Difficulty: Intermediate | Prerequisites: functional groups, Lewis structures, electronegativity, acid-base concepts.
Substitution reactions are one of the four core reaction types in organic chemistry, alongside addition, elimination and rearrangement. They sit at the heart of Organic Chemistry I because almost every later topic (elimination, synthesis design, biochemistry) builds on understanding how and why a nucleophile replaces a leaving group. If you are comfortable drawing Lewis structures and identifying electron-rich versus electron-poor sites, you have what you need. The SN1/SN2 distinction is one of the most heavily tested topics in any introductory organic chemistry course.
In a substitution reaction, a nucleophile kicks out a leaving group and takes its place on the carbon skeleton. SN2 does this in one concerted step (backside attack, inversion, strong nucleophile required). SN1 does it in two steps via a carbocation intermediate (racemisation, favoured by weak nucleophiles and stable carbocations).
Nucleophile
An electron-rich species that donates a pair of electrons to an electron-poor atom to form a new covalent bond.
Think of it as the attacker in the reaction, the one with electrons to spare.
Electrophile
An electron-poor species that accepts a pair of electrons from a nucleophile.
In simple terms, this is the carbon (or other atom) that gets attacked because it is partially positive.
Leaving group
An atom or group that departs from the substrate, taking the bonding electrons with it.
Think of it as the group that gets replaced. Good leaving groups are stable once they leave (e.g. halide ions, water).
Substrate
The organic molecule that contains the electrophilic carbon and the leaving group.
In simple terms, this is the molecule that undergoes the reaction.
Carbocation
A carbon atom bearing a formal positive charge because it has only three bonds and an empty p orbital.
Think of it as a carbon that has temporarily lost its leaving group and is desperate for electrons.
Backside attack
Nucleophilic attack from the side opposite the leaving group, characteristic of SN2.
In simple terms, the nucleophile approaches from behind, which flips the stereochemistry like an umbrella inverting in the wind.
Inversion of configuration (Walden inversion)
The reversal of stereochemistry at the carbon centre during an SN2 reaction.
Think of it as the carbon's groups flipping to the opposite spatial arrangement.
Racemisation
The formation of a roughly equal mixture of both enantiomers, resulting in loss of optical activity.
In simple terms, the product is a 50:50 mix of mirror-image molecules because the carbocation intermediate is flat and can be attacked from either face.
Concerted reaction
A reaction in which bond breaking and bond forming happen simultaneously in a single step.
Think of it as everything happening at once, with no intermediate.
Rate law
A mathematical expression that relates the reaction rate to the concentrations of the reactants.
In simple terms, the rate law tells you which species' concentrations matter for how fast the reaction goes.
Mechanism overview
The nucleophile attacks the electrophilic carbon from the backside (180° opposite the leaving group) while the leaving group departs, all in a single concerted step.
There is no intermediate. The reaction passes through a single transition state in which the carbon is partially bonded to both the nucleophile and the leaving group.
Rate law
Rate = k[nucleophile][substrate]
The reaction is second order overall: the rate depends on the concentration of both the nucleophile and the electrophile (substrate).
Stereochemistry
Always produces inversion of configuration at the carbon centre (Walden inversion).
If the starting material is R, the product is S, and vice versa.
Substrate effects
SN2 works best on methyl and primary substrates, where the backside of the carbon is unhindered.
Secondary substrates react slowly. Tertiary substrates essentially do not undergo SN2 because steric crowding blocks the nucleophile's approach.
Reactivity order: methyl > 1° > 2° >> 3° (3° is effectively zero for SN2).
Nucleophile requirements
A strong, unhindered nucleophile favours SN2 (e.g. OH⁻, CN⁻, I⁻, RS⁻).
Solvent effects
Polar aprotic solvents (DMSO, DMF, acetone) favour SN2 because they do not solvate the nucleophile, leaving it free to attack.
Example
I⁻ + CH₃OH → CH₃I + OH⁻ (one-step displacement with inversion).
Mechanism overview
Step 1: The leaving group departs on its own, forming a planar carbocation intermediate. This is the slow, rate-determining step.
Step 2: The nucleophile attacks the carbocation to form the product. This is the fast step.
Rate law
Rate = k[substrate]
The reaction is first order. Only the substrate concentration affects the rate, because the nucleophile is not involved in the rate-determining step.
Stereochemistry
The carbocation intermediate is sp2-hybridised and planar, so the nucleophile can attack from either face.
This leads to racemisation: a roughly equal mixture of retention and inversion products.
In practice, the mixture is often not perfectly 50:50 because the departing leaving group can partially block one face (ion-pair effect), giving a slight excess of inversion.
Substrate effects
SN1 is favoured by substrates that form stable carbocations.
Reactivity order: 3° > 2° >> 1° > methyl (methyl and primary carbocations are too unstable to form under normal conditions).
Allylic and benzylic substrates are also good SN1 candidates because resonance stabilises their carbocations.
Nucleophile requirements
A weak nucleophile is sufficient (e.g. water, alcohols) because the nucleophile does not participate in the rate-determining step.
Solvent effects
Polar protic solvents (water, methanol, ethanol) favour SN1 because they stabilise the carbocation intermediate and the departing leaving group through solvation.
Rearrangements
Because SN1 passes through a carbocation, hydride shifts and methyl shifts can occur to form a more stable carbocation before the nucleophile attacks. This is a distinctive feature of SN1 that does not happen in SN2.
Example
(CH₃)₃CBr + CH₃OH → (CH₃)₃COCH₃ + HBr (two-step process via a tertiary carbocation).
