Generic Sn2 Substitution

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

Generic SN2 on Alkyl Halides (NaOCH₃ / NaOEt / NaOH / NaSH / NaSR) | OrgoSolver

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

This unified guide covers concerted SN2 substitution on alkyl halides (R–X; X = Cl, Br, I) for the most common one-click presets: NaOCH₃, NaOEt, NaOEt/heat, NaOH, NaSH, and NaSR. Each preset uses the same three-step SN2 template—backside approach, single transition state, Walden inversion—while reagent overlays flag solvent, temperature, and E2 risk. Primary substrates are the fastest; secondary demand tight control of solvent and temperature; tertiary do not undergo SN2 and default to elimination or ionization pathways. Allylic and benzylic halides are exceptionally reactive and may show minor SN2′ contributions (toggle overlay available).


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/MS/OTf) behave like superb halides.
  • Solvent: Polar aprotic (DMSO, DMF, MeCN, acetone) accelerates SN2; protic slows attack and encourages SN1/E1.
  • Competition: Strong bases + heat push E2; ionizing media favor SN1/E1. NaSH/NaSR minimize elimination thanks to low basicity.


Mechanism (Shared SN2 Template)

Each preset reuses the same two-panel SN2 sequence; the artwork simply swaps in the relevant reagent.

  1. 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
  2. Product (Walden inversion) – Only the substituted product is shown; the halide leaves off-screen so the panel stays focused on inversion.
    Illustration: /assets/reaction-library/alkyl-halide-sn2-naome/alkyl-halide-sn2-naome-step-02.svg

NaOEt (Δ) reminder: Heating a strong alkoxide tips the balance toward elimination; emphasize temperature control when you prefer substitution.

Allylic option: Allylic halides can access SN2′ pathways. Call out the alternate step when drawing allylic substrates so students expect double-bond migration.

Reagent-specific steps

The figures below pair each reagent with the shared template so learners can connect the preset button to the exact steps shown in the Mechanism Solver.

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: methoxypropane product with inversion
Step 2 — Walden inversion delivers 1-methoxypropane and Br⁻.

NaOEt (secondary SN2 with E2 risk)

NaOEt SN2 Step 1: backside attack on a secondary bromide
Step 1 — Ethoxide attacks anti to bromide; keep the mixture cool to prevent elimination from taking over.
NaOEt SN2 Step 2: inverted secondary ether product
Step 2 — Inversion yields the secondary ether; confirm the name with the IUPAC Namer.

NaOH (aqueous substitution)

NaOH SN2 Step 1: hydroxide attack on a primary chloride
Step 1 — Hydroxide attacks in a polar aprotic medium (use DMSO/DMF in lab to suppress elimination).
NaOH SN2 Step 2: inverted primary alcohol
Step 2 — The primary alcohol forms with inversion at Cα.

NaSH (thiol formation)

NaSH SN2 Step 1: HS attack
Step 1 — Hydrosulfide approaches backside; low basicity keeps E2 minimal.
NaSH SN2 Step 2: propyl thiol product
Step 2 — 1-propanethiol appears with Walden inversion.

NaSR (generic thiolate)

NaSR SN2 Step 1: thiolate attack
Step 1 — Thiolate attacks; optional SN2′ overlay appears for allylic inputs.
NaSR SN2 Step 2: thioether product
Step 2 — Thioether (R′S–Pr) forms; use the IUPAC Namer to practice naming thioethers.

NaCN (ambident substitution)

NaCN SN2 Step 1: cyanide attack
Step 1 — Cyanide attacks carbon; highlight that C-bound attack dominates in polar aprotic solvents.
NaCN SN2 Step 2: nitrile product
Step 2 — Nitrile forms (great handle for hydrolysis/reduction later).

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 secondary bromide
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.
  • Emphasize Walden inversion (draw the product with inverted wedge/dash).
  • Primary halides react rapidly; secondary require careful solvent/temperature; tertiary cannot do SN2.
  • Polar aprotic solvents accelerate substitution; protic solvents slow SN2 and promote SN1/E1.
  • Allylic/benzylic halides may give small SN2′ fractions—call them out if present.
  • Strong base plus heat (as with NaOEt under reflux) raises the chance of E2; softer nucleophiles such as NaSH, NaSR, NaCN, and NaN₃ keep substitution dominant.
  • 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 Reagent panel showing sodium methoxide Product panel showing the methyl ether obtained after SN2 inversion
Primary bromide + NaOCH₃ (DMSO, 0 °C → room temperature) — methyl ether with clean inversion; cool conditions ward off elimination.
Reactant panel showing a secondary linear bromide kept cold Reagent panel showing sodium ethoxide Product panel showing the inverted secondary ether
Secondary linear bromide + NaOEt (MeCN, 0 °C) — secondary ether via substitution; polar aprotic solvent keeps the base nucleophilic while minimizing E2.
Reactant panel showing a hindered bicyclic chloride Reagent panel showing sodium hydrosulfide Product panel showing the bicyclic thioether
Bicyclic chloride + NaSH (DMF, room temperature) — thioether formation; the soft, weakly basic nucleophile excels on hindered frameworks.
Reactant panel showing a bicyclic bromide primed for azide substitution Reagent panel showing sodium azide Product panel showing the installed azide handle
Cycloalkyl bromide + NaN₃ (DMF, room temperature) — alkyl azide produced as a latent amine handle; remind students about potential SN2′ pathways on allylic systems.
Reactant panel showing a secondary bromide ready for halide exchange Reagent panel showing sodium iodide in acetone Product panel showing the improved iodide leaving group
Secondary bromide + NaI (acetone, reflux) — halide exchange that precipitates NaBr and upgrades the leaving group for future reactions.


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: strong nucleophiles, weak bases → superb SN2 selectivity.
  • Solvents: Polar aprotic accelerate SN2; protic solvents hamper Nu⁻ and encourage elimination/ionization.
  • Allylic/benzylic: Rapid SN2; note potential SN2′ (allylic) or SN1 (benzylic in protic solvent).
  • Vinyl/aryl halides: Do not undergo SN2 at sp² carbon—explicitly ruled out in template prechecks.


Practical Tips & Pitfalls

  • Use cool, polar aprotic conditions for substitution on secondary halides; warming in protic solvents leads to E2/SN1.
  • Label inversion explicitly for stereochemical questions.
  • For NaOEt/heat, warn students that elimination dominates unless conditions are softened.
  • NaSH/NaSR solutions smell; mention lab safety (closed setups, proper waste).
  • Dry glassware for alkoxide reactions; moisture quenches Nu⁻ and promotes solvolysis.
  • Avoid neopentyl unless you can activate the electrophile (e.g., convert to sulfonate; even then slow).
  • Mention optional halide swap presets (NaI/NaBr) if students need to upgrade leaving groups.


Exam-Style Summary

SN2 on alkyl halides is a one-step backside displacement that inverts configuration. Methyl and primary react rapidly; secondary depend on solvent, base strength, and temperature; tertiary do not undergo SN2. NaOEt at reflux tips toward E2, whereas NaSH/NaSR keep substitution dominant. Draw the single concerted arrow push, mark inversion, and comment on competing pathways.


Interactive Toolbox

  • Mechanism Solver — step through each preset to see the exact SN2 steps used in class.
  • Reaction Solver — explore how substrate class, nucleophile strength, solvent, and temperature affect substitution versus elimination.
  • IUPAC Namer — practice naming ethers, alcohols, thioethers, and azides produced by these substitutions.


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