Reaction Mechanisms Summary

Reaction mechanisms describe the step-by-step bond-breaking and bond-forming processes by which reactants are converted into products. At A-Level, you need to be able to draw, describe, and recognise several key mechanism types. This lesson consolidates all the organic mechanisms required for the Edexcel specification.

Curly Arrow Conventions

Before reviewing the mechanisms, it is essential to understand curly arrow notation:

A full curly arrow represents the movement of a pair of electrons
A half curly arrow represents the movement of a single electron (used only in free radical mechanisms)
Curly arrows always start from the electron source (a lone pair, a bond, or a negative charge) and point to the electron sink (an atom or bond accepting the electrons)
Each curly arrow must start and finish precisely -- examiners mark this strictly

Common Curly Arrow Errors

Error	Why It Loses Marks
Arrow starting from a positive charge	Positive sites are electron-poor, not electron-rich
Arrow starting from the atom rather than the lone pair or bond	Arrows must start from the electron source
Arrow pointing to a bond rather than an atom	Electrons go to atoms, not bonds
Missing half-arrows in radical mechanisms	Radical steps involve single electron movement
Arrow going the wrong way (sink to source)	Always source to sink

Mechanism Classification Flowchart

flowchart TD
    A[Identify the starting material] --> B{What type of compound?}
    B -->|Alkane + Halogen + UV| C[Free Radical Substitution]
    B -->|Alkene + HX or X2| D[Electrophilic Addition]
    B -->|Halogenoalkane| E{What reagent and solvent?}
    E -->|"Nucleophile (OH-/CN-/NH3)<br>in aqueous solution, heat"| F[Nucleophilic Substitution]
    E -->|"NaOH in ethanol, heat"| G[Elimination]
    B -->|Aldehyde/Ketone + nucleophile| H[Nucleophilic Addition]
    B -->|Acyl chloride/Anhydride + nucleophile| I[Nucleophilic Addition-Elimination]
    B -->|Carboxylic acid + Alcohol + H2SO4| J[Esterification / Condensation]

1. Free Radical Substitution

Where it occurs: Alkanes reacting with halogens (e.g., CH4 + Cl2) in the presence of UV light

Three stages:

Initiation

UV light provides the energy to break the Cl-Cl bond homolytically (evenly), producing two chlorine free radicals:

Cl2 --> 2Cl*

Homolytic fission means each atom gets one electron from the bond. Half curly arrows are used to show each electron going to one atom.

Propagation

Two propagation steps form a chain reaction:

Cl* + CH4 --> CH3 + HCl (chlorine radical abstracts H from methane) CH3 + Cl2 --> CH3Cl + Cl (methyl radical reacts with Cl2, regenerating Cl)

The regenerated Cl* radical feeds back into the first propagation step, continuing the chain. This is why a single photon of UV light can initiate a chain that produces thousands of product molecules.

Termination

Two radicals combine, ending the chain:

Cl* + Cl* --> Cl2 CH3 + Cl --> CH3Cl *CH3 + *CH3 --> C2H6

Why a mixture of products forms: The product CH3Cl still has C-H bonds, so it can undergo further substitution in the propagation steps:

CH3Cl --> CH2Cl2 --> CHCl3 --> CCl4

This is a major limitation of free radical substitution -- selectivity is poor. Using excess alkane minimises poly-substitution.

How to recognise: Alkane + halogen + UV light --> free radical substitution

2. Electrophilic Addition

Where it occurs: Alkenes reacting with electrophiles such as HBr, Br2, or H2SO4

Mechanism (HBr + ethene as example):

The electron-rich C=C double bond (high electron density in the pi bond) attracts the electrophile HBr
The pi electrons of C=C attack the delta+ H of HBr, forming a C-H bond and breaking the H-Br bond (heterolytic fission). A carbocation intermediate forms on the other carbon
Br- (now acting as a nucleophile) attacks the positively charged carbocation, forming the C-Br bond

Product: Bromoethane (CH3CH2Br)

Markovnikov's Rule and Carbocation Stability

With unsymmetrical alkenes (e.g., propene), the H adds to the carbon with more hydrogens, and the halide adds to the carbon with fewer hydrogens. This produces the more stable carbocation intermediate.

Carbocation stability order: Tertiary > Secondary > Primary

A tertiary carbocation is stabilised by three alkyl groups pushing electron density toward the positive centre (positive inductive effect), partially neutralising the charge. A primary carbocation has only one such group.

