Source: Organic Chemistry, The Ohio State University (Baldwin)
Tags: SN1, nucleophilic substitution, solvolysis, carbocation, unimolecular, leaving group, stereochemistry, racemic mixture, carbocation stability, hyperconjugation, resonance
Difficulty: Intermediate | Prerequisites: Chapter 6 (SN2 mechanism, nucleophiles, leaving groups)
Big picture: This is the second of the two nucleophilic substitution mechanisms you need to know. Where SN2 is a one-step, backside-attack process, SN1 is a stepwise mechanism that goes through a carbocation intermediate. The distinction matters because SN1 and SN2 favour different substrates, different solvents, and produce different stereochemical outcomes. If you understand when and why a carbocation forms, you can predict which pathway a reaction will follow, and that is the central skill tested in this chapter.
SN1 is a two-step (three-step with neutralisation) nucleophilic substitution: the leaving group departs first to form a carbocation, then a nucleophile attacks. Because the carbocation is planar, the nucleophile can attack from either face, giving racemic products. SN1 is favoured by weak nucleophiles, good leaving groups, polar protic solvents, and substrates that form stable (2° or 3°) carbocations.
Nucleophilic substitution
A reaction in which a nucleophile replaces a leaving group on a carbon. Think of it as a molecular swap: the nucleophile comes in, the leaving group goes out.
SN1 (substitution, nucleophilic, unimolecular)
A nucleophilic substitution mechanism whose rate depends only on the concentration of the substrate, not the nucleophile. In simple terms, the slow step is the substrate falling apart on its own.
Solvolysis
An SN1 reaction in which the solvent itself acts as the nucleophile. Hydrolysis is the specific case where the solvent is water.
Carbocation
A carbon atom bearing a formal positive charge, with only three bonds and an empty p orbital. Think of it as an electron-hungry intermediate, flat and sp² hybridised.
Leaving group (LG)
The atom or group that departs with the bonding electrons. Weaker bases make better leaving groups because they are more stable once they leave (e.g. Br⁻ is a better LG than HO⁻).
Racemic mixture
A 50:50 mix of both enantiomers of a chiral product. In simple terms, you get equal amounts of the "left-handed" and "right-handed" versions.
Hyperconjugation
Stabilisation of a carbocation by overlap of adjacent C–H (or C–C) sigma bonds with the empty p orbital on the positively charged carbon. This is why more substituted carbocations are more stable: more neighbouring bonds can donate electron density.
Resonance stabilisation (of carbocations)
When a carbocation is adjacent to a pi system or lone pair, the positive charge can be delocalised across multiple atoms. A primary carbocation with resonance (e.g. allylic or benzylic) can rival or exceed the stability of a secondary carbocation without resonance.
Rate-determining step (RDS)
The slowest step in a multi-step mechanism, which controls the overall reaction rate. For SN1, the RDS is carbocation formation.
The mechanism proceeds through three distinct steps:
Step 1, Carbocation formation (RDS): The leaving group departs, taking the bonding electrons with it. This is the slow step and determines the overall rate.
Step 2, Nucleophilic attack: The nucleophile donates a pair of electrons to the electron-deficient carbocation. This is fast.
Step 3, Acid-base chemistry (ABC) / Neutralisation: If the nucleophile was neutral (e.g. water or methanol), the product carries a positive charge and must be deprotonated. The strongest base in solution does this. Check: H₃O⁺ is a stronger acid than Br⁻ is a base, so ABC is favourable.
The rate law is: Rate = k[substrate]. This is unimolecular because only the substrate is involved in the rate-determining step.
Starting from a tertiary alkyl bromide in water:
The C–Br bond breaks heterolytically to form a 3° carbocation and Br⁻
Water attacks the carbocation (nucleophilic attack)
A proton is lost to give the alcohol product (ABC step)
The overall result: Br is replaced by OH, with water acting as both nucleophile and solvent.
The energy diagram for SN1 has two transition states and one intermediate:
The first energy hill (TS1) is the highest, corresponding to the RDS (carbocation formation)
The intermediate sits in an energy well between TS1 and TS2
TS2 (nucleophilic attack) is lower in energy, so that step is faster
For an exothermic SN1 reaction (ΔH negative), the products sit lower in energy than the starting materials. For an endothermic one (ΔH positive), products sit higher.
Key question: "For ΔH, which is more stable?" Compare the energy of starting materials to products. Whichever is lower in energy is more stable, and that direction is thermodynamically favoured.
When the leaving group departs, the carbon goes from sp³ (tetrahedral) to sp² (trigonal planar). The empty p orbital sticks out above and below the plane.
The nucleophile can attack from the top face or the bottom face with roughly equal probability
This produces a racemic mixture (equal amounts of R and S products)
If the starting material is a single enantiomer, you lose that stereochemical information
If the substrate has other stereocentres that are not involved in the reaction, those are preserved. The result may be a mixture of diastereomers rather than enantiomers.
The stability order is:
CH₃⁺ (methyl) <<< 1° (primary) <<<< 2° (secondary) < 3° (tertiary)
Methyl and primary carbocations are so unstable they rarely form. This means SN1 does not occur at methyl or primary carbons.
Secondary carbocations do form but are less stable than tertiary.
Tertiary carbocations are the most stable and form most readily.
All carbocations are highly reactive, even the stable ones.
