Difficulty: Intermediate | Prerequisites: Lewis structures, electronegativity, acid-base chemistry, orbital hybridisation (sp, sp², sp³), basic stereochemistry (R/S configuration).
Nucleophilic substitution is one of the most fundamental reaction types in organic chemistry and sits at the heart of how carbon-based molecules are transformed. This topic covers how a nucleophile replaces a leaving group on a carbon centre, with particular focus on the SN2 (bimolecular) mechanism. You will need a solid grasp of Lewis acid-base theory, bond polarity, and stereochemistry before tackling this material. Understanding SN2 deeply is essential because it underpins functional group interconversions you will use throughout the rest of the course and in synthesis problems.
In an SN2 reaction, a nucleophile attacks an electrophilic carbon from the back side in a single concerted step, kicking out the leaving group and inverting the stereochemistry. The rate depends on the concentration of both the substrate and the nucleophile (second-order kinetics). SN2 works best on methyl and primary substrates with strong nucleophiles in polar aprotic solvents.
Nucleophile (Nuc)
A Lewis base that donates a lone pair of electrons to an electrophilic centre. In simple terms, this is the "attacker" in a substitution reaction, the species with electrons to share.
Electrophile
A species that accepts an electron pair; it has an energetically accessible empty or partially empty orbital and usually carries a partial or full positive charge. Think of it as the carbon that gets attacked.
Leaving group (LG)
The atom or group that departs with the bonding electron pair during substitution. A good leaving group stabilises negative charge well and is typically a weak base (e.g. I⁻, Br⁻, H₂O).
Transition state (TS)
A potential energy maximum on the reaction coordinate where bonds are partially formed and partially broken. It exists only fleetingly, on the timescale of a bond vibration, and cannot be isolated.
Reaction intermediate
A local potential energy minimum between two transition states. Intermediates are real species but are typically unstable and high in energy. Note: SN2 has no intermediate; SN1 does (a carbocation).
Rate-limiting step (rate-determining step)
The elementary step with the highest activation energy (ΔG‡) and therefore the slowest rate. The overall reaction can go no faster than this step.
SN2 reaction
Substitution, Nucleophilic, Bimolecular. A concerted, one-step mechanism in which the nucleophile attacks the substrate at the same time the leaving group departs. Think of it as a simultaneous swap: the nucleophile pushes in from the back while the leaving group exits from the front.
Backside attack
The nucleophile approaches the electrophilic carbon from the side directly opposite the leaving group, at 180°. This geometry is required for SN2 and is why inversion of configuration occurs.
Inversion of configuration (Walden inversion)
The stereochemical outcome of an SN2 reaction: the spatial arrangement of substituents around the electrophilic carbon flips, like an umbrella inverting in the wind.
Stereospecific reaction
A reaction in which different stereoisomers of the starting material give different stereoisomeric products. SN2 is stereospecific because it always inverts configuration.
Before diving into nucleophilic substitution specifically, it helps to see where it fits among the four broad reaction classes in organic chemistry.
Substitution – one functional group replaces another on a saturated carbon. Characteristic of alkyl halides and other sp³ substrates.
Addition – two molecules combine into one. Characteristic of compounds with multiple bonds (alkenes, alkynes, carbonyls).
Elimination – the reverse of addition. A small molecule (e.g. HBr, H₂O) is lost, generating a new pi bond.
Rearrangement – bonds within a single molecule reorganise. The product is a constitutional isomer of the starting material.
Substitution and elimination often compete with each other when an alkyl halide is treated with a nucleophile or base. Knowing which pathway dominates is a core exam skill.
The SN2 mechanism is a single, concerted step. The nucleophile donates a lone pair to the electrophilic carbon (the carbon bonded to the leaving group), forming a new bond at the same time the C–LG bond breaks. There is no intermediate.
The carbon-leaving group bond is polarised: carbon carries a partial positive charge (δ+), making it electrophilic and attractive to nucleophiles. The nucleophile approaches from the back side of the carbon, directly opposite the leaving group (180° angle). This geometry is called backside attack.
At the transition state, both the nucleophile and the leaving group are partially bonded to the central carbon. The carbon is briefly pentacoordinate. Partial negative charges (δ−) sit on both the incoming nucleophile and the departing leaving group.
Because there is no intermediate, the energy diagram for SN2 shows a single energy maximum (one transition state) between reactants and products.
The experimentally observed rate law for SN2 is second order:
rate = k[substrate][nucleophile]
Both the substrate and the nucleophile appear in the rate expression, which confirms they must collide in the rate-limiting (and only) step. This is what "bimolecular" means: two molecules participate in the rate-determining step.
For example, the reaction of CH₃Cl with OH⁻ in water follows rate = k[CH₃Cl][OH⁻]. The activation energy (ΔG‡) for this reaction is about 103 kJ/mol, and the overall free energy change (ΔG°) is about −100 kJ/mol, making products thermodynamically favoured.
SN2 reactions are stereospecific. The backside attack forces the electrophilic carbon to undergo inversion of configuration, sometimes called Walden inversion.
Picture an umbrella flipping inside out: the three substituents on the electrophilic carbon "flip" to the other side as the nucleophile pushes in and the leaving group departs.
If you start with an (R)-configured substrate and the nucleophile and leaving group have the same CIP priority ranking relative to the other substituents, the product will be (S).
If the CIP priorities change because the new group has a different priority from the old leaving group, you must reassign R/S from scratch. The spatial arrangement still inverts, but the label may or may not change.
Different stereoisomers of the starting material produce different stereoisomeric products. This is what "stereospecific" means: the mechanism guarantees 100% selectivity for one stereochemical outcome.