Alkene	Major Product	Carbocation Formed	Stability
Propene + HBr	2-bromopropane	Secondary (on C2)	More stable
But-1-ene + HBr	2-bromobutane	Secondary (on C2)	More stable
Symmetrical (ethene)	Bromoethane	Primary (only option)	N/A

How to recognise: Alkene + HX, H2SO4, or X2 --> electrophilic addition

3. Nucleophilic Substitution

Where it occurs: Halogenoalkanes reacting with nucleophiles (OH-, CN-, NH3)

There are two sub-types, and which one operates depends on the structure of the halogenoalkane:

SN2 (bimolecular nucleophilic substitution)

Occurs with primary halogenoalkanes
One step: The nucleophile attacks the delta+ carbon from the opposite side to the halide (backside attack), forming a new bond while the C-halide bond breaks simultaneously
Rate depends on the concentration of both the halogenoalkane and the nucleophile: Rate = k[halogenoalkane][nucleophile]
Leads to inversion of configuration (the groups around the carbon flip like an umbrella turning inside out)

SN1 (unimolecular nucleophilic substitution)

Occurs with tertiary halogenoalkanes
Two steps:
- Step 1: The C-halide bond breaks heterolytically, forming a carbocation and a halide ion (slow, rate-determining step)
- Step 2: The nucleophile attacks the carbocation rapidly
Rate depends only on the concentration of the halogenoalkane: Rate = k[halogenoalkane]
Leads to a racemic mixture (the nucleophile can attack from either side of the flat, planar carbocation)

Why Do Primary Go SN2 and Tertiary Go SN1?

Factor	Primary	Tertiary
Steric hindrance at delta+ C	Low -- nucleophile can approach	High -- three bulky groups block backside attack
Carbocation stability	Primary carbocation very unstable -- cannot form	Tertiary carbocation stable -- can form readily
Dominant pathway	SN2 (direct displacement)	SN1 (via carbocation)

Secondary halogenoalkanes can undergo a mixture of SN1 and SN2.

How to recognise: Halogenoalkane + nucleophile (OH-/CN-/NH3) + heat --> nucleophilic substitution

4. Nucleophilic Addition

Where it occurs: Aldehydes and ketones reacting with nucleophiles (HCN/CN-, NaBH4)

Mechanism (HCN + carbonyl):

CN- (the nucleophile) uses its lone pair on carbon to attack the delta+ carbonyl carbon
The C=O pi bond breaks, and both electrons move onto the oxygen, forming an alkoxide (negative charge on O)
The alkoxide is protonated by HCN to form the hydroxynitrile product, and CN- is regenerated (so KCN is catalytic)

Key feature: The nucleophile adds to the molecule and nothing leaves -- this distinguishes it from nucleophilic substitution (where a leaving group departs) and from addition-elimination (where a leaving group departs after addition).

How to recognise: Aldehyde/ketone + nucleophile (CN-, H- from NaBH4) --> nucleophilic addition

5. Nucleophilic Addition-Elimination

Where it occurs: Acyl chlorides (and acid anhydrides) reacting with nucleophiles (H2O, ROH, NH3, RNH2)

Mechanism:

Addition: The nucleophile attacks the delta+ carbonyl carbon. The C=O pi bond breaks (electrons onto O), forming a tetrahedral intermediate
Elimination: The leaving group (Cl- from acyl chloride, or RCOO- from anhydride) departs, and the C=O bond reforms

The tetrahedral intermediate must be drawn -- skipping it loses marks.

Why does this happen with acyl chlorides but not aldehydes/ketones? Because Cl- is a good leaving group. In aldehydes and ketones, the groups bonded to the carbonyl carbon (H or R) are not good leaving groups -- they cannot leave as stable anions. So the mechanism stops at addition.

How to recognise: Acyl chloride/anhydride + nucleophile --> nucleophilic addition-elimination

6. Elimination

Where it occurs: Halogenoalkanes heated with NaOH in ethanol (rather than aqueous NaOH)

Mechanism:

OH- acts as a base (not a nucleophile in this case) and removes a hydrogen atom from a carbon adjacent to the C-halide bond
The electrons from the C-H bond form a C=C double bond
The halide leaves as a halide ion

Product: An alkene

Key distinction -- substitution vs elimination:

Condition	OH- Acts As	Product	Mechanism
NaOH in water, heat	Nucleophile	Alcohol	Substitution
NaOH in ethanol, heat	Base	Alkene	Elimination

The solvent determines which pathway dominates. This is one of the most commonly examined distinctions in organic chemistry.

How to recognise: Halogenoalkane + NaOH in ethanol + heat --> elimination

7. Esterification (Condensation)

Where it occurs: Carboxylic acid + alcohol with concentrated H2SO4 catalyst, heated under reflux

The overall process involves the -OH from the carboxylic acid reacting with the H from the alcohol, losing water and forming an ester bond (-COO-). This is a reversible equilibrium reaction.

