Reaction Mechanisms — Curly Arrows and Electron Movement

Spec Mapping — OCR H432 Module 4.1.1 — Basic concepts of organic chemistry, covering homolytic and heterolytic bond fission, the use of full-headed (double-barb) curly arrows for movement of electron pairs and half-headed (single-barb / fishhook) arrows for movement of single electrons, and the definitions of nucleophile, electrophile and radical (refer to the official OCR H432 specification document for exact wording).

A reaction mechanism is a step-by-step account of how a reaction proceeds at the molecular level: which bonds break, which bonds form, and how electrons move. The curly arrow is the most important symbol in organic chemistry — it shows exactly where electrons are moving from and to. Curly-arrow convention was largely developed by Sir Robert Robinson in the 1920s as part of his foundational work on the electronic theory of organic mechanism — work that earned him the 1947 Nobel Prize in Chemistry. Every mechanism question on OCR Paper 2 expects a technically correct curly-arrow diagram, and Paper 3 frequently demands curly arrows in synoptic contexts. This lesson is the operating manual: it defines homolytic vs heterolytic bond fission, sets out the rules of arrow-pushing, defines nucleophile / electrophile / radical, and walks through three worked examples that prefigure every reaction in Modules 4, 5 and 6 — nucleophilic substitution, electrophilic addition to alkenes, electrophilic aromatic substitution, esterification, and free-radical substitution.

Key Mechanism: a curly arrow represents the movement of a pair of electrons (full-headed, double-barbed) or a single electron (half-headed, single-barbed/fishhook). The arrow starts at the source of the electrons (a lone pair, a sigma bond, or a pi bond) and ends at the destination (an atom, the centre of a new bond, or another bond). Arrows never start from an empty orbital, a positive charge, or a hydrogen atom alone. The arrow encodes both the direction of electron flow and, by convention, the rate-determining or product-determining step.

1. Why Mechanisms Matter

Without a mechanism, a reaction is just reactants → products. With a mechanism, we can:

Explain why certain products form and others do not (e.g., Markovnikov's rule for HBr addition to propene; the SN2 vs SN1 split for haloalkane hydrolysis).
Predict the product of a reaction we haven't seen before — once the mechanistic class is identified, the regiochemistry and stereochemistry usually follow.
Rationalise the influence of solvent (polar vs non-polar), temperature, and concentration on the rate and product distribution.
Understand reactive intermediates (carbocations, carbanions, radicals) and transition states.
Plan multi-step synthesis routes — you cannot design a synthesis without knowing which functional-group transformations are mechanistically compatible.

OCR examines mechanism diagrams at every level: a 2-mark "draw the curly arrows" question on Paper 2, a 6-mark "explain Markovnikov selectivity using the mechanism" on Paper 3, and a 12-mark synthesis-route question that demands mechanism awareness throughout.

2. Bond Breaking — Homolytic vs Heterolytic Fission

A covalent bond is a pair of shared electrons. When a bond breaks, those two electrons must go somewhere. There are two possibilities — and the choice is determined by the polarity of the bond and the conditions.

2.1 Homolytic Fission

In homolytic fission, each atom takes one electron from the broken bond. This forms two radicals — species with an unpaired electron.

Key Definition — Homolytic fission: Breaking of a covalent bond in which one electron goes to each of the two atoms originally bonded, forming two radicals.

Shown in curly arrow notation using half-headed (single-barb / fishhook) arrows. Two fishhook arrows are drawn, each starting at the centre of the bond and pointing toward one of the atoms:

Each fishhook arrow moves a single electron. Homolytic fission typically happens when:

The bond is non-polar (e.g., Cl–Cl, Br–Br, peroxide O–O).
There is enough energy to break the bond (heat at >500 K, or UV light around 320 nm for Cl–Cl).
The resulting radicals are reasonably stable (delocalised or sterically protected radicals are favoured).

A classic example: Cl₂ absorbs UV light and undergoes homolytic fission to give two chlorine radicals, Cl•. This is the initiation step of free-radical substitution of methane by chlorine.

2.2 Heterolytic Fission

In heterolytic fission, both electrons from the broken bond go to one of the atoms, forming a pair of ions.

Key Definition — Heterolytic fission: Breaking of a covalent bond in which both electrons go to one of the two atoms originally bonded, forming a cation and an anion.

Shown in curly arrow notation using a single full-headed (double-barb) arrow starting at the bond and ending at the atom that takes both electrons:

$\text{A}-\text{B} \longrightarrow \text{A}^+ + :\text{B}^-$

The curly arrow starts at the bond and ends at the atom that takes both electrons. Heterolytic fission is favoured when the bond is polar, because the more electronegative atom is already pulling the electron pair toward itself in the ground state. For example, C–Br in bromomethane breaks heterolytically to give CH₃⁺ and Br⁻ (the mechanism of nucleophilic substitution, SN1 form; SN2 has the C–Br bond breaking concertedly with the new C–nucleophile bond forming, but the electrons still end up on Br).

3. Curly Arrow Rules — The Golden Rules

A curly arrow represents the movement of a pair of electrons (full-headed arrow) or a single electron (half-headed arrow) in a mechanism.