What stabilises carbocations:
Hyperconjugation: Adjacent C–H or C–C sigma bonds overlap with the empty p orbital, donating electron density. More substituents on the cation carbon means more hyperconjugation, which is why 3° > 2° > 1°.
Resonance: If the carbocation is next to a double bond or a heteroatom lone pair, the charge delocalises. A primary carbocation with resonance can be comparable to a secondary carbocation without it. Resonance adds roughly one level of stability.
Four conditions push a reaction towards the SN1 pathway:
Weak nucleophiles: Species like H₂O, CH₃OH, and CH₃SH are not strong enough to force a backside attack (SN2). They wait for the carbocation to form, then attack.
Polar protic solvents: These stabilise the polar intermediates (the carbocation and the departing anion) through solvation. This lowers the energy of the intermediate and increases the rate of SN1.
Better leaving group, faster reaction: The rate depends on carbocation formation, and a good leaving group makes it easier to break the C–LG bond. Weaker bases are better leaving groups.
More stable carbocation, faster rate: A more stable carbocation means the RDS has a lower activation energy. Tertiary substrates react fastest by SN1; primary substrates essentially do not react by SN1 at all.
When writing out the ABC step or any proton transfer in a mechanism:
Always check that the ABC step is thermodynamically favourable
Never create a strong base in acidic solution
Never create a strong acid in basic solution
Once you create a certain type of charge, stick with that charge: neutral products from a neutral starting nucleophile, charged products from a charged nucleophile
"SN1 can happen at a primary carbon." It cannot, under normal conditions. Primary carbocations are too unstable to form. The only exception is if the primary carbon has resonance stabilisation (allylic or benzylic), which is a special case.
"SN1 gives inversion of configuration like SN2." SN1 gives a racemic mixture (both retention and inversion), not clean inversion. The planar carbocation is attacked from both faces.
"A stronger nucleophile speeds up SN1." The nucleophile does not appear in the rate law. Changing the nucleophile's strength has no effect on the rate of an SN1 reaction.
"Polar aprotic solvents favour SN1." Polar protic solvents (water, alcohols) favour SN1 because they stabilise the charged intermediates through hydrogen bonding. Polar aprotic solvents favour SN2.
⚠️ You will be asked to draw the full three-step mechanism for SN1 reactions. Do not skip the ABC step.
⚠️ The rate law (Rate = k[substrate]) is commonly tested. Know that it is unimolecular and why.
⚠️ Stereochemistry questions will ask what happens at the stereocentre: the answer for SN1 is always racemisation (loss of optical activity, mixture of enantiomers).
⚠️ Carbocation stability ranking (methyl < 1° < 2° < 3°) and the two stabilisation mechanisms (hyperconjugation, resonance) appear on nearly every exam.
⚠️ Know the difference between polar protic (favours SN1/E1) and polar aprotic (favours SN2/E2) solvents.
True or False: The rate of an SN1 reaction depends on the concentration of the nucleophile.
Fill in the blank: The intermediate in an SN1 reaction is a _______, which has _______ geometry.
True or False: SN1 reactions at a stereocentre produce a single enantiomer.
Fill in the blank: Carbocation stability increases with more substituents because of _______.
True or False: A primary alkyl halide readily undergoes SN1.
Q: What are the three steps of the SN1 mechanism, and which is rate-determining?
A: (1) Carbocation formation (rate-determining), (2) nucleophilic attack, (3) acid-base chemistry / deprotonation. Step 1 is the RDS.
Q: Why does SN1 produce a racemic mixture?
A: The carbocation intermediate is sp² hybridised and planar. The nucleophile can attack from either face of the plane with equal probability, giving a 50:50 mixture of enantiomers.
Q: Rank the following carbocations in order of increasing stability: CH₃⁺, (CH₃)₃C⁺, (CH₃)₂CH⁺
A: CH₃⁺ (methyl, least stable) < (CH₃)₂CH⁺ (secondary) < (CH₃)₃C⁺ (tertiary, most stable).
Q: A secondary alkyl bromide is dissolved in methanol. No strong nucleophile is added. What mechanism is most likely, and what is the nucleophile?
A: SN1. Methanol (CH₃OH) is a weak nucleophile and a polar protic solvent, both of which favour SN1. Methanol is both the solvent and the nucleophile (solvolysis).
Q: Why can a 1° carbocation with resonance stabilisation (e.g. allylic) undergo SN1 while an ordinary 1° carbocation cannot?
A: Resonance delocalises the positive charge across multiple atoms, adding roughly one level of stability. This makes the allylic or benzylic 1° carbocation comparable in energy to a 2° carbocation, which is stable enough to form.
This material connects directly to SN2 (Chapter 6): every substitution problem on the exam asks you to decide between SN1 and SN2 based on the nucleophile strength, substrate class, and solvent. It also sets the stage for E1 elimination (next section), which shares the same first step, carbocation formation, and competes with SN1 under the same conditions.
nucleophilic substitution, SN1 mechanism, unimolecular substitution, solvolysis, hydrolysis, carbocation intermediate, tertiary carbocation, secondary carbocation, racemic mixture, racemisation, loss of stereochemistry, rate-determining step, leaving group ability, polar protic solvent, hyperconjugation, resonance stabilisation, CHEM 2510, organic chemistry chapter 7, Ohio State, Baldwin