How to recognise: Carboxylic acid + alcohol + acid catalyst + reflux --> esterification

Complete Mechanism Reference Table

Mechanism	Starting Material	Reagent	Conditions	Product	Key Feature
Free radical substitution	Alkane	Halogen (Cl2, Br2)	UV light	Halogenoalkane	Radicals, chain reaction
Electrophilic addition	Alkene	HBr, Br2, H2SO4	Room temperature	Halogenoalkane, diol, etc.	Pi bond attacks electrophile
Nucleophilic substitution (SN2)	Primary halogenoalkane	OH-, CN-, NH3	Aqueous, heat	Alcohol, nitrile, amine	One step, inversion
Nucleophilic substitution (SN1)	Tertiary halogenoalkane	OH-, CN-, NH3	Aqueous, heat	Alcohol, nitrile, amine	Two steps, racemic mixture
Nucleophilic addition	Aldehyde/ketone	HCN/KCN, NaBH4	Various	Hydroxynitrile, alcohol	No leaving group
Addition-elimination	Acyl chloride/anhydride	H2O, ROH, NH3, RNH2	Room temperature	Acid, ester, amide	Cl- or RCOO- leaves
Elimination	Halogenoalkane	NaOH	In ethanol, heat	Alkene	OH- acts as base

Common Exam Mistakes

Using full curly arrows in radical mechanisms. Radical steps involve single electrons, so half curly arrows must be used. Full arrows (pair of electrons) are used in ionic mechanisms.
Confusing nucleophilic addition with addition-elimination. The difference is whether a leaving group departs. Aldehydes/ketones: nothing leaves (addition). Acyl chlorides: Cl- leaves (addition-elimination).
Forgetting which solvent gives substitution vs elimination. Aqueous NaOH = nucleophilic substitution. Ethanolic NaOH = elimination. The same reagent gives different products in different solvents.
Drawing curly arrows the wrong way. Always from the electron-rich species (nucleophile, lone pair, bond) to the electron-poor species (electrophile, delta+ atom). Never the other way round.
Not showing the carbocation intermediate in SN1. The two-step process requires showing the carbocation forming in step 1 (rate-determining step) before the nucleophile attacks in step 2.
Not showing the tetrahedral intermediate in addition-elimination. The intermediate must be drawn with four groups around the central carbon, including both the nucleophile and the leaving group, before the leaving group departs.
Applying Markovnikov's rule to symmetrical alkenes. If both carbons of the double bond are equivalent (as in ethene), there is no Markovnikov's rule issue -- only one product forms.

Summary

The ability to draw and identify mechanisms is one of the most heavily tested skills at A-Level. Each mechanism has characteristic reagents, conditions, and arrow patterns. In the exam, read the question carefully to identify the type of starting material and the reagent, and this will guide you to the correct mechanism. Practice drawing each mechanism until the curly arrows become automatic.

A-Level Deep Dive: Reaction Mechanisms Summary

Spec mapping

Edexcel 9CH0 specification, Topics 6, 9, 16, 17 and 18 (synoptic) covers the major mechanism types of A-Level organic chemistry: nucleophilic substitution (SN1 and SN2 in haloalkanes); elimination (E1 and E2 in haloalkanes/alcohols); free radical substitution (alkanes + halogens, UV light); electrophilic addition (alkenes + Br2, HBr, H2O); electrophilic substitution (aromatic — nitration, halogenation, Friedel–Crafts acylation/alkylation); nucleophilic addition (carbonyls + HCN, NaBH4); nucleophilic addition–elimination (acyl chlorides, anhydrides, esters with nucleophiles); and condensation (peptide bonds, polyester/polyamide formation) (refer to the official specification document for exact wording). Examined directly on Paper 2 (Topic-specific) and synoptically on Paper 3 (mechanism identification + curly-arrow diagrams + linkage to kinetics/rate equations from Topic 11). Connects to CP5 (kinetics — rate equations identifying rate-determining steps).

Worked example with full mark scheme

Question (8 marks):

For each of the following reactions, name the mechanism type and predict the rate equation:

(a) (CH3)3C-Br + OH⁻ → (CH3)3C-OH + Br⁻ (in 80% aqueous ethanol, room temperature) (2)

(b) CH3CH2-Br + OH⁻ → CH3CH2-OH + Br⁻ (in dry ethanol, room temperature) (2)

(d) C6H6 + Br2 → C6H5Br + HBr (with FeBr3 catalyst) (2)

Solution with mark scheme:

(a) SN1 (unimolecular nucleophilic substitution). The tertiary carbocation (CH3)3C+ is highly stable (3 alkyl groups donating by +I and hyperconjugation). The C–Br bond ionises in the polar protic solvent (water/ethanol) to form the carbocation, which is rapidly captured by OH⁻ in the second step.

Rate = k[(CH3)3CBr] (first order in haloalkane, zero order in OH⁻).

M1 — mechanism named SN1 with reasoning (tertiary carbocation stable). A1 — rate equation correct.

(b) SN2 (bimolecular nucleophilic substitution). Primary haloalkane; no carbocation can form (would be unstable). OH⁻ attacks from the rear of the C–Br bond, forming a 5-coordinate transition state; Br⁻ leaves. Single-step concerted mechanism with inversion of stereochemistry.

Rate = k[CH3CH2Br][OH⁻] (first order in each reactant).