Difficulty: Intermediate | Prerequisites: Familiarity with Lewis structures, electronegativity, polar bonds, and basic reaction types (Ch. 1–4 material). You should be comfortable identifying nucleophiles and electrophiles before reading these notes.
Nucleophilic substitution is one of the foundational reaction types in organic chemistry. It explains how and why certain functional groups swap places on a carbon skeleton, and it sets the stage for understanding elimination reactions (E1/E2), which compete with substitution under many conditions. This chapter sits at the pivot point of CHEM 2301: everything before it builds the vocabulary, and everything after it uses these mechanisms as a template. If you have been away from the course for a few weeks, make sure you are solid on leaving groups and carbocation stability before diving in.
Nucleophilic substitution reactions replace a leaving group on a carbon with a nucleophile. There are two mechanisms: SN2 (one concerted step, backside attack, inversion of stereochemistry) and SN1 (two steps via a carbocation intermediate, racemisation). Which pathway dominates depends on substrate structure, nucleophile strength, solvent, and leaving group ability.
Substitution reaction
A reaction that follows the general pattern A + B → C + D, where a nucleophile replaces a leaving group on the substrate. Think of it as a straight swap: one group leaves, another takes its place.
Nucleophile (Nu)
An electron-rich species that donates a pair of electrons to an electrophilic carbon. In simple terms, the nucleophile is the "attacker" that moves in to form a new bond.
Leaving group (LG)
The atom or group that departs with the bonding electrons during the reaction. Good leaving groups are stable once they leave (weak bases such as I⁻, Br⁻, Cl⁻, and tosylate). Think of it as the group that is comfortable walking away.
Electrophile / substrate
The carbon bearing the leaving group. It carries a partial positive charge (δ+) because the leaving group is electronegative, making it the target for nucleophilic attack.
SN2 (Substitution Nucleophilic Bimolecular)
A one-step mechanism in which the nucleophile attacks the substrate at the same time as the leaving group departs. "Bimolecular" means two molecules appear in the rate-determining step. In simple terms, bond making and bond breaking happen simultaneously.
SN1 (Substitution Nucleophilic Unimolecular)
A two-step mechanism in which the leaving group departs first to form a carbocation intermediate, and the nucleophile attacks second. "Unimolecular" means only one molecule (the substrate) appears in the rate-determining step. Think of it as "leave first, attack later."
Carbocation
A positively charged carbon intermediate formed in the first step of an SN1 reaction. Stability order: 3° > 2° > 1° > methyl. In simple terms, the more alkyl groups around the positive carbon, the more stable it is (hyperconjugation and inductive effects donate electron density).
Transition state
The highest-energy arrangement along the reaction coordinate, where bonds are partially formed and partially broken. Drawn in square brackets with a double-dagger symbol [‡]. This is a fleeting geometry, not an isolable species.
Rate-determining step (RDS)
The slowest step in a multi-step mechanism, which controls the overall rate of the reaction. Think of it as the bottleneck: the reaction can only go as fast as its slowest step.
Inversion of configuration (Walden inversion)
The stereochemical outcome of an SN2 reaction, where the nucleophile attacks from the back side of the leaving group and the three remaining substituents "flip" like an umbrella turning inside out.
Racemisation
Formation of a roughly equal mixture of R and S enantiomers. This is the typical stereochemical outcome of SN1 reactions, because the planar carbocation intermediate can be attacked from either face.
Tags: SN2, bimolecular substitution, backside attack, Walden inversion, concerted mechanism
One concerted step: the nucleophile attacks the electrophilic carbon at the same time as the leaving group departs.
The transition state shows five groups around carbon: the incoming nucleophile, the outgoing leaving group, and the three remaining substituents. Bonds to the nucleophile and leaving group are both partial (shown as dashed lines).
No intermediate is formed. There is a single transition state on the energy diagram (one energy maximum).
Rate = k[substrate][nucleophile]
The reaction is second order overall, first order in each reactant.
Because both the nucleophile and the substrate appear in the rate law, doubling the concentration of either one doubles the rate.
Backside attack forces inversion of configuration at the electrophilic carbon (Walden inversion).
If the starting material is R, the product is S (and vice versa), assuming the priority assignments do not change.
This is one of the most testable points in the chapter. If a question asks for the stereochemical outcome of an SN2 reaction, the answer is always inversion.
SN2 reactions are fastest at methyl and primary carbons, where steric hindrance is minimal.
Secondary substrates react more slowly.
Tertiary substrates essentially do not undergo SN2 because the three bulky groups block backside attack.
Reactivity order: CH₃X > 1° > 2° >> 3° (no reaction).
