Difficulty: Intermediate | Prerequisites: General chemistry (bonding, Lewis structures, electronegativity), familiarity with functional groups and basic molecular structure.
Organic reaction mechanisms describe how and why bonds break and form during chemical reactions. This topic sits at the heart of Organic Chemistry I: once you understand the logic of mechanisms, you can predict products, explain stereochemistry, and design synthetic routes rather than memorising each reaction in isolation. You should already be comfortable with Lewis structures, electronegativity, and the idea that electrons move from areas of high density to low density. Everything here builds on that foundation and feeds directly into later topics like alkene reactions, carbonyl chemistry, and retrosynthesis.
Organic reactions fall into four broad types: addition, elimination, substitution, and rearrangement. Each proceeds through either polar (heterolytic) or radical (homolytic) bond-breaking steps. The two substitution mechanisms you need cold for exams are SN1 (two-step, carbocation intermediate, racemisation) and SN2 (one-step, backside attack, inversion of configuration).
Addition reaction
Two reactants combine to form a single product. Think of it as snapping two pieces together: nothing leaves, everything ends up in one molecule.
Elimination reaction
A single reactant loses atoms or groups to split into two products (typically a small molecule like water and an unsaturated organic product). In simple terms, the molecule sheds a piece of itself to form a double bond.
Substitution reaction
Two reactants exchange parts, producing two new products. Think of it as a swap: one group leaves and another takes its place on the same carbon.
Rearrangement reaction
A single reactant reorganises its own bonds to produce an isomeric product. In simple terms, the atoms shuffle their connections without anything entering or leaving the molecule.
Nucleophile (Nu⁻)
An electron-rich species that donates a pair of electrons to form a new bond. Think of it as the electron donor, the one doing the attacking.
Electrophile (E⁺)
An electron-poor species that accepts a pair of electrons to form a new bond. In simple terms, it is the electron-hungry target that the nucleophile attacks.
Heterolytic bond cleavage (heterolysis)
Bond breaking in which both bonding electrons move to one atom, producing ions. Think of it as an uneven split: one atom takes everything.
Homolytic bond cleavage (homolysis)
Bond breaking in which each atom retains one electron, producing radicals. In simple terms, the electrons are split equally, one to each side.
Carbocation
A positively charged carbon intermediate formed when a carbon loses a bonding pair (via heterolysis). The more substituted the carbon, the more stable the carbocation (3° > 2° > 1° > methyl).
Transition state
A fleeting, highest-energy arrangement of atoms at the top of an energy barrier. It cannot be isolated; it exists only at the instant bonds are partially broken and partially formed.
Intermediate
A transient species that forms between two transition states in a multi-step mechanism. Unlike a transition state, an intermediate sits in a local energy minimum and, in principle, could be detected.
Activation energy (ΔG‡)
The energy barrier that must be overcome for a reaction to proceed. Higher activation energy means a slower reaction at a given temperature.
Racemisation
Formation of a roughly equal mixture of both enantiomers (a racemic mixture) at a stereogenic centre. In simple terms, the product is a 50/50 mix of mirror-image forms because the nucleophile can attack from either face.
Inversion of configuration (Walden inversion)
A complete flip of stereochemistry at the reacting carbon, like an umbrella turning inside out. This is the hallmark stereochemical outcome of an SN2 reaction.
SN1 (Substitution Nucleophilic Unimolecular)
A two-step substitution mechanism: the leaving group departs first to form a carbocation, then the nucleophile attacks. The rate depends on the concentration of the substrate only.
SN2 (Substitution Nucleophilic Bimolecular)
A one-step (concerted) substitution mechanism: the nucleophile attacks at the same time as the leaving group departs. The rate depends on the concentrations of both the substrate and the nucleophile.
Addition reactions
Two reactants combine into one product; no atoms are lost.
Classic example: hydrogenation of an alkene (C=C + H₂ → C–C with two new C–H bonds).
The pi bond breaks and new sigma bonds form.
Elimination reactions
One reactant splits into two products, typically forming a new pi bond.
Classic example: dehydration of an alcohol (alcohol → alkene + H₂O).
A base or heat removes a proton and a leaving group from adjacent carbons.
