Difficulty: Intermediate
Prerequisites: Chapters 1 to 4 (bonding, Lewis structures, polarity, functional groups) and the SN2 study notes. Understanding carbocation stability from Chapter 4 is especially important here.
SN1 is the second of the two nucleophilic substitution mechanisms covered in Chapter 5 of CHEM 2301. Where SN2 is a one-step, concerted process, SN1 proceeds in two distinct steps through a carbocation intermediate. This mechanism dominates for tertiary substrates and benzylic/allylic systems, particularly in polar protic solvents with weak nucleophiles. Grasping SN1 is essential because you will need to predict when it occurs instead of (or alongside) SN2, and because it introduces the concept of racemisation at a stereogenic centre. If you have not yet reviewed the SN2 study notes, start there.
In an SN1 reaction the leaving group departs first, on its own, forming a carbocation intermediate. The nucleophile then attacks the carbocation in a second, faster step. Because the carbocation is planar, the nucleophile can attack from either face, producing a racemic mixture (equal amounts of both enantiomers) when the carbon was a stereocentre.
SN1 (Substitution Nucleophilic Unimolecular)
A two-step substitution mechanism in which only the substrate participates in the rate-determining step. In simple terms, "unimolecular" means the slow step involves just one molecule (the substrate losing its leaving group).
Carbocation (carbenium ion)
A positively charged carbon species with an empty p orbital and only three bonds. It forms when the leaving group departs in SN1, step 1. Think of it as a carbon that has lost its leaving group and is now electron-deficient, waiting for a nucleophile.
Carbocation stability order
Tertiary (3°) > secondary (2°) > primary (1°) > methyl. More alkyl groups stabilise the positive charge through hyperconjugation and inductive effects. Benzylic and allylic carbocations are also stabilised by resonance.
Intermediate
A real, distinct chemical species that forms during a reaction and exists (briefly) in an energy minimum between two transition states. In SN1, the carbocation is the intermediate. Unlike a transition state, an intermediate can in principle be detected or even isolated.
Racemic mixture (racemate)
A 1:1 mixture of both enantiomers of a chiral product. In SN1, the planar carbocation allows the nucleophile to attack from either face with roughly equal probability, giving a racemic product.
Rate-determining step (slow step)
The step with the highest activation energy, which limits how fast the overall reaction proceeds. In SN1 this is step 1: the departure of the leaving group to form the carbocation.
Polar protic solvent
A solvent that has O-H or N-H bonds and can form hydrogen bonds (water, methanol, ethanol, acetic acid). These solvents stabilise the carbocation intermediate and the departing leaving group through solvation, favouring SN1.
Unimolecular
Only one molecule (the substrate) is involved in the rate-determining step. The nucleophile does not appear in the rate law.
Step 1 (slow, rate-determining): the leaving group departs on its own, taking the bonding electrons with it. This generates a carbocation at the carbon that was bonded to the leaving group.
This step has the highest activation energy and controls the overall rate
The substrate does this without any help from the nucleophile
Step 2 (fast): the nucleophile attacks the carbocation, forming the new bond and giving the substitution product.
Because the carbocation is sp2-hybridised and planar, the nucleophile can approach from either face
The carbocation intermediate is flat (trigonal planar), with an empty p orbital perpendicular to the plane of the three remaining groups
The nucleophile can attack from the top face or the bottom face with roughly equal probability
This produces a racemic mixture: a 1:1 ratio of R and S enantiomers
Contrast with SN2, which gives a single enantiomer (inversion). If you see racemisation, think SN1.
The energy diagram has two peaks (two transition states) with a valley between them
The valley represents the carbocation intermediate, a real species sitting in a local energy minimum
The first peak (T.S. 1) is taller than the second (T.S. 2), because step 1 is the slow step
Starting materials on the left, products on the right
Tertiary or resonance-stabilised substrate: the carbocation must be stable enough to form. Tertiary carbocations are the most stable; benzylic and allylic carbocations benefit from resonance. Primary and methyl carbocations are too unstable for SN1 to proceed.
Weak nucleophile: because the nucleophile is not involved in the rate-determining step, a weak nucleophile (solvent molecules such as water or methanol) is sufficient and typical
Polar protic solvent: solvents like water, methanol, and ethanol stabilise both the carbocation and the departing anion through hydrogen bonding and solvation
Good leaving group: just as in SN2, a better leaving group helps, because step 1 requires L.G. departure
Carbocations can rearrange to form more stable cations via 1,2-hydride shifts or 1,2-methyl shifts
If a secondary carbocation can rearrange to a tertiary one, it often will
This is a classic exam trap: the product may not be the one you first predict, because the carbocation rearranged before the nucleophile attacked
A tertiary alkyl bromide reacts with methoxide (⁻OCH₃) in methanol (CH₃OH)
Step 1: Br⁻ departs, forming a tertiary carbocation
Step 2: ⁻OCH₃ (or CH₃OH acting as nucleophile) attacks the planar carbocation from either face
Product: a racemic mixture of the methyl ether, drawn as both enantiomers
A neopentyl-type iodide (with iodine on a primary carbon adjacent to a quaternary carbon) undergoes SN1
Step 1: I⁻ departs, initially forming a primary carbocation (very unstable)
A 1,2-methyl shift rearranges it to a more stable tertiary carbocation
The nucleophile (thiol, ⁻SH) then attacks the tertiary carbocation
The major product comes from the rearranged (tertiary) carbocation; the minor product from the unrearranged position
SN1 Rate Law
\text{Rate} = k[\text{Substrate}]The rate depends only on the concentration of the substrate, not the nucleophile. This is a first-order rate law. Doubling the substrate concentration doubles the rate; doubling the nucleophile concentration has no effect. The "1" in SN1 refers to unimolecular (one species in the rate-determining step).
