Difficulty: Intermediate | Prerequisites: Functional groups, bond polarity, carbocation stability, leaving groups.
Nucleophilic substitution is one of the most fundamental reaction types in organic chemistry. A nucleophile (an electron-rich species) replaces a leaving group on a carbon atom. This is the mechanism behind a huge range of transformations: converting alkyl halides into alcohols, ethers, nitriles, and more.
You need a solid grasp of carbocation stability, leaving group ability, and solvent effects before this topic will click. If those feel shaky, revisit them first.
There are two main pathways, SN1 and SN2, and telling them apart is one of the most commonly examined skills in introductory organic chemistry.
Nucleophilic substitution swaps a leaving group for a nucleophile on a carbon. SN1 goes through a carbocation intermediate (two steps, favoured by 3° substrates and weak nucleophiles in polar protic solvents). SN2 happens in a single concerted step with backside attack (favoured by methyl/1° substrates and strong nucleophiles in polar aprotic solvents).
Nucleophile
An electron-rich species that donates a pair of electrons to an electrophilic carbon. Think of it as the "attacker" in the reaction, the one bringing electrons to form a new bond.
Electrophile
An electron-poor species that accepts electrons. In substitution reactions, this is the carbon bearing the leaving group.
Leaving group (LG)
The atom or group that departs with the bonding electrons. In simple terms, it is the part that gets kicked off the carbon so the nucleophile can take its place. Good leaving groups are weak bases (e.g. I⁻, Br⁻, Cl⁻, H₂O).
Leaving group ability (order)
I⁻ > Br⁻ > Cl⁻ ≈ H₂O > F⁻ > CH₃CO⁻ > HO⁻. The better the leaving group, the faster it departs.
Carbocation
A positively charged carbon intermediate, formed when the leaving group departs before the nucleophile arrives. In simple terms, it is a carbon that has temporarily lost its partner and is desperate for electrons. Stability order: 3° > 2° > 1° > methyl.
SN1 (substitution, nucleophilic, unimolecular)
A two-step substitution mechanism where the leaving group departs first to form a carbocation, and the nucleophile attacks second. The rate depends only on the substrate concentration.
SN2 (substitution, nucleophilic, bimolecular)
A one-step (concerted) substitution mechanism where the nucleophile attacks at the same time as the leaving group departs. The rate depends on both the substrate and the nucleophile concentrations.
Polar protic solvent
A solvent that can donate hydrogen bonds (e.g. H₂O, MeOH, EtOH). These stabilise carbocations and favour SN1.
Polar aprotic solvent
A solvent that cannot donate hydrogen bonds (e.g. DMSO, DMF, acetone). These leave nucleophiles "naked" and reactive, favouring SN2.
Inversion of stereochemistry (Walden inversion)
The flipping of configuration at the carbon centre during an SN2 reaction, caused by backside attack. Think of it as an umbrella inverting in the wind.
Substrate
Favoured by 3° alkyl halides (tertiary carbons)
2° substrates can also undergo SN1 under the right conditions
Methyl and 1° substrates essentially never go SN1 because their carbocations are too unstable
Nucleophile
Weak nucleophiles (e.g. H₂O, alcohols)
The nucleophile does not appear in the rate-determining step, so its strength does not affect the rate
Solvent
Polar protic solvents: H₂O, MeOH, EtOH
These stabilise the carbocation intermediate through solvation
Mechanism (two steps)
The leaving group departs, forming a carbocation intermediate. This is the slow, rate-determining step.
The nucleophile attacks the carbocation. This is the fast step.
If the nucleophile is a neutral molecule (e.g. H₂O), a third step follows: deprotonation to give the final product.
Stereochemistry
The carbocation is sp² hybridised and planar
The nucleophile can attack from either face
Result: a racemic mixture (a mix of both R and S configurations), or close to it
Rate law
Rate = k[substrate]
Unimolecular: only the substrate concentration matters
Example from the source notes
A 3° alkyl bromide reacts with water. The bromine leaves first, forming a tertiary carbocation. Water attacks the carbocation, then loses a proton. The major product is the alcohol; HBr is the byproduct.
Substrate
Favoured by methyl and 1° (primary) alkyl halides
2° substrates can go SN2 under the right conditions (strong nucleophile, polar aprotic solvent)
3° substrates essentially never go SN2 because steric hindrance blocks backside attack
Nucleophile
Strong nucleophiles required (e.g. CN⁻, I⁻, HO⁻, RO⁻)
Nucleophile strength directly affects the reaction rate
Solvent
Polar aprotic solvents: DMSO, DMF, acetone
These do not solvate the nucleophile, keeping it reactive and "naked"
Mechanism (one step, concerted)
The nucleophile attacks the electrophilic carbon from the back side (opposite the leaving group) at the same time as the leaving group departs.
There is no intermediate. Bond formation and bond breaking happen simultaneously through a single transition state.