Substitution reactions
One group on a molecule is replaced by another.
Two major mechanisms:
SN1: two-step, leaving group departs first, carbocation intermediate.
SN2: one-step (concerted), nucleophile attacks as the leaving group departs.
Classic example: hydroxide ion reacting with methyl iodide (SN2) or water reacting with tert-butyl bromide (SN1).
Rearrangement reactions
The molecular skeleton reorganises to give an isomer of the starting material.
Classic example: 1,2-hydride or 1,2-alkyl shifts in carbocation intermediates, moving to a more stable carbocation.
Polar reactions involve heterolytic cleavage: one atom takes both electrons from the broken bond, generating ions.
Nucleophilic attack: The nucleophile (electron-rich) donates electrons to the electrophile (electron-poor), forming a new covalent bond.
Loss of a leaving group: A leaving group departs with the bonding pair, creating a cation (often a carbocation) at the carbon.
Rearrangements in polar pathways: Carbocations can rearrange via hydride or alkyl shifts to reach a more stable intermediate before the nucleophile attacks.
Curved-arrow notation tracks the electron pair movement from nucleophile to electrophile.
Radical reactions involve homolytic cleavage: each atom gets one electron, producing neutral species with unpaired electrons (radicals).
Initiation: Energy input (UV light or heat) breaks a bond homolytically to generate radicals. Example: Cl₂ → 2 Cl·
Propagation: A radical reacts with a stable molecule, forming a new radical and continuing the chain. Each propagation step produces one radical from another.
Termination: Two radicals combine, destroying both radical centres and ending the chain.
Fishhook (single-barbed) arrows track the movement of single electrons in radical mechanisms.
One-step, concerted mechanism.
Rate = k[substrate][nucleophile] (second-order kinetics, bimolecular).
The nucleophile attacks the carbon from the side opposite the leaving group (backside attack).
Stereochemistry: complete inversion of configuration (Walden inversion).
Favoured by: strong nucleophile, methyl or primary substrate, polar aprotic solvent, good leaving group.
Hindered by: bulky (tertiary) substrates, weak nucleophiles.
Example: OH⁻ + CH₃I → CH₃OH + I⁻
Two-step mechanism: (1) leaving group departs to form a carbocation; (2) nucleophile attacks the carbocation.
Rate = k[substrate] (first-order kinetics, unimolecular rate-determining step).
Stereochemistry: racemisation (nucleophile attacks from either face of the planar carbocation).
Favoured by: stable carbocation (tertiary > secondary), weak nucleophile, polar protic solvent, good leaving group.
Carbocation rearrangements (1,2-shifts) can occur between steps 1 and 2.
Example: (CH₃)₃CBr + H₂O → (CH₃)₃COH + HBr
Energy diagrams (reaction coordinate diagrams) plot free energy against the progress of the reaction.
Activation energy (ΔG‡): the height of the energy barrier from reactants to the transition state. A higher barrier means a slower reaction.
Transition state: the peak (saddle point) on the energy diagram; bonds are partially broken and partially formed.
Intermediate: a valley between two transition states in a multi-step mechanism. More stable than transition states but less stable than reactants or products.
A one-step reaction (e.g. SN2) has one transition state and no intermediate.
A two-step reaction (e.g. SN1) has two transition states with one intermediate (the carbocation) between them.
The rate-determining step is the step with the highest-energy transition state.
Students often think SN1 and SN2 are just "fast" and "slow" versions of the same reaction. They are not. SN1 is a two-step process with a carbocation intermediate; SN2 is a concerted one-step process. The mechanisms, kinetics, stereochemical outcomes, and substrate preferences are all different.
Students often confuse a transition state with an intermediate. A transition state is a fleeting energy maximum that cannot be isolated. An intermediate sits in an energy minimum between two transition states and, in principle, has a finite (if short) lifetime.
Students often assume that SN1 reactions give 100% racemisation. In practice, the leaving group can partially shield one face of the carbocation, leading to slightly more inversion than retention. Exams usually treat it as racemisation, but be alert if a question says "predominant" product.
Students often think that a strong nucleophile always means substitution will win over elimination. Bulky strong bases (like tert-butoxide) favour elimination (E2) even though they are nucleophilic. The steric environment of both the nucleophile/base and the substrate matters.