Energy Diagram (description)
Two humps with a valley in between. The first hump (T.S. 1, slow step) is taller. The valley is the carbocation intermediate. The second hump (T.S. 2, fast step) is shorter. Starting materials sit at the left, products at the right. Label the valley "carbocation intermediate" and the peaks "T.S. 1" and "T.S. 2".
SN1 reactions are common in biological and industrial settings where polar protic environments and tertiary or resonance-stabilised substrates are present. For instance, the solvolysis of tert-butyl chloride in water is a textbook SN1 reaction used in industrial synthesis. In biochemistry, certain enzyme-catalysed glycosyl transfers proceed through oxocarbenium-ion intermediates that resemble SN1 carbocations.
Students often think SN1 gives inversion of stereochemistry, carrying over what they learned from SN2. SN1 gives racemisation (a 1:1 mixture of enantiomers), because the planar carbocation can be attacked from either side.
A frequent error is assuming that a strong nucleophile speeds up SN1. It does not. The nucleophile is absent from the rate law, so its strength or concentration is irrelevant to the rate of an SN1 reaction.
Students sometimes forget that carbocations can rearrange. If a less stable carbocation can shift to a more stable one (e.g. secondary to tertiary via a 1,2-hydride or 1,2-methyl shift), it usually will. The product then comes from the rearranged carbocation, not the original one.
Some students confuse the carbocation intermediate with a transition state. The carbocation is a real species sitting in an energy minimum (a valley on the energy diagram). Transition states sit at energy maxima (peaks) and cannot be isolated.
⚠️ Be ready to draw both steps of the SN1 mechanism, including the carbocation intermediate with its empty p orbital and planar geometry.
⚠️ Expect a problem asking you to identify the stereochemical outcome. If the substrate has a stereocentre, SN1 gives a racemic mixture, not a single enantiomer.
⚠️ Know when to predict SN1 versus SN2. The classic decision tree: tertiary substrate with a weak nucleophile in a polar protic solvent points to SN1. Primary substrate with a strong nucleophile in a polar aprotic solvent points to SN2.
⚠️ Watch for carbocation rearrangements. If a question shows a substrate that would form a secondary carbocation next to a tertiary carbon, consider whether a 1,2-shift will occur before the nucleophile attacks.
⚠️ Energy-diagram questions: SN1 has two transition states and one intermediate. SN2 has one transition state and no intermediate. Being able to sketch and label both diagrams is a frequent exam task.
True or false: SN1 is a concerted, one-step mechanism. ___
The intermediate in an SN1 reaction is a ___.
True or false: increasing the nucleophile concentration increases the rate of an SN1 reaction. ___
An SN1 reaction at a chiral centre produces (inversion / retention / a racemic mixture).
True or false: polar protic solvents favour SN1. ___
Answers: 1. False (it is a two-step mechanism). 2. Carbocation. 3. False (rate depends only on [Substrate]). 4. A racemic mixture. 5. True.
Q: Tert-butyl bromide is dissolved in water. What mechanism operates, and what is the stereochemical outcome?
A: SN1. Tertiary substrate, weak nucleophile (water), polar protic solvent. The product (tert-butanol) forms as a racemic mixture if the carbon was a stereocentre (in this case the carbon is not chiral, so racemisation is not observable, but the mechanism is still SN1).
Q: Draw the two-step mechanism for the SN1 reaction of (R)-3-bromo-3-methylhexane with methanol.
A: Step 1: Br⁻ leaves, forming a tertiary carbocation at C3. Step 2: methanol (CH₃OH) attacks the planar carbocation from either face. The product is a racemic mixture of (R)- and (S)-3-methoxy-3-methylhexane (after deprotonation of the oxonium ion).
Q: Why does SN1 not occur with methyl or primary substrates (under normal conditions)?
A: Methyl and primary carbocations are extremely unstable. The activation energy for forming them is too high, so step 1 does not proceed at a useful rate. SN2 is the pathway for these substrates instead.
Q: A secondary alkyl iodide reacts with ⁻SH in a polar protic solvent. The product is a racemic mixture. What mechanism operated?
A: SN1. Racemisation indicates a planar carbocation intermediate, which is diagnostic of SN1. (If SN2 had occurred, you would see inversion, not racemisation.)
Q: In the lecture example, a primary iodide gave a product from a rearranged tertiary carbocation. Explain.
A: The initial primary carbocation (very unstable) underwent a 1,2-methyl shift to form a much more stable tertiary carbocation. The nucleophile then attacked the rearranged carbocation. The major product therefore has the nucleophile attached at the position of the rearranged (tertiary) carbon, not the original primary carbon.
SN1 pairs directly with SN2 (the companion study notes): exams will ask you to predict which mechanism dominates given a set of conditions. SN1 also competes with E1 (elimination unimolecular), which shares the same first step (carbocation formation) but diverges at step 2 when a base removes a proton instead of a nucleophile attacking. Understanding carbocation stability here feeds directly into electrophilic addition reactions (Chapters 6 to 7), where carbocations also appear as intermediates and can rearrange.
SN1 reaction, substitution nucleophilic unimolecular, carbocation intermediate, racemic mixture, racemisation, carbocation rearrangement, 1,2-hydride shift, 1,2-methyl shift, first-order kinetics, unimolecular rate law, polar protic solvent, tertiary substrate, leaving group, energy diagram two transition states, organic chemistry 1, CHEM 2301 Chapter 5, Hoover, University of Minnesota