H3 Chemistry 9813 · Study focus: H3 Chemistry: Explain substituent effects on substitution rate
H3 Chemistry: Explain substituent effects on substitution rate
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Explain substituent effects on substitution rate
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Specify the pathway before ranking substrates
Alkyl substitution has opposite kinetic consequences for backside SN2 attack and carbocation-forming SN1 ionisation.
A rate ranking is meaningless unless substrate, leaving group, nucleophile and the operative pathway are stated.
Separate steric and electronic effects
Crowding at the reacting carbon obstructs the SN2 approach trajectory.
Alkyl groups stabilise developing or complete carbocation charge, lowering the barrier for SN1 ionisation.
For methyl, primary, secondary and tertiary halides with a fixed strong nucleophile, the SN2 tendency falls as substitution increases.
For matched ionisation conditions and the same leaving group, tertiary carbocation formation is more favourable than secondary, while primary and methyl carbocations are strongly disfavoured.
| Mechanism | Substrate/target | Class | Backside access / ionisation barrier | Relative channel rate | Simple rate law |
|---|---|---|---|---|---|
| SN2 | CH₃Br | methyl | open | 100 | k[RX][Nu] |
| SN2 | CH₃CH₂Br | primary | low crowding | 38 | k[RX][Nu] |
| SN2 | (CH₃)₂CHBr | secondary | moderate crowding | 3 | k[RX][Nu] |
| SN2 | (CH₃)₃CBr | tertiary | blocked | <0.01 | k[RX][Nu] |
| SN1 | CH₃⁺ target | methyl | prohibitive ionisation | negligible | k[RX] |
| SN1 | primary R⁺ | primary | high barrier | low | k[RX] |
| SN1 | secondary R⁺ | secondary | medium barrier | medium | k[RX] |
| SN1 | tertiary R⁺ | tertiary | lower barrier | high | k[RX] |
Text alternative: The text alternative announces the mechanism before each ranking and states why increasing alkyl substitution reverses its effect between the two rows.
Build both matched series
For methyl, primary, secondary and tertiary halides with a fixed strong nucleophile, the SN2 tendency falls as substitution increases.
For matched ionisation conditions and the same leaving group, tertiary carbocation formation is more favourable than secondary, while primary and methyl carbocations are strongly disfavoured.
Backside attack data
With 0.050 mol dm−3 CN−, equal methyl bromide, bromoethane, 2-bromopropane and tert-butyl bromide give relative substitution rates 100 : 38 : 3 : less than 0.01 for the SN2 channel.
The fall follows increasing steric obstruction at the carbon bearing Br.
- Draw the backside approach for bromoethane and tert-butyl bromide and identify the steric collision in the tertiary case.
Open the feedback checkpoint after attempting
- Credit an approach opposite C–Br and three methyl groups shielding the tertiary carbon; do not draw a tertiary SN2 intermediate.
Carbocation stability series
Ionisation of tert-butyl bromide produces a tertiary carbocation stabilised by three alkyl substituents.
Ionisation of bromoethane would produce a much less stable primary carbocation and a higher barrier.
- Rank tert-butyl bromide, 2-bromopropane and bromoethane for a stated SN1 ionisation and justify the extremes.
Open the feedback checkpoint after attempting
- tert-Butyl > secondary >> primary; connect substitution to carbocation/developing-charge stabilisation.
Detect branching beyond the first carbon
Neopentyl bromide is primary at the reacting carbon but unusually slow in SN2 because adjacent branching blocks backside access.
Classification as ‘primary’ is therefore a starting point, not a substitute for inspecting the whole local geometry.
- Compare SN2 attack on 1-bromobutane and neopentyl bromide with CN−.
Open the feedback checkpoint after attempting
- Choose 1-bromobutane; the adjacent tert-butyl framework of neopentyl bromide creates severe steric obstruction despite primary classification.
Repair one universal rate order
Tertiary is not universally fastest: it favours SN1 but blocks SN2.
Primary is not universally fastest: it favours SN2 but gives an unstable carbocation for SN1.
- Correct: ‘More alkyl groups always increase substitution rate.’
Open the feedback checkpoint after attempting
- State opposite pathway effects—alkyl substitution stabilises SN1 charge development but hinders SN2 backside approach.
Assess matched substrate effects
Complete the twelve substrate diagnostics before opening the unseen branching rate set.
After the different cyclopropyl-free retest, connect the substrate trends to complete SN1 and SN2 rate laws and profiles.
- Attempt the stored substituent-rate assessment and mark pathway, steric/electronic cause and controlled variables.
Open the feedback checkpoint after attempting
- Use only the matched evidence supplied, then test the proposed pathway against concentration changes and profile topology.