H3 Chemistry 9813 · Study focus: H3 Chemistry: Explain nucleophile effects on substitution rate
H3 Chemistry: Explain nucleophile 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 nucleophile effects on substitution rate
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Hold the substrate constant
Compare nucleophiles using the same haloalkane, leaving group, temperature and stated medium so that the nucleophile is the intended variable.
For an SN2 reaction, both nucleophile identity and nucleophile concentration can affect rate because nucleophile attack occurs in the rate-determining elementary step.
Separate nucleophilicity from concentration
A nucleophile donates an electron pair to the electrophilic carbon; the curved arrow starts at that pair and ends at carbon.
Nucleophilicity describes reaction tendency under matched conditions, whereas [Nu] is an independently varied concentration in rate = k[RX][Nu].
At constant [1-bromopropane], doubling [CN−] doubles the initial SN2 rate and supports first order in cyanide.
For a genuinely unimolecular SN1 ionisation, changing nucleophile concentration does not change the ionisation rate even though the nucleophile is required later to trap the carbocation.
| Substrate | T / K | [RX] / mol dm⁻³ | Nucleophile | [Nu] / mol dm⁻³ | Substitution rate / 10⁻⁴ mol dm⁻³ s⁻¹ | Product scope |
|---|---|---|---|---|---|---|
| 1-bromopropane | 298 | 0.05 | CN⁻ | 0.04 | 1.2 | substitution only |
| 1-bromopropane | 298 | 0.05 | CN⁻ | 0.08 | 2.4 | substitution only |
| 1-bromopropane | 298 | 0.05 | CN⁻ | 0.12 | 3.6 | substitution only |
| 1-bromopropane | 298 | 0.05 | OH⁻ | 0.05 | 4.8 | illustrative substitution-product rate; elimination excluded |
| 1-bromopropane | 298 | 0.05 | H₂O | 0.05 | 0.05 | illustrative substitution-product rate; elimination excluded |
| 1-bromopropane | 298 | 0.05 | CH₃O⁻ | 0.05 | 6.2 | illustrative substitution-product rate; elimination excluded |
| 1-bromopropane | 298 | 0.05 | tert-butoxide | 0.05 | 0.6 | illustrative substitution-product rate; elimination excluded |
Text alternative: The text alternative lists every concentration and rate, states which variables are fixed, and names the proportional trend and steric comparison.
Read matched kinetic evidence
At constant [1-bromopropane], doubling [CN−] doubles the initial SN2 rate and supports first order in cyanide.
For a genuinely unimolecular SN1 ionisation, changing nucleophile concentration does not change the ionisation rate even though the nucleophile is required later to trap the carbocation.
Cyanide concentration series
Runs with [CH3CH2CH2Br] = 0.050 mol dm−3 and [CN−] = 0.040, 0.080 and 0.120 mol dm−3 give initial rates 1.2, 2.4 and 3.6 × 10−4 mol dm−3 s−1.
The proportional rate increase identifies first order in CN− for the matched SN2 experiment.
- Calculate rate/[CN−] for all three runs and state what remains constant.
Open the feedback checkpoint after attempting
- Each quotient is 3.0 × 10−3 s−1 for the fixed substrate concentration; this is k[RX], not the bimolecular rate constant k itself.
Compare charged and neutral donors
Hydroxide and water both donate an oxygen lone pair, but OH− is normally the more effective nucleophile in a matched comparison with a primary bromoalkane.
Do not claim that charge alone fixes every nucleophile order; use the supplied identities and controlled conditions.
- Predict which matched reaction of 1-bromobutane is faster at equal nucleophile concentration: OH− or H2O, and draw the attack arrow.
Open the feedback checkpoint after attempting
- Choose OH−; draw the arrow from an oxygen lone pair to the carbon bearing Br and a simultaneous C–Br bond arrow to Br−.
Distinguish steric access
Methoxide attacks an unhindered primary carbon more readily by SN2 than bulky tert-butoxide under a fixed comparison where substitution is measured.
A rate conclusion must not silently count elimination product as substitution product.
- Given separate substitution-product rates for CH3O− and (CH3)3CO− with 1-iodobutane, justify the faster SN2 nucleophile without invoking solvent effects.
Open the feedback checkpoint after attempting
- Credit reduced steric obstruction to carbon attack for methoxide and use only the measured substitution rates.
Repair the ‘nucleophile never matters’ claim
Nucleophile concentration appears in the SN2 rate law because bond formation occurs in the single concerted step.
Nucleophile concentration is absent from the simple SN1 rate law because ionisation occurs before nucleophile capture.
- Correct: ‘Since the nucleophile is not in the haloalkane, changing [Nu−] cannot affect substitution rate.’
Open the feedback checkpoint after attempting
- State the mechanism dependency: rate ∝ [Nu−] for SN2 but the simple SN1 ionisation rate is independent of [Nu].
Assess nucleophile evidence
Finish the twelve matched-case diagnostics before opening the unseen azide experiment.
After the delayed thiolate case, isolate leaving-group effects with substrate and nucleophile controlled.
- Attempt the stored nucleophile assessment and mark the rate order, numerical constant and curved-arrow evidence.
Open the feedback checkpoint after attempting
- Do not proceed from a strength label alone; next compare C–X bond strength and the stability of each departing species.