Stereochemistry
100% inversion of configuration (Walden inversion)
Backside attack forces the other three groups to flip, like an umbrella inverting
If the starting material is R, the product is S (and vice versa)
Rate law
Rate = k[substrate][nucleophile]
Bimolecular: both concentrations matter
Example from the source notes
A 1° substrate with both Br and I as potential leaving groups reacts with NaCN. I⁻ is a better leaving group than Br⁻, so the iodide is displaced. CN⁻ attacks from the back side in a single step. The major product has the CN group where I was, with inverted stereochemistry. NaI is the byproduct.
Feature | SN1 | SN2 |
|---|---|---|
Steps | Two (carbocation intermediate) | One (concerted) |
Substrate | 3° (best), 2° (possible) | Methyl (best), 1° (best), 2° (possible) |
Nucleophile | Weak (H₂O, ROH) | Strong (CN⁻, I⁻, HO⁻, RO⁻) |
Solvent | Polar protic (H₂O, MeOH, EtOH) | Polar aprotic (DMSO, DMF, acetone) |
Rate law | Rate = k[substrate] | Rate = k[substrate][nucleophile] |
Stereochemistry | Racemisation (mix of R and S) | 100% inversion |
Rearrangements | Yes (carbocation can rearrange) | No (no intermediate to rearrange) |
Steric effects | Less sensitive to steric bulk | Very sensitive (3° blocked) |
Students often think SN1 and SN2 can both happen equally well with any substrate. They cannot. Substrate structure is the single most important factor in determining the mechanism.
Students often confuse "polar protic" with "polar aprotic." Protic solvents have O-H or N-H bonds and can hydrogen bond. Aprotic solvents lack these. Getting this wrong flips your mechanism prediction.
Students sometimes think SN2 gives a racemic mixture. It does not. SN2 gives 100% inversion. Only SN1 gives racemisation (because the planar carbocation can be attacked from either side).
Students often forget that 2° substrates are the ambiguous case. They can go either SN1 or SN2 depending on the nucleophile and solvent. Exam questions love to test this grey area.
⚠️ Predicting the mechanism (SN1 vs SN2) given a substrate, nucleophile, and solvent is one of the most common exam question types.
⚠️ Drawing the product with correct stereochemistry (inversion for SN2, racemisation for SN1) is routinely tested.
⚠️ Leaving group ability order (I⁻ > Br⁻ > Cl⁻ ≈ H₂O > F⁻ > CH₃CO⁻ > HO⁻) appears frequently. Know it cold.
⚠️ Questions often give you a 2° substrate deliberately to test whether you can reason through the ambiguity.
True or false: SN1 reactions always produce a single enantiomer.
Fill in the blank: SN2 reactions are favoured by ______ solvents.
True or false: The rate of an SN2 reaction depends on the concentration of the nucleophile.
Fill in the blank: The best substrates for SN1 are ______ alkyl halides.
True or false: Backside attack in SN2 leads to retention of configuration.
Answers: 1. False (SN1 gives a racemic mixture). 2. Polar aprotic. 3. True. 4. Tertiary (3°). 5. False (backside attack gives inversion, not retention).
Q: A tertiary alkyl bromide is dissolved in water. What mechanism will occur, and what is the expected stereochemical outcome?
A: SN1. Water is a weak nucleophile and a polar protic solvent, and the substrate is 3°. The carbocation intermediate is planar, so the product will be a roughly racemic mixture of enantiomers.
Q: Methyl iodide is treated with NaCN in DMSO. Predict the mechanism and the product.
A: SN2. The substrate is methyl (no steric hindrance), CN⁻ is a strong nucleophile, and DMSO is a polar aprotic solvent. The product is methyl cyanide (acetonitrile), with NaI as a byproduct.
Q: Why does a 3° substrate not undergo SN2?
A: The three bulky groups around the electrophilic carbon block the nucleophile from performing backside attack. Steric hindrance makes the SN2 transition state too high in energy.
Q: Rank the following leaving groups from best to worst: Cl⁻, I⁻, HO⁻, Br⁻.
A: I⁻ > Br⁻ > Cl⁻ > HO⁻. Better leaving groups are weaker bases and have more polarisable electron clouds.
Q: A secondary alkyl chloride is treated with a strong nucleophile in DMF. What mechanism is most likely?
A: SN2. Although 2° substrates are ambiguous, the strong nucleophile and polar aprotic solvent (DMF) push the reaction toward SN2.
This connects directly to elimination reactions (E1 and E2), which compete with substitution under many of the same conditions. Strong bases that are also strong nucleophiles can trigger elimination instead of substitution, especially with bulky substrates.
Carbocation stability, which drives SN1, also matters for electrophilic addition reactions and for understanding rearrangements (hydride and methyl shifts).
Leaving group chemistry recurs in acyl substitution (carboxylic acid derivatives), where the nucleophile replaces a leaving group on a carbonyl carbon rather than a saturated carbon.
Nucleophilic substitution, SN1, SN2, alkyl halide reactions, leaving group ability, carbocation stability, polar protic solvent, polar aprotic solvent, Walden inversion, backside attack, racemisation, stereochemistry of substitution, nucleophile strength, DMSO, DMF, acetone, methanol, ethanol, weak nucleophile, strong nucleophile, organic chemistry substitution, concerted mechanism, unimolecular, bimolecular, rate law.