Nucleophilic substitution is a fundamental transformation of haloalkanes, allowing predictable changes to molecular structure through well-defined mechanisms involving electron-pair donation and bond reorganisation.

Nucleophiles: Nature and Reactivity

Nucleophiles play a central role in substitution reactions because they provide an electron pair to form a new covalent bond. They commonly bear lone pairs and may be neutral or negatively charged.

Nucleophiles target electrophilic centres, and in haloalkanes the carbon attached to the halogen is strongly polarised. The electronegative halogen withdraws electron density, creating a δ+ carbon susceptible to attack. Haloalkanes such as chloroethane, bromoethane, and iodoethane therefore undergo substitution when exposed to suitable nucleophiles.

Factors Affecting Nucleophilic Strength

Nucleophilic strength depends on:

• Charge – negatively charged species (e.g. OH⁻, CN⁻) are generally stronger nucleophiles than neutral molecules.

• Electronegativity – nucleophiles with lower electronegativity donate electron density more readily.

• Solvent – polar protic solvents can hinder nucleophiles through hydrogen bonding, while polar aprotic solvents enhance nucleophilicity for many anions.

• Steric hindrance – bulky nucleophiles attack electrophilic centres less effectively.

These factors influence rate, but under OCR requirements, emphasis remains on substitution involving aqueous alkali and primary haloalkanes.

Nucleophilic Substitution of Primary Haloalkanes

Primary haloalkanes undergo nucleophilic substitution through the S(_N)2 mechanism. The specification requires students to show the substitution mechanism using curly arrows and apply correct dipoles.

This image illustrates concerted SN2 attack with curly arrows showing electron movement, a pentacoordinate transition state, and formation of methanol and bromide. Source

Polarity of the C–X Bond

In haloalkanes, the halogen (X) is more electronegative than carbon. This produces:

• Cδ+, an electrophilic carbon centre

• Xδ−, capable of leaving as a halide ion

This polarisation is essential because the carbon atom has an accessible δ+ site for nucleophilic attack.

The S(_N)2 Mechanism: Key Features

The mechanism for primary haloalkanes with aqueous alkali proceeds in one concerted step, meaning bond breaking and bond forming occur simultaneously.

The reaction involves:

• A nucleophile such as OH⁻ from aqueous alkali

• A primary haloalkane (e.g. bromoethane)

• Formation of an alcohol and release of a halide ion

This scheme shows hydroxide attacking bromoethane in one step to produce ethanol and bromide, emphasising simultaneous bond-making and bond-breaking. Source

Important features of the S(_N)2 process include:

• Back-side attack: the nucleophile approaches the carbon opposite the C–X bond.

• Transition state formation: carbon simultaneously bonds partially to the nucleophile and halogen.

• Inversion of configuration at carbon, though OCR does not require stereochemical drawings.

Step-by-Step Description of the Required Mechanism

1. Nucleophilic Approach

The OH⁻ nucleophile approaches the δ+ carbon from the opposite side of the halogen. This direction is essential because it minimises repulsion between the nucleophile and the electron-rich halogen.

2. Curly Arrow Representation

A curly arrow originates from the lone pair on the nucleophile and points towards the carbon atom. A second curly arrow emerges from the C–X bond to the halogen, representing bond cleavage.

3. Formation of the Transition State

As the nucleophile forms a new bond with carbon, the C–X bond weakens. In the transition state:

• Both nucleophile and halogen are partially bonded to carbon.

• The carbon adopts a geometry with five partial bonds momentarily.

• Example identity of species is not required beyond primary haloalkanes and aqueous alkali.

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4. Product Formation

The halogen leaves as X⁻ (a halide ion), while the nucleophile fully bonds to carbon, forming an alcohol. The reaction therefore converts a haloalkane into an alcohol, aligning with fundamental synthetic pathways introduced in Module 4.

Why Primary Haloalkanes Use S(_N)2

OCR expects students to understand that primary haloalkanes favour the S(_N)2 mechanism due to minimal steric hindrance. Secondary haloalkanes may undergo either S(_N)1 or S(_N)2, and tertiary haloalkanes typically undergo S(_N)1, but these comparisons fall outside the direct specification for this subsubtopic.

Primary haloalkanes’ relatively open structure allows the nucleophile to approach directly along the required trajectory for back-side attack. The mechanism therefore remains both predictable and high-yielding under aqueous alkali conditions.

Relevance of Dipoles in Mechanistic Diagrams

When drawing the mechanism, OCR requires:

This diagram highlights dipole placement and curly arrow conventions in SN2 mechanisms, illustrating nucleophilic attack and departure of the leaving group. Source

• δ+ clearly placed on carbon

• δ− on the halogen

• Curly arrows accurately directed: lone pair → carbon; C–X bond → halogen

Correct placement of dipoles is not optional; it demonstrates mechanistic understanding rather than memorised arrows.

Key Mechanistic Bullet Points for OCR Practice

• Nucleophile: electron pair donor attacking Cδ+

• Primary haloalkane: undergoes S(_N)2 substitution

• Aqueous alkali: common nucleophile source (OH⁻)

• Curly arrows: from nucleophile to carbon; from C–X bond to halogen

• Products: alcohol + halide ion

• Back-side attack: essential for mechanism

• Polarisation: C–X bond dipole drives reactivity