Generic Sn2 Substitution

Alkyl Halide Reactions: Generic SN2 (NaOCH3 / NaOEt / NaOH / NaSH / NaSR)

This guide covers concerted SN2 substitution on alkyl halides (R–X; X = Cl, Br, I) with NaOCH₃, NaOEt, NaOH, NaSH, and NaSR, plus cyanide, azide and iodide examples. Backside attack and leaving-group departure occur in one step through a single transition state, with inversion when the reacting carbon is stereogenic. Unhindered primary substrates generally react readily; secondary substrates face more elimination competition, and tertiary substrates do not undergo ordinary SN2. Allylic halides can also give SN2′ substitution with double-bond migration; benzylic substrates can undergo rapid substitution but are not generically SN2′ cases.

Quick Summary

Mechanism class: Concerted SN2 backside displacement (single transition state, no carbocation).
Stereochemistry: Walden inversion at the reacting stereocenter.
Substrate scope: Methyl ≫ primary > secondary (condition-dependent); tertiary ≠ SN2. Allylic/benzylic excellent; beware neopentyl slowdown.
Leaving group hierarchy: I > Br ≫ Cl ≫ F; sulfonates (OTs/OMs/OTf) behave like superb halides.
Solvent: Polar aprotic (DMSO, DMF, MeCN, acetone) accelerates SN2; protic solvents solvate anions and can slow SN2; SN1/E1 also require a substrate capable of ionization.
Competition: Strong bases + heat push E2; ionizing media favor SN1/E1. NaSH/NaSR usually reduce E2 competition because they are less basic than alkoxides.

Mechanism (Shared SN2 Template)

Each reagent is illustrated in two panels: the concerted electron flow and the resulting product. The panels do not represent separate elementary steps.

Backside attack & leaving-group departure – Nu⁻ attacks anti to the C–X bond while the leaving group departs in the same step (single concerted event).
Illustration: /assets/reaction-library/alkyl-halide-sn2-naome/alkyl-halide-sn2-naome-step-01.svg
Product – Only the substituted product is shown; the halide is omitted. Inversion is observable when substitution occurs at a stereogenic carbon; an attacked CH₂ or a carbon with two identical groups is not a stereocenter.
Illustration: /assets/reaction-library/alkyl-halide-sn2-naome/alkyl-halide-sn2-naome-step-02.svg

NaOEt (Δ) reminder: Heating with a strong alkoxide can increase elimination competition; emphasize temperature control when you prefer substitution.

Allylic option: Allylic halides can access SN2′ pathways with double-bond migration.

Reagent-specific steps

The figures below show the nucleophile and product for each reagent. All illustrate the same concerted SN2 electron flow.

NaOCH₃ (primary SN2)

NaOCH3 SN2 Step 1: backside attack on 1-bromopropane — Step 1 — Methoxide approaches anti to the C–Br bond; electrons flow to Br⁻.

NaOCH3 SN2 Step 2: 1-methoxypropane product — Step 2 — 1-Methoxypropane forms; Br⁻ is omitted. The attacked CH₂ is not a stereocenter.

NaOEt (secondary SN2 with E2 risk)

NaOEt SN2 Step 1: backside attack on a secondary bromide — Step 1 — Ethoxide attacks anti to bromide; cool conditions can favor substitution, but E2 can still compete.

NaOEt SN2 Step 2: 2-ethoxypropane product — Step 2 — 2-Ethoxypropane forms. Its reacting carbon has two identical methyl groups, so no configuration is assigned.

NaOH (aqueous substitution)

NaOH SN2 Step 1: hydroxide attack on bromoethane — Step 1 — Hydroxide attacks bromoethane as C–Br breaks. Polar aprotic solvents can enhance anionic nucleophilicity, but do not generally suppress E2 from a strong base.

NaOH SN2 Step 2: ethanol product — Step 2 — Ethanol forms; the attacked CH₂ is not a stereocenter.

