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SN2 is one-step back-side attack, blocked by crowding, so it favours methyl and primary substrates; SN1 goes through a carbocation stabilized by carbon groups, so it favours tertiary ones; for secondary substrates the nucleophile and solvent decide.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will classify a substrate's carbon, choose SN1 or SN2 from its structure, rank SN2 rates, sort features between the mechanisms, and decide a secondary substrate's mechanism from its conditions.
You know the SN1 and SN2 rate laws and what each does to a stereocentre. This lesson is about choosing between them from the substrate's structure.
The substrate is the molecule attacked. Its carbon carrying the leaving group is methyl (no carbons attached), primary (one), secondary (two) or tertiary (three). Steric hindrance is crowding that blocks an approach. A polar protic solvent (water, alcohols) has O–H bonds that stabilize ions.
SN2 — the nucleophile attacks the carbon's back side as the leaving group leaves, in one step. Every carbon group on that carbon gets in the way:
methyl > primary > secondary >> tertiary (almost no SN2).
SN1 — the leaving group departs first, leaving a carbocation, which the nucleophile then catches. Carbon groups stabilize a cation by pushing electron density towards it, so:
tertiary > secondary >> primary, methyl (almost no SN1).
The two trends run in opposite directions, so the class of the carbon usually decides:
| Class | Mechanism |
|---|---|
| methyl, primary | SN2 |
| secondary | either: conditions decide |
| tertiary | SN1 |
Another way: steps
To choose:
A secondary substrate sits between the two trends. A strong nucleophile such as cyanide or iodide in acetone, a polar solvent without O–H bonds, pushes in from the back: SN2, with inversion. A weak nucleophile such as water or ethanol, which is also the solvent and stabilizes ions, lets the substrate ionize first: SN1, with racemization. So 2-bromobutane gives inverted product with iodide in acetone and largely racemic product warmed in water.
A stronger nucleophile always proves an SN1 mechanism. A strong nucleophile favours SN2.
Tertiary substrates react fastest by SN2. They barely react by SN2 at all.
One mechanism applies to every alkyl halide. The class decides, and for secondary the conditions do.
Primary substrates go SN1. Their cations are too unstable.
The C–Br carbon has one carbon neighbour: primary.
Classify first.
Primary: open at the back, a poor cation. So SN2.
Structure decides.
Cyanide attacks in one step, giving propanenitrile with rate = k[CH3CH2Br][CN−].
One step, second order.
What class is the C–Cl carbon?
Tertiary: three carbon neighbours.
Which mechanism does a tertiary carbon favour?
SN1, through the stable tert-butyl cation.
In bromomethane, CH3Br, how many carbon atoms are bonded to the carbon that carries the halogen?
Answer:
bromomethane, CH3Br, reacts with a nucleophile. Which substitution mechanism does its structure favour?
Rank these bromides from fastest to slowest reaction by SN2 with iodide: 2-bromo-2-methylpropane, bromoethane, bromomethane, 2-bromopropane.
Number the steps in order (write the number in the box):
Match each feature to the mechanism it belongs to.
| SN2 | SN1 | |
|---|---|---|
| rate = k[substrate][nucleophile] | ||
| inversion at the carbon | ||
| a carbocation intermediate | ||
| fastest for tertiary substrates |
2-bromobutane, a secondary bromide, is warmed in water with no other reagent. Water is a weak nucleophile and a polar protic solvent. Which mechanism is favoured, and what does it predict for a single enantiomer of the starting material?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Each substrate reacts with a good nucleophile. Classify it and give the substitution mechanism its structure favours.
| class | mechanism | |
|---|---|---|
| bromomethane | ||
| 1-chloropropane | ||
| 2-bromo-2-methylpropane |
You can choose a substitution mechanism from a structure. Tell someone why tertiary halides barely react by SN2. Next: the elimination reactions that compete with substitution.
8. Your turn: 2-chloro-2-methylpropane in water, step 3