Difficulty: Intermediate | Prerequisites: Understanding of nucleophiles, electrophiles, leaving groups, and the four organic reaction classes. Review the companion notes on Organic Reaction Types and Mechanisms first if those concepts are not yet solid.
Substitution reactions are one of the most heavily tested topics in Organic Chemistry I. When a nucleophile replaces a leaving group on a carbon, the reaction can follow one of two mechanistic pathways: SN2 (bimolecular, one concerted step) or SN1 (unimolecular, two steps via a carbocation intermediate). Knowing which pathway a given substrate and nucleophile will favour is a core skill that comes up repeatedly in exams, problem sets, and later in synthesis design. This topic builds directly on the nucleophile, electrophile, and leaving-group concepts from the introductory reaction-types material.
SN2 is a one-step mechanism where the nucleophile attacks the back side of the electrophilic carbon at the same time the leaving group departs, causing inversion of stereochemistry. SN1 is a two-step mechanism where the leaving group departs first to form a carbocation, and then the nucleophile attacks, often giving a racemic (50/50) mixture. Whether SN1 or SN2 dominates depends on the substrate structure, the nucleophile strength, the solvent, and the leaving group.
SN2 (Substitution, Nucleophilic, Bimolecular)
A one-step substitution mechanism in which the nucleophile attacks the substrate at the same time the leaving group departs. The "2" refers to the fact that the rate depends on the concentration of both the nucleophile and the substrate. Think of it as a simultaneous swap: one in, one out, in a single motion.
SN1 (Substitution, Nucleophilic, Unimolecular)
A two-step substitution mechanism in which the leaving group departs first to form a carbocation intermediate, and then the nucleophile attacks. The "1" means the rate depends on the concentration of the substrate only. Think of it as a two-phase move: the leaving group walks out, then the nucleophile walks in.
Backside attack
In SN2, the nucleophile approaches from the side opposite the leaving group. This geometry is required because the nucleophile's electrons need to enter the antibonding orbital of the C–LG bond. In simple terms, the nucleophile comes in the back door.
Walden inversion (inversion of configuration)
The stereochemical outcome of an SN2 reaction: the three groups on the electrophilic carbon flip to the other side, like an umbrella turning inside out in a gust of wind.
Carbocation
A carbon atom bearing a positive charge and only three bonds (sp2 hybridised, planar). It is the intermediate in SN1 reactions. Stability order: tertiary > secondary > primary > methyl.
Racemic mixture (racemate)
A 50:50 mixture of two enantiomers. SN1 reactions at a stereogenic centre tend toward racemic products because the planar carbocation can be attacked from either face.
Rate law
The mathematical relationship between reaction rate and the concentrations of the species involved. SN2 rate = k[nucleophile][substrate]. SN1 rate = k[substrate].
Rate-determining step
The slowest step in a multi-step mechanism, which sets the overall rate. In SN1, this is the first step (leaving group departure to form the carbocation).
Overview
SN2 stands for Substitution, Nucleophilic, Bimolecular. It is a one-step, concerted mechanism. Bond formation (nucleophile to carbon) and bond breaking (carbon to leaving group) happen simultaneously.
Mechanism in detail
The nucleophile approaches the electrophilic carbon from the side opposite the leaving group (backside attack).
In the transition state, the carbon is partially bonded to both the nucleophile and the leaving group. The three remaining groups are coplanar with the carbon in a trigonal bipyramidal arrangement.
As the nucleophile fully bonds, the leaving group departs. The three substituents invert, like an umbrella flipping inside out.
Example
OH⁻ + CH₃I → CH₃OH + I⁻
Hydroxide (nucleophile) attacks methyl iodide from the back side. Iodide (leaving group) departs. One step, no intermediate.
Key features of SN2
Rate law: rate = k[Nu⁻][substrate]. Both species are involved in the rate-determining (and only) step.
Stereochemistry: inversion of configuration at the carbon. If the starting material is R, the product is S, and vice versa.
Substrate effects: works best on unhindered substrates. Methyl > primary > secondary. Tertiary substrates essentially do not undergo SN2 because the three bulky groups block backside attack.
Nucleophile: a strong nucleophile is needed to drive the reaction.
Solvent: polar aprotic solvents (acetone, DMSO, DMF) are best. They dissolve ionic nucleophiles without solvating and weakening them.