NaSH (thiol formation)

NaSH SN2 Step 1: HS attack — Step 1 — Hydrosulfide approaches backside; its lower basicity usually reduces E2 competition.

NaSH SN2 Step 2: propyl thiol product — Step 2 — Propane-1-thiol forms; the attacked CH₂ is not a stereocenter.

NaSR (generic thiolate)

NaSR SN2 Step 1: thiolate attack — Step 1 — Methanethiolate attacks the primary bromide to form a C–S bond.

NaSR SN2 Step 2: thioether product — Step 2 — Methyl propyl sulfide forms; use the IUPAC Namer to practice naming thioethers.

NaCN (ambident substitution)

NaCN SN2 Step 1: cyanide attack — Step 1 — Cyanide attacks carbon; C-bound attack dominates in polar aprotic solvents.

NaCN SN2 Step 2: nitrile product — Step 2 — Nitrile forms. The reacting carbon is CH₂; the remote, unspecified stereocenter is not inverted or racemized by this displacement. The pair represents its two possible starting configurations.

NaN₃ (latent amine handle)

NaN3 SN2 Step 1: azide attack — Step 1 — Azide attacks anti to the leaving group and can access an SN2′ pathway on allylic substrates.

NaN3 SN2 Step 2: alkyl azide product — Step 2 — Alkyl azide is produced; reduction → amine is a common follow-up.

NaI (acetone) — halide exchange

NaI SN2 Step 1: iodide attack on primary 1-chloro-4-methylpentane — Step 1 — Iodide performs SN2; NaCl precipitates in acetone, driving the exchange.

NaI SN2 Step 2: alkyl iodide product — Step 2 — Alkyl iodide emerges as a superior leaving group for subsequent chemistry.

Draw a single step illustration: Nu⁻ arrow into Cα, Cα–X bond pushing electrons onto X.
At a stereogenic reacting carbon, show inversion with the appropriate wedge/dash; do not assign inversion to an achiral CH₂.
Unhindered primary halides often react readily; secondary require careful solvent/temperature; tertiary cannot do SN2.
Polar aprotic solvents accelerate substitution; protic solvents can slow anionic SN2; SN1/E1 require suitable ionization-prone substrates.
Allylic halides may give SN2′ products; the ratio depends on the nucleophile, substrate and conditions. Benzylic substitution is a separate case.
Strong base plus heat (as with NaOEt under reflux) raises the chance of E2; less basic nucleophiles such as NaSH, NaSR, NaCN and NaN₃ often favor substitution on accessible substrates.
Halide exchange with iodide (NaI in acetone) upgrades sluggish chlorides or bromides before further transformations.

Worked Examples

Reactant panel showing a primary alkyl bromide poised for SN2 attack — Primary 1-bromopropane + NaOCH₃ (typical DMSO, 0 °C → room temperature) — 1-methoxypropane. No reacting stereocenter is present; E2 remains a possible competing pathway.

Reagent panel showing sodium methoxide — Primary 1-bromopropane + NaOCH₃ (typical DMSO, 0 °C → room temperature) — 1-methoxypropane. No reacting stereocenter is present; E2 remains a possible competing pathway.

Reactant panel showing primary 1-bromo-3-methylpentane — Primary 1-bromo-3-methylpentane + NaOEt (typical MeCN, 0 °C) — 1-ethoxy-3-methylpentane by substitution at CH₂Br. The remote stereocenter is unspecified and is not the site of inversion; polar aprotic solvent can also accelerate competing E2.

Reagent panel showing sodium ethoxide — Primary 1-bromo-3-methylpentane + NaOEt (typical MeCN, 0 °C) — 1-ethoxy-3-methylpentane by substitution at CH₂Br. The remote stereocenter is unspecified and is not the site of inversion; polar aprotic solvent can also accelerate competing E2.

Reactant panel showing primary cyclohexylmethyl chloride — Cyclohexylmethyl chloride + NaSH (typical DMF, room temperature) — cyclohexylmethanethiol. HS⁻ supplies SH, giving a thiol rather than a thioether; the ring is monocyclic and attack occurs at the exocyclic CH₂.