⚠️ SN1 vs SN2 comparison questions appear on nearly every Organic Chemistry I exam. Know the substrate preference (SN2: methyl/primary; SN1: tertiary), the stereochemical outcome (inversion vs racemisation), the kinetics (second-order vs first-order), and the solvent preference (polar aprotic vs polar protic).
⚠️ Drawing correct curved arrows is often worth marks on its own. For polar mechanisms, use double-barbed arrows showing electron pair movement from nucleophile to electrophile. For radical mechanisms, use single-barbed (fishhook) arrows.
⚠️ Energy diagram interpretation is a common exam question. You may be asked to identify the rate-determining step (highest transition state), the number of intermediates, or whether the reaction is exergonic or endergonic from the diagram alone.
⚠️ Carbocation rearrangements in SN1 pathways are a classic exam trap. If a secondary carbocation can shift to a tertiary one via a 1,2-hydride or 1,2-methyl shift, you are expected to show the rearrangement and predict the rearranged product.
⚠️ Classifying a reaction as addition, elimination, substitution, or rearrangement from a balanced equation is frequently tested as a warm-up question. Count the number of reactants and products and check whether atoms were gained, lost, swapped, or shuffled.
True or False: An SN2 reaction produces a racemic mixture of products. (Answer: False. SN2 gives complete inversion of configuration.)
Fill in the blank: In homolytic bond cleavage, each atom receives ______ electron(s) from the broken bond. (Answer: one.)
True or False: A transition state can be isolated and characterised spectroscopically. (Answer: False. Only intermediates occupy energy minima; transition states are fleeting maxima.)
Fill in the blank: The rate law for an SN1 reaction depends only on the concentration of the ______. (Answer: substrate.)
True or False: Tertiary substrates are the best candidates for SN2 reactions. (Answer: False. Tertiary substrates are too sterically hindered for SN2; they favour SN1 or elimination.)
Q: Classify the following reaction and name its type: CH₂=CH₂ + HBr → CH₃CH₂Br.
A: This is an addition reaction. Two reactants (the alkene and HBr) combine to form a single product. The pi bond breaks and new C–H and C–Br sigma bonds form.
Q: In an SN2 reaction of (R)-2-bromobutane with hydroxide ion, predict the stereochemical outcome of the product.
A: The product is (S)-2-butanol. SN2 proceeds with complete inversion of configuration (Walden inversion), so the R starting material gives the S product.
Q: Why does tert-butyl bromide react with water primarily via an SN1 mechanism rather than SN2?
A: The tertiary carbon is too sterically hindered for backside attack by the nucleophile (SN2 is blocked). The tertiary carbocation that forms after the leaving group departs is relatively stable, making the SN1 pathway energetically accessible.
Q: Draw the energy diagram for a two-step reaction. How many transition states and intermediates does it contain?
A: A two-step reaction has two transition states (two energy maxima) and one intermediate (one energy minimum between the two peaks). The rate-determining step is the one with the higher transition-state energy.
Q: During an SN1 reaction with a secondary substrate, a student obtains an unexpected product in which a methyl group has migrated. Explain what happened.
A: A 1,2-methyl shift (carbocation rearrangement) occurred. The initial secondary carbocation rearranged to a more stable tertiary carbocation before the nucleophile attacked, giving the rearranged product.
Q: What distinguishes the arrow notation used in polar mechanisms from that used in radical mechanisms?
A: Polar mechanisms use double-barbed (full) curved arrows to show the movement of an electron pair. Radical mechanisms use single-barbed (fishhook) arrows to show the movement of a single electron.
This material connects directly to alkene and alkyne addition reactions, where you will apply the same polar and radical mechanism logic to predict regiochemistry (Markovnikov vs anti-Markovnikov) and stereochemistry. It also feeds into elimination reactions (E1 and E2), which compete with SN1 and SN2 under similar conditions, so understanding one pair helps you predict when the other wins. Later in the course, carbonyl chemistry (aldehydes, ketones, carboxylic acid derivatives) reuses nucleophilic attack as the central mechanistic step, making this the foundation for the entire second half of organic chemistry.
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