→R-SH+ \text{NaSH} \to \text{R-SH}; +NaI→R-I+ \text{NaI} \to \text{R-I} (Finkelstein).

Ambident nucleophiles

KCN→R-C≡N;AgCN→R-N≡C;KNO2→R-NO2;AgNO2→R-O-N=O\text{KCN} \to \text{R-C≡N};\quad \text{AgCN} \to \text{R-N≡C};\qquad \text{KNO}_2 \to \text{R-NO}_2;\quad \text{AgNO}_2 \to \text{R-O-N=O}

Worked example

Give the products of 1-bromopropane with (i) alcoholic KCN, (ii) AgNO₂, (iii) silver acetate.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Halogen Derivatives of AlkanesMODERATE
Identify the product obtained in the following reaction. CH3CH2Br+CH3COOAg→Δx+AgBr\text{CH}_{3}\text{CH}_{2}\text{Br} + \text{CH}_{3}\text{COOAg} \xrightarrow{\Delta} x + \text{AgBr}

[Q74 · 15th May Shift 1 · 2023]

Writing the ester the wrong way round

The acyl part comes from the silver salt, the alkyl from the halide: silver ACETATE + ETHYL bromide is CH₃COO–C₂H₅, ethyl acetate — not ethyl-COO-methyl.
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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 (2)

  • SN1 Against SN2: Mechanism, Rate Order and Stereochemistry

    Rate orders

    SN1: 3∘>2∘>1∘>CH3X;SN2: CH3X>1∘>2∘>3∘\text{SN1: } 3^\circ > 2^\circ > 1^\circ > \text{CH}_3\text{X};\qquad \text{SN2: } \text{CH}_3\text{X} > 1^\circ > 2^\circ > 3^\circ
  • Which Nucleophile Gives Which Product

    Ambident nucleophiles

    KCN→R-C≡N;AgCN→R-N≡C;KNO2→R-NO2;AgNO2→R-O-N=O\text{KCN} \to \text{R-C≡N};\quad \text{AgCN} \to \text{R-N≡C};\qquad \text{KNO}_2 \to \text{R-NO}_2;\quad \text{AgNO}_2 \to \text{R-O-N=O}

Watch out for (2)

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