Reagent panel showing sodium hydrosulfide — Cyclohexylmethyl chloride + NaSH (typical DMF, room temperature) — cyclohexylmethanethiol. HS⁻ supplies SH, giving a thiol rather than a thioether; the ring is monocyclic and attack occurs at the exocyclic CH₂.

Reactant panel showing secondary bromocyclohexane — Bromocyclohexane + NaN₃ (typical DMF, room temperature) — cyclohexyl azide, a latent amine handle. This is a secondary cycloalkyl substrate; cyclohexane conformation affects backside access. There is no allylic double bond or reacting stereocenter in this example.

Reagent panel showing sodium azide — Bromocyclohexane + NaN₃ (typical DMF, room temperature) — cyclohexyl azide, a latent amine handle. This is a secondary cycloalkyl substrate; cyclohexane conformation affects backside access. There is no allylic double bond or reacting stereocenter in this example.

Reactant panel showing primary cyclohexylmethyl bromide — Primary cyclohexylmethyl bromide + NaI (typical acetone, reflux) — cyclohexylmethyl iodide. NaBr precipitation favors this exchange; the heated conditions illustrate that SN2 can occur at reflux.

Reagent panel showing sodium iodide in acetone — Primary cyclohexylmethyl bromide + NaI (typical acetone, reflux) — cyclohexylmethyl iodide. NaBr precipitation favors this exchange; the heated conditions illustrate that SN2 can occur at reflux.

Scope & Limitations

Best substrates: Methyl, unhindered primary, benzylic, allylic. Secondary workable with control.
Poor substrates: Tertiary (SN2 forbidden), heavily β-branched, neopentyl (extremely slow).
Leaving groups: I > Br ≫ Cl; convert chlorides to bromides/iodides first if necessary.
Nucleophile strength & basicity:
- Alkoxides/hydroxide: strong nucleophiles, strong bases → E2 competition rises with temperature.
- Hydrosulfide/thiolates: good nucleophiles, less basic than alkoxides → often favor SN2 on accessible substrates.
Solvents: Polar aprotic accelerate SN2; protic solvents solvate anionic nucleophiles; competition also depends on substrate and basicity.
Allylic/benzylic: Rapid SN2; note potential SN2′ (allylic) or SN1 (benzylic in protic solvent).
Vinyl/aryl halides: Do not undergo ordinary SN2 at sp² carbon.

Practical Tips & Pitfalls

Cool, polar aprotic conditions can favor substitution on suitable secondary halides, but solvent and temperature do not remove E2 competition from strong bases.
Label inversion explicitly for stereochemical questions.
For NaOEt/heat, assess the substrate and β-hydrogens: heating can increase E2 competition but is not an elimination-only switch.
NaSH/NaSR solutions smell; mention lab safety (closed setups, proper waste).
Alkoxide reactions often use dry apparatus and solvent. Water changes alkoxide/hydroxide speciation and solvation; it does not remove all nucleophilicity.
Neopentyl substrates remain strongly hindered even with a better leaving group; changing to a sulfonate does not remove the β-branching.
A suitable halide exchange, such as NaI in acetone, can replace a chloride or bromide; account for the substrate and salt solubility.

Exam-Style Summary

SN2 on alkyl halides is a one-step backside displacement that inverts configuration at a stereogenic reacting carbon. Methyl and unhindered primary substrates generally react readily; secondary depend on solvent, base strength, and temperature; tertiary do not undergo SN2. NaOEt can promote E2, including without heating; NaSH/NaSR are less basic and often favor substitution on accessible substrates. Draw the single concerted arrow push, mark inversion, and comment on competing pathways.

Interactive Toolbox

Mechanism Solver — try this example with 1-bromopropane and NaOCH₃, or explore reaction mechanisms.
Reaction Solver — explore reaction predictions for alkyl halides.
IUPAC Namer — practice naming ethers, alcohols, thioethers, and azides produced by these substitutions.

Study Tools