A strong nucleophile is required. Strong nucleophiles are typically negatively charged species: OH⁻, CN⁻, RS⁻, I⁻, N₃⁻.
Neutral molecules (H₂O, ROH) are generally too weak to drive an SN2 reaction.
Polar aprotic solvents favour SN2: DMSO, DMF, acetone, acetonitrile.
These solvents do not hydrogen-bond to and therefore do not stabilise (solvate) the nucleophile, leaving it "naked" and highly reactive.
Polar protic solvents (water, alcohols) slow SN2 because they solvate and stabilise the nucleophile, reducing its reactivity.
Better leaving groups make both SN1 and SN2 faster.
Leaving group ability roughly follows the trend: I⁻ > Br⁻ > Cl⁻ >> F⁻. Tosylate (OTs) and mesylate (OMs) are also excellent leaving groups.
The leaving group must be a weak base (stable with the bonding electrons).
Tags: SN1, unimolecular substitution, carbocation intermediate, racemisation, two-step mechanism
Two discrete steps.
Step 1 (slow, rate-determining): The leaving group departs on its own to form a carbocation intermediate. No nucleophile is involved yet.
Step 2 (fast): The nucleophile attacks the planar carbocation to form the product.
The energy diagram shows two transition states and one intermediate (a valley between two peaks).
Rate = k[substrate]
The reaction is first order overall. The nucleophile does not appear in the rate law because it is not involved in the rate-determining step.
Doubling the nucleophile concentration has no effect on the rate. Doubling the substrate concentration doubles the rate.
The carbocation intermediate is sp2 hybridised and planar.
The nucleophile can attack from either face of the plane, leading to a roughly equal mixture of R and S products (racemisation).
In practice, there is often a slight excess of inversion product because the leaving group partially shields one face as it departs (ion-pair effect), but for exam purposes the answer is "racemic mixture" unless told otherwise.
SN1 is fastest at tertiary carbons, where the carbocation intermediate is most stable (3° > 2° >> 1° > methyl).
Primary and methyl substrates do not undergo SN1 under normal conditions because 1° and methyl carbocations are far too unstable.
Secondary substrates can go either SN1 or SN2, depending on other conditions (nucleophile strength, solvent).
Allylic and benzylic substrates are special cases: even primary allylic/benzylic substrates can undergo SN1 because their carbocations are stabilised by resonance.
SN1 does not require a strong nucleophile. Weak nucleophiles (H₂O, ROH) work well because the carbocation is a powerful electrophile.
If a strong nucleophile is present with a 3° substrate, elimination (E2) tends to compete or dominate.
Polar protic solvents favour SN1: water, methanol, ethanol, acetic acid.
These solvents stabilise both the carbocation intermediate and the leaving group anion through solvation (hydrogen bonding and ion-dipole interactions).
Polar aprotic solvents do not favour SN1 because they are poor at stabilising the cation.
Same trend as SN2: better leaving groups accelerate the reaction.
The leaving group must depart without assistance from the nucleophile, so its ability to stabilise the negative charge it carries away is critical.
SN2 rate law: Rate = k [substrate] [nucleophile]
SN1 rate law: Rate = k [substrate]
Leaving group ability (best to worst): I⁻ > Br⁻ > Cl⁻ >> F⁻ (Tosylate and mesylate sit alongside I⁻ and Br⁻ as excellent leaving groups.)
Carbocation stability order: 3° > 2° > 1° > methyl (Also: benzylic ≈ allylic carbocations gain extra stability through resonance delocalisation.)
Nucleophile strength in a polar aprotic solvent (approximate): CN⁻ > I⁻ > OH⁻ > Br⁻ > Cl⁻
SN2 substrate reactivity: CH₃X > 1° > 2° >> 3° (no reaction)
SN1 substrate reactivity: 3° > 2° >> 1° > methyl (no reaction)
SN2 reactions are central to pharmaceutical synthesis, where chemists need precise control of stereochemistry. When a drug must be a single enantiomer (which is most modern drugs), the predictable inversion of SN2 lets the chemist set the correct configuration at a chiral centre.
SN1 reactions show up in biological systems. Solvolysis of alkyl halides in aqueous or alcoholic environments is essentially SN1 chemistry, and the formation of carbocation intermediates is also relevant to terpene biosynthesis, where enzymes guide carbocation rearrangements to build complex natural products.
Students often think "bimolecular" means two molecules of the same species react. It does not. It means the rate-determining step involves two distinct reactant molecules (one substrate, one nucleophile).
Students frequently assume a secondary substrate must be SN1 or must be SN2. It can be either, and the outcome depends on the nucleophile strength and solvent. Secondary substrates are the "grey zone."
