Reaction Mechanisms: Nucleophilic Substitution and Elimination

Reaction Mechanisms: Nucleophilic Substitution and Elimination

Understanding reaction mechanisms is essential at A-Level. You must be able to draw curly arrows showing the movement of electron pairs, identify intermediates, and explain the factors that determine which pathway a reaction follows.

Curly Arrow Conventions

A curly arrow represents the movement of a pair of electrons:

• The tail shows where the electrons come from
• The head shows where the electrons go to
• A full curly arrow = movement of two electrons (a pair)
• A half curly arrow (fish-hook) = movement of one electron (used in radical mechanisms)

Nucleophilic Substitution

A nucleophile is an electron-pair donor — a species with a lone pair that it can donate to an electron-deficient (δ⁺) carbon atom. Common nucleophiles include OH⁻, CN⁻, NH₃, and H₂O.

In nucleophilic substitution, a nucleophile replaces a leaving group bonded to a carbon. The leaving group departs with the bonding pair of electrons.

There are two mechanisms: SN1 and SN2.

SN2 Mechanism (Bimolecular Nucleophilic Substitution)

One step — the nucleophile attacks at the same time as the leaving group departs.

Example: 1-bromobutane + NaOH → butan-1-ol + NaBr

CH₃CH₂CH₂CH₂Br + OH⁻ → CH₃CH₂CH₂CH₂OH + Br⁻

Mechanism (curly arrows):

1. The lone pair on OH⁻ forms a curly arrow towards the δ⁺ carbon bonded to Br

2. Simultaneously, the C–Br bond breaks heterolytically — a curly arrow from the C–Br bond to Br

3. A single transition state forms (not an intermediate) in which OH is partially bonded to C and Br is partially leaving — shown in square brackets with a double-dagger symbol ‡

Key features of SN2:

• Rate = k[halogenoalkane][nucleophile] — second order overall (bimolecular)
• Backside attack — the nucleophile attacks the carbon from the opposite side to the leaving group
• Results in inversion of configuration at the chiral centre (Walden inversion)
• Favoured by primary halogenoalkanes (less steric hindrance around the carbon)
• Also favoured by strong nucleophiles, polar aprotic solvents, and good leaving groups

SN1 Mechanism (Unimolecular Nucleophilic Substitution)

Two steps — the leaving group departs first, forming a carbocation intermediate, then the nucleophile attacks.

Example: 2-bromo-2-methylpropane + NaOH → 2-methylpropan-2-ol + NaBr

(CH₃)₃CBr + OH⁻ → (CH₃)₃COH + Br⁻

Mechanism (curly arrows):

Step 1 (slow, rate-determining):

• A curly arrow from the C–Br bond to Br — the bond breaks heterolytically
• A planar tertiary carbocation (CH₃)₃C⁺ forms, plus Br⁻

Step 2 (fast):

• A curly arrow from the lone pair on OH⁻ to the positive carbon of the carbocation
• The C–O bond forms, giving the alcohol product

Key features of SN1:

• Rate = k[halogenoalkane] — first order (unimolecular — only the halogenoalkane is in the rate-determining step)
• A planar carbocation intermediate forms, so the nucleophile can attack from either side
• This produces a racemic mixture if the carbon was a chiral centre
• Favoured by tertiary halogenoalkanes (the carbocation is stabilised by the inductive effect of three alkyl groups)
• Also favoured by polar protic solvents (which stabilise ions) and weak nucleophiles

Carbocation Stability

Tertiary > Secondary > Primary > Methyl

Alkyl groups are electron-releasing (positive inductive effect), which stabilises the positive charge by spreading it over a larger area. This is why tertiary carbocations form more readily.

Summary: SN1 vs SN2

Factor	SN1	SN2
Substrate	Tertiary (best)	Primary (best), methyl
Rate law	First order	Second order
Mechanism	Two steps via carbocation	One step (concerted)
Stereochemistry	Racemisation	Inversion
Nucleophile	Weak nucleophile favours	Strong nucleophile favours

Secondary halogenoalkanes may react by either mechanism, depending on conditions.

Elimination Reactions

In an elimination reaction, a small molecule (usually HBr, HCl, or H₂O) is removed from the substrate, forming a C=C double bond (an alkene).

Example: Ethanol heated with concentrated H₂SO₄ at 170 °C:

CH₃CH₂OH → CH₂=CH₂ + H₂O

Halogenoalkane elimination — heated with NaOH in ethanol:

CH₃CHBrCH₃ + NaOH (in ethanol, heat) → CH₃CH=CH₂ + NaBr + H₂O

Mechanism (E2 — bimolecular elimination):

1. OH⁻ acts as a base (not a nucleophile) — it abstracts a hydrogen from a carbon adjacent to the one bearing the leaving group

2. Curly arrow: lone pair on OH⁻ → H atom on the β-carbon

3. Curly arrow: C–H bond breaks → electrons form the C=C π bond

4. Curly arrow: C–Br bond breaks → Br⁻ departs

5. All three curly arrows are drawn in one step (concerted E2)

Substitution vs Elimination — Competition

Nucleophilic substitution and elimination compete when a halogenoalkane is treated with OH⁻:

Condition	Favours
Aqueous NaOH, warm	Substitution (OH⁻ acts as nucleophile)
Ethanolic NaOH, heat	Elimination (OH⁻ acts as base)
Primary substrate	Substitution (SN2)
Tertiary substrate	Elimination (steric hindrance blocks SN2, and stable carbocation favours E1)
Strong base, high temperature	Elimination
Weak base, low temperature	Substitution

Exam Tips

• Always draw curly arrows from the electron-rich species to the electron-poor species
• Curly arrows must start from a lone pair or a bond, and end at an atom or a bond
• Show all relevant partial charges (δ⁺ and δ⁻) on the substrate
• For SN1, clearly label Step 1 as the slow / rate-determining step
• For elimination, the OH⁻ attacks a hydrogen (it is acting as a base), not the carbon
• If a question asks you to predict whether substitution or elimination will dominate, consider substrate class (1°/2°/3°), solvent, and temperature