SN2 reactions are enormously useful for building new functional groups from alkyl halides (R–LG). The nucleophile determines the product:
OH⁻ → Alcohol (R–OH)
OR⁻ → Ether (R–OR) — this is the Williamson ether synthesis
RCO₂⁻ → Ester (R–OOCR)
SH⁻ → Thiol (R–SH)
SR⁻ → Thioether (R–SR)
RC≡C⁻ → Alkyne (R–C≡CR) — useful for extending carbon chains
CN⁻ → Nitrile (R–CN) — adds one carbon to the chain
Each of these transformations replaces the leaving group with the nucleophile in a single step with inversion of stereochemistry. This table of transformations is worth memorising; it shows up repeatedly in synthesis problems throughout organic chemistry.
SN2 rate law:
rate = k[substrate][nucleophile]
Second-order overall: first order in substrate, first order in nucleophile.
Free energy of activation (ΔG‡):
Determines the rate constant k (kinetics). A lower ΔG‡ means a faster reaction.
Free energy of reaction (ΔG°):
Determines the thermodynamic favourability (equilibrium). A negative ΔG° means the products are more stable than the reactants (K > 1).
Remember: ΔG‡ tells you about speed; ΔG° tells you about extent. They are independent of each other.
SN2 reactions are used throughout synthetic chemistry to build drug molecules, agrochemicals, and materials. The Williamson ether synthesis (alkoxide + alkyl halide → ether) is one of the most widely used lab methods for making ethers, including those found in pharmaceuticals.
Cyanide displacement (R–X + CN⁻ → R–CN) is a key step in extending carbon chains during total synthesis. The nitrile product can then be hydrolysed to a carboxylic acid or reduced to an amine, making this a versatile building block reaction.
Students often think SN2 produces a mixture of retained and inverted stereochemistry. It does not. SN2 always gives complete inversion, never retention or racemisation.
Students frequently confuse nucleophilicity with basicity. Both involve electron donation, but basicity is a thermodynamic property (how well a species bonds to H⁺), while nucleophilicity is a kinetic property (how quickly it attacks carbon). A bulky, strong base like tert-butoxide is a poor nucleophile.
A common error is thinking that SN2 can occur on tertiary substrates if you use a strong enough nucleophile. Steric hindrance at a 3° carbon makes SN2 essentially impossible regardless of nucleophile strength.
Students sometimes forget that SN2 does not occur at sp² or sp-hybridised carbons. The substrate must have an sp³ carbon bearing the leaving group.
⚠️ You will almost certainly be asked to draw the mechanism of an SN2 reaction, including curved arrows showing electron flow from nucleophile to electrophilic carbon and from C–LG bond to the leaving group.
⚠️ Predicting stereochemistry is a frequent exam target. Given a substrate with defined stereochemistry, you must be able to draw the product with inverted configuration.
⚠️ Expect questions asking you to distinguish SN1 from SN2 based on substrate structure, rate law, or stereochemical outcome.
⚠️ Functional group transformation questions ("what nucleophile would you use to convert R–Br to R–CN?") draw directly from the SN2 reaction table.
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 SN2 is rate = k[][]. (substrate, nucleophile)
True or False: SN2 at a tertiary carbon is faster than at a primary carbon. (False. Steric hindrance makes SN2 at 3° essentially impossible.)
Fill in the blank: SN2 always produces ____ of stereochemical configuration. (inversion)
True or False: A good leaving group is typically a strong base. (False. Good leaving groups are weak bases.)
Q: A secondary alkyl iodide is treated with NaCN in DMSO. What is the product and what mechanism operates?
A: The product is a nitrile (R–CN). The mechanism is SN2. CN⁻ is a strong nucleophile, DMSO is a polar aprotic solvent (favours SN2), and secondary substrates can undergo SN2 when the nucleophile is strong.
Q: (R)-2-bromobutane is treated with NaOH. Predict the configuration of the alcohol product.
A: The product is (S)-2-butanol. SN2 proceeds with inversion of configuration, so R becomes S (assuming the relative CIP priorities of OH and Br are similar enough that the label flips).
Q: Why does SN2 not occur at a tertiary carbon?
A: The three bulky alkyl groups surrounding the electrophilic carbon block the nucleophile from accessing the back side. Steric hindrance raises the activation energy to a prohibitive level.
Q: Rank the following substrates in order of decreasing SN2 reactivity: CH₃Br, (CH₃)₂CHBr, CH₃CH₂Br, (CH₃)₃CBr.
A: CH₃Br > CH₃CH₂Br > (CH₃)₂CHBr > (CH₃)₃CBr. Reactivity decreases as substitution increases because steric hindrance increases.
Q: Explain why I⁻ is a better leaving group than F⁻.
A: I⁻ is a much weaker base than F⁻, so it stabilises the negative charge more effectively after departure. The C–I bond is also weaker and longer, making it easier to break. Leaving group ability follows I⁻ > Br⁻ > Cl⁻ >> F⁻.
This material connects directly to elimination reactions (E1 and E2), which compete with substitution and are typically covered next. The same factors (substrate structure, base/nucleophile strength, solvent) determine whether substitution or elimination dominates.
SN2 functional group transformations (particularly Williamson ether synthesis and cyanide displacement) reappear throughout synthesis and retrosynthesis problems in later chapters.
Stereochemistry concepts here (inversion, stereospecificity) build on the R/S and chirality material from earlier in the course and prepare you for stereoselective reactions in later units.
SN2, nucleophilic substitution bimolecular, backside attack, Walden inversion, inversion of configuration, stereospecific reaction, second-order kinetics, rate law, nucleophile, electrophile, leaving group, alkyl halide substitution, Williamson ether synthesis, cyanide displacement, transition state, activation energy, ΔG‡, ΔG°, concerted mechanism, steric hindrance, polar aprotic solvent, DMSO, DMF, acetone, Lewis base, electron pair donor