Overview
SN1 stands for Substitution, Nucleophilic, Unimolecular. It proceeds in two steps. The leaving group departs first to form a carbocation intermediate, and then the nucleophile attacks.
Mechanism in detail
Step 1 (slow, rate-determining): the leaving group departs with its bonding electrons, generating a planar carbocation at the electrophilic carbon. This step requires energy to break the C–LG bond without any assistance from a nucleophile.
Step 2 (fast): the nucleophile attacks the carbocation. Because the carbocation is sp2-hybridised and planar, the nucleophile can approach from either face.
Example
CH₃C(Br)HCH₃ + H₂O → CH₃C(OH)HCH₃ + Br⁻
2-Bromobutane loses bromide to form a secondary carbocation. Water then attacks from either side.
Key features of SN1
Rate law: rate = k[substrate]. The nucleophile does not appear in the rate law because it is not involved in the slow step.
Stereochemistry: attack on the planar carbocation from both faces produces a mixture of retention and inversion. At a stereogenic centre, this typically gives a racemic or near-racemic mixture.
Substrate effects: works best on substrates that form stable carbocations. Tertiary > secondary. Primary and methyl substrates essentially do not undergo SN1 because their carbocations are too unstable.
Nucleophile: even a weak nucleophile (like water or an alcohol) is sufficient, because the rate-determining step does not involve the nucleophile.
Solvent: polar protic solvents (water, alcohols) stabilise both the leaving group (as an anion) and the carbocation intermediate through solvation.
Carbocation stability and rearrangements
If a less stable carbocation can rearrange (via a 1,2-hydride shift or 1,2-methyl shift) to a more stable one, it will. This can change the product of an SN1 reaction. Always check whether a rearrangement is possible when a secondary carbocation forms adjacent to a tertiary carbon.
When you see a substitution reaction on an exam, the key question is: will it follow SN1 or SN2? Four factors guide the prediction.
1. Substrate structure (most important factor)
Methyl and primary substrates: SN2 (unhindered, backside attack is easy; carbocation would be too unstable for SN1).
Tertiary substrates: SN1 (too sterically hindered for backside attack; tertiary carbocation is stable enough to form).
Secondary substrates: borderline. Can go either way depending on the nucleophile and solvent.
2. Nucleophile strength
Strong nucleophile (e.g. OH⁻, CN⁻, RS⁻, I⁻): favours SN2.
Weak nucleophile (e.g. H₂O, ROH): favours SN1.
3. Solvent
Polar aprotic (DMSO, DMF, acetone): favours SN2. These solvents do not solvate the nucleophile, leaving it free and reactive.
Polar protic (water, methanol, ethanol): favours SN1. These solvents stabilise the carbocation intermediate and solvate the leaving group.
4. Leaving group
A good leaving group is needed for both pathways. Weak bases make the best leaving groups (I⁻ > Br⁻ > Cl⁻ > F⁻). Tosylate (OTs) and mesylate (OMs) are also excellent.
The leaving group does not strongly differentiate between SN1 and SN2, but a very good leaving group accelerates both.
Decision shortcut
Methyl or 1° substrate + strong nucleophile + polar aprotic solvent → SN2.
3° substrate + weak nucleophile + polar protic solvent → SN1.
2° substrate: look at the nucleophile and solvent to tip the balance.
Rate laws
SN2: rate = k[nucleophile][substrate]
SN1: rate = k[substrate]
Energy diagram, SN2
A single energy peak (one transition state). Reactants climb to the transition state, then drop to products. No intermediate.
Energy diagram, SN1
Two energy peaks separated by a valley. The first peak (rate-determining) represents leaving-group departure. The valley is the carbocation intermediate. The second peak represents nucleophilic attack.
Stereochemistry summary
SN2: 100% inversion of configuration (Walden inversion).
SN1: racemisation (or near-racemisation). The planar carbocation is attacked from both faces.
SN2 reactions are used routinely in the pharmaceutical and agrochemical industries to attach specific functional groups to molecular scaffolds with precise stereochemical control. The Walden inversion ensures that a single enantiomer of a drug can be produced, which matters because the two mirror-image forms of a drug often have very different biological effects. SN1 chemistry appears in biological systems too: certain enzymatic reactions proceed through carbocation-like intermediates, including steps in cholesterol biosynthesis.
Students often think SN1 always gives a perfectly 50:50 racemic mixture. In practice, the leaving group can partially shield one face of the carbocation as it departs (ion pairing), so there may be a slight excess of inversion. Exams usually treat it as racemic unless told otherwise.