When deciding which mechanism applies, work through these factors in order:
Factor | Favours SN2 | Favours SN1 |
|---|---|---|
Substrate | Methyl, 1° | 3°, allylic, benzylic |
Nucleophile | Strong (OH⁻, CN⁻, I⁻) | Weak (H₂O, ROH) |
Solvent | Polar aprotic (DMSO, DMF) | Polar protic (H₂O, MeOH) |
Rate law | Rate = k[Nuc][Sub] | Rate = k[Sub] |
Steps | One (concerted) | Two (via carbocation) |
Stereochemistry | Inversion | Racemisation |
Rearrangements | Never | Possible |
Secondary substrates are the tricky middle ground. They can go either way depending on the nucleophile strength and solvent.
SN2 rate law
\text{Rate} = k[\text{Nuc}][\text{Sub}]SN1 rate law
\text{Rate} = k[\text{Sub}]Energy diagram notes
SN2: one energy maximum (the transition state). The diagram shows a single hump.
SN1: two energy maxima separated by a valley (the carbocation intermediate). The first hump is taller because the rate-determining step is leaving-group departure.
Activation energy (ΔG‡) is the height of the tallest peak above the starting materials. A lower ΔG‡ means a faster reaction.
Overall energy change (ΔG) is the difference between products and reactants. Negative ΔG means the reaction is thermodynamically favourable.
SN2 reactions are used in pharmaceutical synthesis to introduce specific functional groups with precise stereochemistry, for example when building single-enantiomer drugs. SN1 reactions are relevant in biological systems: the solvolysis of alkyl halides in aqueous environments mirrors how certain enzyme-catalysed substitutions proceed through carbocation-like intermediates.
Students often think SN1 always gives a perfect 50:50 racemic mixture. It does not. Ion-pair shielding often gives a slight excess of inversion product.
Students often confuse "strong nucleophile" with "strong base." They overlap but are not the same. A good nucleophile is defined by how fast it attacks carbon; a strong base is defined by how readily it abstracts a proton. I⁻ is a strong nucleophile but a weak base.
Students sometimes assume that a primary substrate always goes SN2. If the nucleophile is very weak and the solvent is polar protic, even a primary substrate may react sluggishly or follow a borderline pathway.
Students often forget that SN1 can produce rearranged products. If your product has a different carbon skeleton from your starting material, a carbocation rearrangement (hydride or methyl shift) is likely responsible.
⚠️ Predicting SN1 vs SN2 is one of the most common exam question types. Know the decision framework cold: substrate, nucleophile, solvent.
⚠️ Drawing the full arrow-pushing mechanism for both SN1 and SN2 is frequently tested. For SN2, show the nucleophile's lone pair attacking the carbon and the leaving group departing in one step. For SN1, show the leaving group departing first, then the nucleophile attacking the carbocation.
⚠️ Stereochemistry outcomes: expect questions that give you a chiral starting material and ask for the product's configuration. SN2 = inversion. SN1 = racemisation (with possible slight inversion excess).
⚠️ Carbocation rearrangements in SN1: if the question shows a product with a shifted carbon skeleton, look for a hydride or methyl shift.
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 SN1 is Rate = k[______]. (substrate)
True or false: A tertiary substrate is a good candidate for SN2. (False, steric hindrance blocks backside attack.)
True or false: Polar aprotic solvents favour SN1 reactions. (False, they favour SN2.)
Fill in the blank: SN2 produces ______ of configuration at the carbon centre. (inversion)
Q: A primary alkyl bromide is treated with NaCN in DMSO. Predict the mechanism (SN1 or SN2), and explain your reasoning.
A: SN2. The substrate is primary (low steric hindrance), CN⁻ is a strong nucleophile, and DMSO is a polar aprotic solvent. All three factors favour SN2. The product will show inversion of configuration at the reacting carbon.
Q: A tertiary alkyl chloride is dissolved in methanol. What substitution mechanism is expected, and what is the stereochemical outcome?
A: SN1. The substrate is tertiary (stable carbocation), methanol is a weak nucleophile, and methanol is a polar protic solvent. The product will be a roughly racemic mixture of the methyl ether, with possible slight excess of inversion.
Q: During an SN1 reaction, the product has a different carbon skeleton from the starting material. What happened?
A: A carbocation rearrangement occurred. After the leaving group departed, a hydride shift or methyl shift converted the initial carbocation into a more stable one before the nucleophile attacked.
Q: Write the rate law for an SN2 reaction between hydroxide ion and methyl iodide. How would doubling the concentration of hydroxide affect the rate?
A: Rate = k[OH⁻][CH₃I]. Doubling [OH⁻] would double the reaction rate, because the rate depends on both reactant concentrations.
Substitution reactions compete directly with elimination reactions (E1 and E2). The same factors that determine SN1 vs SN2 also determine whether substitution or elimination wins. This connects to the elimination topic because strong, bulky bases favour E2 over SN2, and high temperatures push SN1 substrates toward E1.
Carbocation rearrangements in SN1 connect to the broader topic of carbocation stability, which appears again in electrophilic addition to alkenes and in Friedel-Crafts reactions.
The concept of nucleophilicity versus basicity links back to acid-base chemistry and will appear again in carbonyl chemistry later in the course.
SN1, SN2, nucleophilic substitution, bimolecular substitution, unimolecular substitution, backside attack, Walden inversion, inversion of configuration, racemisation, carbocation intermediate, leaving group, nucleophile, electrophile, substrate, polar aprotic solvent, polar protic solvent, DMSO, DMF, rate law, second order kinetics, first order kinetics, concerted mechanism, steric hindrance, methyl shift, hydride shift, carbocation rearrangement, organic chemistry mechanisms, CHEM 101, Organic Chemistry I