Follow these rules strictly — OCR mark schemes penalise every violation:

The arrow starts at the source of the electrons (a lone pair on an atom, a sigma bond, or a pi bond).
The arrow ends at the destination of the electrons (an atom, or the centre of a new bond, or another existing bond).
For a pair of electrons, use a full-headed arrow (double-barb).
For a single electron, use a half-headed arrow (single-barb / fishhook).
Arrows never start from a positive charge, an empty orbital, or a hydrogen atom alone. (A proton-transfer step has the arrow starting at the X–H bond pair, not at H.)
Every arrow must account for both where the electrons came from and where they go — no arrows ending in mid-air.
Charges must balance on both sides of each arrow-pushing step. If a neutral nucleophile attacks a neutral electrophile and creates a new bond, the nucleophile becomes positive — unless something else simultaneously leaves with a negative charge.
The number of arrows equals the number of bonds breaking or forming in that step. Concerted SN2 has 2 arrows; addition-elimination on a carbonyl typically has 2-3 arrows per step; electrophilic addition usually has 2 arrows.

4. Three Key Species: Nucleophiles, Electrophiles, and Radicals

4.1 Nucleophiles

Key Definition — Nucleophile: An electron pair donor (a species attracted to a region of positive or partial positive charge).

Examples: OH⁻, CN⁻, NH₃, H₂O, Cl⁻, Br⁻. All have a lone pair of electrons to donate.

In a mechanism, a nucleophile is always drawn with its lone pair showing, and a curly arrow is drawn from the lone pair to the electrophilic carbon.

4.2 Electrophiles

Key Definition — Electrophile: An electron pair acceptor (a species attracted to a region of high electron density).

Electrophiles are typically atoms or groups bearing a full or partial positive charge. Examples: H⁺, NO₂⁺, the carbon of C=O, the carbon of a C–Br bond (δ+), the Br⁺ formed when Br₂ is polarised by a C=C bond.

In a mechanism, an electrophile is the target of a curly arrow — a lone pair or a π-bond attacks it.

4.3 Radicals

Key Definition — Radical: A species with an unpaired electron.

Radicals are denoted by a dot, e.g., Cl•, CH₃•. They are typically produced by homolytic fission and are involved in free radical substitution (Lesson 6).

graph TD
  A[Reactive species] --> B["Nucleophile<br/>donates e- pair"]
  A --> C["Electrophile<br/>accepts e- pair"]
  A --> D["Radical<br/>unpaired e-"]
  B --> E[e.g., OH⁻, NH3, H2O, CN⁻]
  C --> F[e.g., H⁺, NO2⁺, δ+ C]
  D --> G[e.g., Cl•, CH3•]

5. Worked Example 1 — Nucleophilic Substitution of Bromoethane by Hydroxide (SN2)

This reaction makes ethanol from bromoethane using aqueous KOH; the most-examined mechanism at A-Level.

Overall: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

Mechanism (SN2, single step):

The hydroxide lone pair attacks the δ+ carbon of the C–Br bond. Simultaneously, the C–Br bond pair shifts entirely onto Br, which leaves as Br⁻. The two events are concerted — they happen in the same step at the same transition state.

Two curly arrows:

From the lone pair on HO⁻ to the carbon (forms new C–O bond).
From the C–Br bond to the bromine (breaks old C–Br bond).

Both arrows are full-headed (electron pairs moving). The two events happen simultaneously at one transition state (SN2 = Substitution, Nucleophilic, bimolecular). The reaction proceeds with inversion of configuration at the carbon — the nucleophile attacks from the opposite face from the leaving group, like an umbrella being inverted by a strong wind.

6. Worked Example 2 — Electrophilic Addition of HBr to Propene

Propene (CH₃CH=CH₂) reacts with hydrogen bromide to give 2-bromopropane as the major product (Markovnikov regioselectivity). The mechanism has two steps and goes through a carbocation intermediate.

Step 1 — protonation of the alkene: The pi-electrons of the C=C bond attack H⁺ from HBr. Simultaneously, the H–Br bond breaks heterolytically with the electrons going to Br.

Arrow 1: From the C=C pi bond to one of the two carbons (specifically, the terminal CH₂ in propene — the choice gives Markovnikov regioselectivity, see below). Arrow 2: From the H–Br bond to Br.

This produces a secondary carbocation (CH₃–⁺CH–CH₃, more stable than primary CH₃–CH₂–⁺CH₂) and Br⁻.

Step 2 — nucleophilic capture: Br⁻ attacks the carbocation, forming the new C–Br bond.

Arrow: From a lone pair on Br⁻ to the cationic carbon.

The Markovnikov regioselectivity is explained mechanistically: protonation could in principle give either a primary or secondary carbocation; the secondary carbocation is more stable (3° > 2° > 1° in carbocation stability, due to hyperconjugation and inductive electron donation from neighbouring C–H bonds); the more stable intermediate forms preferentially; Br⁻ then attacks where the cation lived. The H ends up on the C that already had more H, and the Br ends up on the C with fewer H — Markovnikov's empirical rule of 1869 (Vladimir Markovnikov).

7. Worked Example 3 — Bond Polarisation Drives Electrophile Attack

In bromomethane, the C–Br bond is polar because Br (electronegativity 2.96) is more electronegative than C (2.55), giving a $\delta^+$ carbon and $\delta^-$ bromine. The electrophilic carbon is the site of nucleophilic attack. The same logic explains why:

Lesson 4: Reaction Mechanisms — Curly Arrows and Electron Movement