In any pair of SN2 reactions, the faster one is determined by three factors: how crowded the carbon being attacked is, how good the nucleophile is, and how easily the leaving group departs. Once you know how each factor speeds up or slows down the reaction, you can compare almost any two SN2 reactions at a glance.
Steric Hindrance Is the Biggest Factor
The SN2 mechanism requires a nucleophile to attack the back side of a carbon, directly opposite the leaving group. Anything that blocks that approach slows the reaction dramatically. The general speed ranking by substrate type is:
- Methyl halides (CH₃X): Fastest. Three small hydrogen atoms surround the carbon, leaving the back side wide open.
- Primary halides (RCH₂X): Fast. One alkyl group adds some bulk but doesn’t block attack much.
- Secondary halides (R₂CHX): Slow. Two alkyl groups crowd the carbon significantly.
- Tertiary halides (R₃CX): Essentially no SN2 reaction. Three alkyl groups block backside attack almost completely.
So if one reaction in your pair uses a methyl substrate and the other uses a secondary substrate, the methyl substrate reacts faster every time. This single comparison resolves most textbook pairs.
Branching Near the Reaction Site Matters Too
It’s not just the carbon being attacked that matters. Bulky groups on the neighboring carbon (the beta position) also block the nucleophile’s approach. Neopentyl bromide is a dramatic example: the carbon bearing the bromine is technically primary, yet it reacts about 100,000 times slower than a typical primary alkyl bromide. The massive tert-butyl group next door blocks backside attack so effectively that SN2 reactions on neopentyl systems barely happen at all. If your pair includes a neopentyl-type substrate against a simple primary substrate, the simple primary one is far faster.
Aryl and Vinyl Halides Don’t React by SN2
If one member of the pair is an aryl halide (halogen bonded to a benzene ring) or a vinyl halide (halogen bonded to a double-bond carbon), that reaction effectively doesn’t proceed by SN2. The nucleophile would need to attack through the plane of the ring or double bond, which is sterically impossible. The leaving group is also held more tightly on these sp²-hybridized carbons. In a comparison, any standard alkyl halide wins.
Stronger Nucleophiles Speed Things Up
A better nucleophile attacks the carbon faster, lowering the energy barrier for the reaction. Two principles help you rank nucleophiles quickly.
First, a negatively charged species is always a stronger nucleophile than its neutral counterpart. Hydroxide (OH⁻) is a much better nucleophile than water (H₂O). If one reaction in your pair uses a charged nucleophile and the other uses a neutral one, the charged nucleophile wins.
Second, within the same row of the periodic table, nucleophilicity tracks with basicity when steric effects are equal. A stronger base donates electrons more readily, making it a better nucleophile. For example, comparing neutral alcohols, a simple alkoxide is more nucleophilic than a carboxylic acid because it’s more basic.
Going down a column of the periodic table adds a twist. Larger atoms hold their electrons more loosely, which makes them more nucleophilic even though they’re weaker bases. Sulfur-based nucleophiles are stronger than oxygen-based ones, and phosphorus-based nucleophiles are stronger than nitrogen-based ones. So iodide (I⁻) is a better nucleophile than fluoride (F⁻) in a protic solvent, despite fluoride being a far stronger base.
Better Leaving Groups Make Faster Reactions
The leaving group must accept the bonding electrons and depart as a stable species. Weak bases are good at this because they’re comfortable holding onto electrons. The strength of a leaving group correlates with the pKa of its conjugate acid: the lower the pKa, the better the leaving group.
For the halogens, the ranking from best to worst leaving group is:
- Iodide (I⁻): Best. Conjugate acid HI has a pKa of about −9.
- Bromide (Br⁻): Very good.
- Chloride (Cl⁻): Good. Conjugate acid HCl has a pKa of about −7.
- Fluoride (F⁻): Terrible. SN2 reactions with fluoroalkanes are rarely observed.
If two reactions are identical except one has a bromide leaving group and the other has a chloride, the bromide version is faster. Tosylate and mesylate groups are also excellent leaving groups you may see in comparison pairs, roughly on par with iodide and bromide.
Solvent Choice Can Flip the Outcome
Solvents affect SN2 rates more than most students expect. The key distinction is between protic solvents (like water or alcohols, which have O-H or N-H bonds) and polar aprotic solvents (like DMSO, DMF, or acetonitrile, which are polar but lack those bonds).
Protic solvents slow SN2 reactions. They form a shell of hydrogen bonds around the nucleophile, stabilizing it and making it less reactive. The nucleophile has to shed that solvent shell before it can attack, which costs energy.
Polar aprotic solvents speed SN2 reactions. They dissolve ionic compounds well because they solvate the positive metal cation, but they leave the nucleophilic anion relatively bare and unsolvated. That exposed, high-energy nucleophile attacks the substrate much more aggressively. If your pair compares the same reaction in two different solvents, the one in a polar aprotic solvent (DMSO, DMF, acetonitrile) is faster.
How to Compare Any Pair Step by Step
When you’re given two SN2 reactions and asked which is faster, work through the differences systematically. Most pairs differ in only one variable, which makes the comparison straightforward. Here’s how to think through it:
- Different substrates, same everything else: The less sterically hindered substrate reacts faster. Methyl beats primary, primary beats secondary. Tertiary substrates don’t do SN2 at all.
- Different leaving groups, same everything else: The weaker base leaves faster. Iodide beats bromide beats chloride. Fluoride barely reacts.
- Different nucleophiles, same everything else: The stronger nucleophile (charged over neutral, larger atom in protic solvent, more basic if same size) gives the faster reaction.
- Different solvents, same everything else: Polar aprotic solvents give faster SN2 rates than protic solvents.
When the pair differs in more than one variable, steric hindrance at the substrate usually dominates. A methyl halide with a mediocre nucleophile still reacts faster by SN2 than a secondary halide with a strong nucleophile. Substrate structure is the factor that causes the largest rate differences, sometimes by factors of thousands or more, while nucleophile and leaving group differences typically cause smaller (though still significant) rate changes.