SN1 vs SN2 practice problems, with worked answers

Four checks for any SN1 vs SN2 problem

Run these in order. Most exam problems are settled by the first two.

1. Substrate. Find the carbon holding the leaving group. Methyl or primary points to SN2. Tertiary rules out SN2 and points to SN1. Secondary could go either way, so the next checks decide it. Allylic and benzylic substrates can do both.
2. Nucleophile. A strong, negatively charged nucleophile (I⁻, CN⁻, N₃⁻, HS⁻, RS⁻) pushes toward SN2. A weak, neutral one (water, an alcohol) can't force a backside attack, so it waits for a carbocation: SN1. If the reagent is a strong or bulky base, think about elimination too.
3. Solvent. Polar aprotic solvents (DMSO, DMF, acetone, acetonitrile) speed up SN2. Polar protic solvents (water, methanol, ethanol) help SN1 by stabilizing the ions. When the solvent is also the only nucleophile, it's called solvolysis and is usually SN1.
4. Stereochemistry and rearrangement. SN2 inverts the reacting carbon. SN1 gives a mix of both configurations and can rearrange to a more stable carbocation before the nucleophile adds.

The comparison to remember

A nucleophile donates an electron pair to make a bond. A leaving group takes the electrons from its old bond. For ordinary alkyl substrates, compare these patterns before predicting a product:

The 1 and 2 in the names count the molecules in the slow step, not the number of steps or arrows.

SN2: backside attack and inversion

Draw one curved arrow from a nucleophile's lone pair to the carbon bearing the leaving group. Draw a second from the carbon–leaving-group bond to the leaving-group atom. Both arrows belong to the same step: carbon never becomes a free carbocation or a stable five-bond intermediate.

The incoming nucleophile approaches opposite the leaving group, producing inversion at that carbon. If it is a stereocenter, track its three-dimensional geometry. Do not automatically change an R label to S: replacing the leaving group can also change the priority order used to assign those labels. See OpenStax's SN2 mechanism.

SN1: a carbocation changes the possibilities

First, the carbon–leaving-group bond breaks heterolytically: both bonding electrons go to the leaving group. This slow ionization forms a carbocation. Next, a nucleophile supplies an electron pair to that electron-poor carbon. If water attacks, oxygen initially has three bonds and a positive formal charge; deprotonation then gives a neutral alcohol.

A simple carbocation is approximately planar at the positive carbon, so attack can occur from either face. Starting with one enantiomer at the reacting center can therefore yield both configurations. In theory that gives a 50:50 racemic mix. Real SN1 reactions often give more of the inverted product, because the leaving group hangs around for a moment and blocks one face. Other stereocenters can also make the faces unequal.

Carbocations may rearrange by a neighboring hydride or alkyl shift when a more favorable carbocation becomes accessible. Check for this before attaching the nucleophile; rearrangement is possible, not compulsory. See the SN1 stereochemistry discussion.

Use substrate, reagent, and solvent together

SN2 needs access to carbon. Methyl and unhindered primary substrates are good candidates; secondary substrates are more difficult, and ordinary tertiary substrates block backside attack. Even a primary neopentyl substrate reacts slowly because neighboring branching crowds the approach.

SN1 needs favorable carbocation formation. Ordinary methyl and primary carbocations are too unstable for the usual pathway. Allylic and benzylic substrates are exceptions because resonance can stabilize the positive charge, including in some primary systems. Secondary substrates require closer attention to conditions.

Polar protic solvents, such as water and methanol, stabilize ions and commonly support SN1 ionization. They also hydrogen-bond to anionic nucleophiles, often slowing SN2. Polar aprotic solvents, such as DMSO and acetonitrile, generally make anionic nucleophiles more effective for SN2. Solvent alone cannot choose the mechanism. These substrate and solvent effects are developed in OpenStax's SN2 characteristics.

A good nucleophile need not be a strong base: iodide is a useful example. Strong bases can instead cause elimination, especially with secondary or tertiary substrates. A bulky strong base such as tert-butoxide often favors E2. SN1 conditions can also give E1 products.

Worked examples, step by step

Each example runs the four checks, then gives the product.

Practice problems

Pick an answer, then check it. A wrong answer tells you what went wrong, so you can try again. Each problem also has a full solution.

Frequently asked questions

How do I tell SN1 from SN2 on an exam?

Start with the substrate. Methyl and primary go SN2, and tertiary goes SN1 if it substitutes at all. For secondary substrates, look at the nucleophile and solvent: a strong nucleophile in a polar aprotic solvent means SN2, and a weak nucleophile in a polar protic solvent means SN1.

Is a secondary alkyl halide SN1 or SN2?

It can be either. Secondary substrates follow the conditions: iodide, cyanide, or azide in acetone or DMSO gives SN2 with inversion, while water or an alcohol as the solvent gives SN1 and possibly rearrangement. A strong base can push it toward E2 instead.

Does a good leaving group guarantee substitution?

No. It helps both substitution and many eliminations. Bromide, iodide, and sulfonates are common good leaving groups. An alcohol's unactivated OH group is usually poor; protonation or other activation changes its behavior.

Can SN2 happen in a protic solvent?

Yes. “Favored” describes a trend, not a requirement. Substrate, nucleophile, solvent, and competing pathways must be considered together.

Do these rules apply to bromobenzene?

Not directly. A halogen bonded directly to an aromatic or alkene carbon does not follow ordinary alkyl SN1/SN2 rules. Benzylic means the reacting carbon is next to the ring.