Questions: SN1 Mechanism, Kinetics, and Factors Affecting Reactivity

Question 1

You triple the nucleophile concentration in an SN1 reaction while holding substrate concentration and temperature constant. What happens to the reaction rate?

• A. The rate triples, confirming bimolecular kinetics
• B. The rate increases but less than threefold, due to partial nucleophile involvement
• C. The rate does not change, because the nucleophile is not involved in the rate-determining step
• D. The rate decreases, because strong nucleophiles push the mechanism toward SN2

Answer: (C)

SN1 kinetics are first-order: rate = k[substrate]. The nucleophile only participates in the fast second step — attacking the already-formed carbocation — not the slow, rate-determining ionization step. Changing nucleophile concentration therefore has no effect on how quickly carbocations form. This is the sharpest experimental test for SN1 vs. SN2: if nucleophile concentration is irrelevant to rate, you have first-order, unimolecular kinetics. Option D describes what happens mechanistically if you use a strong nucleophile, but that shifts the mechanism to SN2 — it doesn't decrease the SN1 rate.

Question 2

A chiral tertiary alkyl bromide is dissolved in aqueous ethanol and undergoes SN1 hydrolysis. What is the expected stereochemical outcome at the former stereocenter?

• A. Complete inversion of configuration (Walden inversion), as in SN2
• B. Complete retention of configuration, because water attacks the same face the bromide left
• C. Predominantly racemization, with a possible slight excess of inversion product
• D. A 100:0 ratio of enantiomers determined by the chirality of the solvent

Answer: (C)

The key is the planar, sp²-hybridized carbocation intermediate. Because it is flat, the nucleophile can attack from either face with roughly equal probability, producing a near-equal mixture of R and S products — racemization. In practice, the departing bromide may partially shield the face it left, creating a slight excess of the inversion product (ion-pair effects). This distinguishes SN1 from SN2 (which gives clean inversion) and from retention (which would require an unusual double-inversion mechanism). Racemization is a diagnostic hallmark of SN1 at a stereocenter.

Question 3

Polar protic solvents such as water and methanol favor SN1 reactions by stabilizing both the developing carbocation and the departing leaving group through solvation and hydrogen bonding.

• T. True
• F. False

Answer: True

Polar protic solvents accelerate SN1 because ionization — forming a separated carbocation and anion — requires stabilizing both charges. Protic solvents hydrogen-bond to the leaving group anion and solvate the developing positive charge, lowering the energy of the ionization transition state. This is why tertiary alkyl halides that barely react in acetone or DMSO react readily in methanol or water. Polar *aprotic* solvents (DMSO, acetone, DMF) lack this hydrogen-bonding ability and are better for SN2, where they enhance nucleophile reactivity without solvating the TS.

Question 4

A strong nucleophile such as hydroxide (OH⁻) at high concentration will accelerate an SN1 reaction because it rapidly captures the carbocation intermediate in the second step.

• T. True
• F. False

Answer: False

This is a tempting but false inference. A strong nucleophile does not speed up SN1 because the second step is already fast — the carbocation is highly reactive and is captured quickly by whatever nucleophile is present. More importantly, a high concentration of a strong nucleophile tends to *redirect* the mechanism toward SN2, where the nucleophile attacks the substrate in a concerted, backside displacement before full carbocation formation. Strong nucleophiles favor SN2; weak or neutral nucleophiles (water, alcohols) favor SN1.

Question 5

Why does substrate structure (primary vs. secondary vs. tertiary) have such a dramatic effect on SN1 rates, but the effect of substrate structure on SN2 rates runs in the opposite direction?

Model answer: In SN1, the rate-determining step is carbocation formation. Tertiary carbocations are stabilized by three alkyl groups through hyperconjugation and inductive donation, making ionization easy. Secondary are borderline; primary carbocations are too unstable to form under normal conditions, so primary substrates essentially never undergo SN1. In SN2, the nucleophile attacks the back face of the carbon in a concerted step. Tertiary carbons are sterically hindered — three large groups block backside approach — so SN2 is fastest for primary substrates and essentially impossible for tertiary ones. The two mechanisms thus have inverted structural preferences, which is the main tool for predicting which pathway dominates.

The structural preference reversal is the central organizing fact of nucleophilic substitution. Tertiary substrates → SN1; primary substrates → SN2; secondary substrates → context-dependent (solvent, nucleophile, temperature matter). Recognizing which mechanism operates allows prediction of rate, stereochemical outcome (racemization vs. inversion), and product distribution, including the competing elimination reactions that become important for tertiary substrates.