Hydroxide swaps for bromine in every halogenoalkane, but not always the same way. Primary ones react in a single concerted step; tertiary ones fall apart first into a carbocation. The structure decides the route.
SN2 (substitution, nucleophilic, bimolecular) happens in one step: the nucleophile attacks the δ+ carbon from the side opposite the halogen while the C–X bond breaks, through a transition state with partial bonds. It is typical of primary halogenoalkanes, and its rate depends on both reactants. SN1 happens in two steps: the C–X bond breaks heterolytically (slowly) to give a carbocation, which the nucleophile then attacks (quickly). It is typical of tertiary halogenoalkanes, because the tertiary carbocation is stabilised by three alkyl groups and the crowded carbon blocks a backside attack. Its rate depends only on the halogenoalkane.
Step through both mechanisms in the simulator, first with bromoethane and then with 2-bromo-2-methylpropane. Watch where the nucleophile approaches from.
| SN2 | SN1 | |
|---|---|---|
| typical substrate | primary | tertiary |
| steps | one; transition state | two; carbocation intermediate |
| rate equation | rate = k[RX][OH−] | rate = k[RX] |
| curly arrows | OH− lone pair to Cδ+; C–X bond to X | step 1: C–X bond to X; step 2: OH− lone pair to C+ |
| chiral substrate gives | inverted product | racemic mixture (planar carbocation attacked from either face) |
Secondary halogenoalkanes can react by either mechanism.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which mechanism is typical of a primary halogenoalkane with hydroxide ions?
Which describes the SN1 mechanism?
What is the rate equation for the SN2 hydrolysis of bromoethane?
Why is a tertiary carbocation more stable than a primary one?
Compare the rates of the three classes in the hydrolysis simulator.
Curly arrows start from the lone pair on OH−, not from the minus sign, and the arrow from the C–X bond must point to the halogen. In SN2 draw the transition state in square brackets with partial bonds and an overall negative charge; SN2 has no carbocation.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Why do tertiary halogenoalkanes not react by SN2?
A single enantiomer of 2-bromobutane is hydrolysed by an SN1 mechanism. What is the product?
In the SN2 mechanism, where does the first curly arrow start?
The rate of hydrolysis of 2-bromo-2-methylpropane is unchanged when [OH−] is doubled. What does this show?
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