Explain nucleophile effects on substitution rate

Compare nucleophiles using the same haloalkane, leaving group, temperature and stated medium so that the nucleophile is the intended variable.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Explain nucleophile effects on substitution rate

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].

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.

Separate two experiments that are often mixed up: changing nucleophile concentration tests the rate law, while changing nucleophile identity compares how readily different electron-pair donors attack under matched conditions.

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.

Try this

Calculate rate/[CN⁻] for all three runs and state what remains constant.

Check your answer

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

Supplied comparison: 1-bromobutane with OH⁻ or with H₂O at equal nucleophile concentration and matched conditions. Both donate an oxygen lone pair.

Compare how available each oxygen lone pair is. Do not claim that charge alone fixes every nucleophile order; use the supplied identities and controlled conditions.

Try this

Predict which matched reaction of 1-bromobutane is faster at equal nucleophile concentration: OH⁻ or H₂O, and draw the attack arrow.

Check your answer

OH⁻ is faster: its negative charge makes its oxygen lone pair more available than water's. Draw the arrow from an oxygen lone pair to the carbon bearing Br, with a simultaneous arrow from the C–Br bond to Br⁻.

Distinguish steric access

Supplied data: separate substitution-product rates for CH₃O⁻ and (CH₃)₃CO⁻ with 1-iodobutane under a fixed comparison.

Compare how easily each oxygen can reach the back of the primary carbon. A rate conclusion must not silently count elimination product as substitution product.

Try this

Given separate substitution-product rates for CH₃O− and (CH₃)₃CO− with 1-iodobutane, justify the faster SN2 nucleophile without invoking solvent effects.

Check your answer

Methoxide is the faster SN2 nucleophile: its single methyl group leaves the oxygen free to reach the carbon, while the three methyl groups of tert-butoxide obstruct backside approach. The conclusion rests only on the measured substitution rates.

Correct 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.

Try this

Correct: ‘Since the nucleophile is not in the haloalkane, changing [Nu⁻] cannot affect substitution rate.’

Check your answer

State the mechanism dependency: rate ∝ [Nu⁻] for SN2 but the simple SN1 ionisation rate is independent of [Nu].

Assess nucleophile evidence

Answer the check questions matched-case check questions before trying the unseen azide experiment.

Later, try the thiolate case; isolate leaving-group effects with substrate and nucleophile controlled.

For rate data, show the concentration factor and rate factor before stating the order. Then connect that order to whether the nucleophile appears in the rate-determining step and draw the attack arrow from the electron pair.

Try this

Attempt the nucleophile final practice question and mark the rate order, numerical constant and curved-arrow evidence.

Check your answer

Do not proceed from a strength label alone; next compare C–X bond strength and the stability of each departing species.