Students confuse "unimolecular" with "one step." SN1 is unimolecular (one species in the rate-determining step), but it has two steps overall.
Students sometimes assume a good nucleophile guarantees SN2. Steric hindrance at the substrate can override nucleophile strength: a tertiary substrate will not undergo SN2 regardless of how strong the nucleophile is.
Students forget to check for carbocation rearrangements in SN1. If a secondary carbocation can shift to a more stable tertiary one, the product will reflect the rearranged structure.
⚠️ "Given this substrate, nucleophile, and solvent, will the reaction proceed by SN1 or SN2?" is one of the most common exam questions in Organic Chemistry I. The substrate structure is the first thing to check.
⚠️ You will be asked to predict the stereochemical outcome. SN2 = inversion. SN1 = racemisation. Know why in each case.
⚠️ Drawing the full mechanism with curved arrows (SN2: one step; SN1: two steps) is a standard exam task. Practise until the arrow-pushing is automatic.
⚠️ Expect at least one question where a carbocation rearrangement changes the expected SN1 product. Check for 1,2-hydride and 1,2-methyl shifts whenever a secondary carbocation forms.
⚠️ Solvent choice questions ("why polar aprotic for SN2?") test whether you understand how solvation affects nucleophile reactivity.
True or false: SN2 reactions proceed through a carbocation intermediate. (False. SN2 is concerted, with no intermediate.)
Fill in the blank: The rate law for an SN1 reaction is rate = k[______]. (substrate)
True or false: A tertiary alkyl halide is a good substrate for SN2. (False. Steric hindrance blocks backside attack.)
Fill in the blank: In an SN2 reaction, the stereochemistry at the reaction centre undergoes ______. (inversion of configuration / Walden inversion)
True or false: Polar protic solvents favour SN2 over SN1. (False. Polar protic solvents favour SN1.)
Q: (R)-2-bromobutane is treated with NaOH in DMSO. What mechanism operates, and what is the stereochemistry of the product?
A: SN2. NaOH is a strong nucleophile, the substrate is secondary (borderline, but the strong nucleophile and polar aprotic solvent tip it to SN2), and DMSO is polar aprotic. The product is (S)-2-butanol, formed by inversion of configuration.
Q: (R)-3-bromo-3-methylhexane is dissolved in water. What mechanism operates, and what is the expected stereochemical outcome?
A: SN1. The substrate is tertiary (no SN2 possible), water is a weak nucleophile, and the solvent is polar protic. The carbocation intermediate is planar, so water attacks from both faces. The product is a near-racemic mixture of (R)- and (S)-3-methyl-3-hexanol.
Q: Why does SN2 favour polar aprotic solvents while SN1 favours polar protic solvents?
A: In a polar aprotic solvent, cations are solvated but anions (including nucleophiles) are relatively free, making them more reactive for backside attack (SN2). In a polar protic solvent, the solvent stabilises both the departing anion and the carbocation intermediate through hydrogen bonding and ion-dipole interactions, which helps the SN1 pathway.
Q: 2-bromo-2-methylpropane is treated with methanol. Draw the SN1 mechanism and identify any possible rearrangement.
A: Step 1: the C–Br bond breaks heterolytically to give a tertiary carbocation, (CH₃)₃C⁺, and Br⁻. Step 2: methanol attacks the carbocation to give (CH₃)₃COCH₃ (after deprotonation). No rearrangement occurs here because the carbocation is already tertiary (the most stable it can be without ring expansion).
Q: Rank the following substrates in order of decreasing SN2 reactivity: CH₃Br, (CH₃)₂CHBr, CH₃CH₂Br, (CH₃)₃CBr.
A: CH₃Br > CH₃CH₂Br > (CH₃)₂CHBr > (CH₃)₃CBr. As branching increases, steric hindrance blocks the nucleophile's backside approach, slowing SN2.
SN1 and SN2 compete directly with elimination reactions (E1 and E2). In many exam questions, the same substrate and conditions could give a substitution or an elimination product, and you will need to predict which dominates. The factors (substrate, base/nucleophile strength, temperature) overlap heavily with what you have learned here.
The stereochemistry concepts (inversion, racemisation) connect to the chirality and stereoisomerism material from earlier in the course. Carbocation stability and rearrangements reappear in electrophilic addition reactions to alkenes.
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