A common error is drawing SN2 on a tertiary substrate. Tertiary carbons are too sterically hindered for backside attack. If the substrate is tertiary, think SN1 (or elimination).
Students sometimes confuse inversion with retention. SN2 always gives inversion. If you see a product with retained configuration from a substitution, that is not a simple SN2.
⚠️ Expect to be given a substrate, nucleophile, and solvent and asked to predict SN1 vs SN2. Walk through the decision tree: substrate class first, then nucleophile strength, then solvent.
⚠️ Drawing the SN2 transition state is a classic exam question. Show the nucleophile and leaving group on opposite sides of the carbon, both connected by dashed bonds, with square brackets and the double-dagger symbol. Include the correct charge.
⚠️ Stereochemistry questions are near-guaranteed. For SN2, assign R/S to the starting material and confirm that the product has the opposite configuration. For SN1, state that a racemic mixture forms.
⚠️ Rate law problems: be ready to predict what happens to the rate when you double or halve the concentration of the substrate or nucleophile. The answer depends entirely on which mechanism is operating.
⚠️ Questions that give you a tertiary alkyl halide with a strong nucleophile in a polar aprotic solvent are testing whether you recognise the conflict: the substrate points to SN1, but the nucleophile and solvent point to SN2. In practice, elimination (E2) dominates here. Flag it.
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 only)
True or false: A tertiary alkyl halide is a good substrate for SN2. (False. Tertiary substrates are too sterically hindered for SN2.)
Fill in the blank: The stereochemical result of an SN2 reaction is ______ of configuration. (inversion)
True or false: Polar protic solvents favour SN2 reactions. (False. Polar protic solvents favour SN1. Polar aprotic solvents favour SN2.)
Q: A primary alkyl bromide is treated with NaCN in DMSO. Predict the mechanism, product, and stereochemical outcome.
A: SN2. The substrate is primary (favours SN2), CN⁻ is a strong nucleophile, and DMSO is a polar aprotic solvent. The product is an alkyl nitrile (R–CN) with inversion of configuration at the electrophilic carbon. Br⁻ is the leaving group.
Q: tert-Butyl bromide is dissolved in methanol. What mechanism operates, and what is the major product?
A: SN1. The substrate is tertiary (cannot undergo SN2), and methanol is a polar protic solvent that stabilises the carbocation. The methanol acts as the nucleophile after the carbocation forms. The major product is tert-butyl methyl ether. A racemic mixture results if the carbon bearing the leaving group is a stereocentre.
Q: Why does doubling the concentration of the nucleophile have no effect on the rate of an SN1 reaction?
A: The nucleophile does not participate in the rate-determining step of SN1. The slow step is the departure of the leaving group (unimolecular), so the rate depends only on the substrate concentration.
Q: Draw the transition state for the SN2 reaction between OH⁻ and 2-iodobutane.
A: Place OH on one side and I on the other side of the central carbon, both connected by dashed bonds. The three remaining groups (H, CH₃, CH₂CH₃) fan out in a plane perpendicular to the Nu–C–LG axis. Enclose everything in square brackets with a ‡ and a ⁻ charge (the overall charge of the transition state).
Q: A secondary alkyl chloride is treated with H₂O in a polar protic solvent. SN1 or SN2?
A: SN1 is favoured. Water is a weak nucleophile (neutral, no charge), and the polar protic solvent stabilises the carbocation intermediate. The product is a secondary alcohol, formed as a racemic mixture.
This material connects directly to elimination reactions (E1 and E2), which are covered next and compete with substitution under many of the same conditions. The decision tree for SN1/SN2/E1/E2 is the single most important framework in CHEM 2301, and it builds entirely on the principles in these notes.
Carbocation stability, introduced here through SN1, also underpins Markovnikov's rule in alkene addition reactions (Ch. 6–7) and carbocation rearrangements (hydride and methyl shifts).
The stereochemistry concepts (inversion, racemisation, chiral centres) link back to the stereochemistry chapter (Ch. 4) and forward to any reaction where configuration at a carbon matters.
SN1, SN2, nucleophilic substitution, substitution reactions, backside attack, Walden inversion, inversion of configuration, racemisation, racemic mixture, carbocation, carbocation stability, leaving group, leaving group ability, nucleophile, electrophile, substrate, bimolecular, unimolecular, rate-determining step, rate law, transition state, concerted mechanism, polar aprotic solvent, polar protic solvent, DMSO, DMF, acetone, methanol, steric hindrance, primary substrate, secondary substrate, tertiary substrate, methyl substrate, allylic, benzylic, tosylate, mesylate, elimination vs substitution, E1, E2, organic chemistry I, CHEM 2301, Chapter 5, LO 6c, LO 6d