Questions: Nucleophilicity, Basicity, and Leaving Group Ability

Question 1

In a protic solvent like water, you need to perform an SN2 reaction. You have two candidate nucleophiles: fluoride (F⁻) and iodide (I⁻). Which is the better nucleophile, and why?

• A. Fluoride, because it is the stronger base and basicity always determines nucleophilicity
• B. Iodide, because it is larger and less tightly solvated, leaving its electrons more available for attack
• C. Fluoride, because higher charge density means stronger electron donation to carbon
• D. They are equally reactive; both carry a −1 charge, which is the only relevant factor

Answer: (B)

In protic solvents, small anions like F⁻ are tightly surrounded by hydrogen bonds that bury their electron density — nucleophilicity drops even though basicity is high. Larger I⁻ is only weakly solvated, so its diffuse electron cloud remains available for attack at carbon. This is the classic divergence between basicity (thermodynamic) and nucleophilicity (kinetic): F⁻ is the stronger base but I⁻ is the stronger nucleophile in water. The trend in protic solvents is I⁻ > Br⁻ > Cl⁻ > F⁻ — the reverse of basicity order.

Question 2

An alcohol (R-OH) reacts very slowly in SN2 reactions until a strong acid is added to protonate it first. The rate acceleration occurs because:

• A. Protonation increases the nucleophilicity of the attacking nucleophile by raising the solution pH
• B. Protonation converts OH⁻ (a strong base, poor leaving group) into H₂O (a weak base, excellent leaving group)
• C. Protonation increases the electrophilicity of the carbon center by withdrawing electron density inductively
• D. Protonation changes the hybridization of the α-carbon from sp³ to sp²

Answer: (B)

Leaving group ability mirrors conjugate acid weakness: the lower the pKa of the leaving group's conjugate acid, the better the leaving group. H₂O (pKa of H₃O⁺ ≈ −1.7) is a far weaker base than OH⁻ (pKa of H₂O ≈ 15.7), so water departs readily while hydroxide clings to carbon. Protonating the alcohol converts the poor leaving group (OH⁻) into an excellent one (H₂O). This is the key role of acid catalysis in substitution and elimination reactions — it activates the leaving group, not the nucleophile.

Question 3

In polar aprotic solvents (e.g., DMSO or DMF), fluoride is a better nucleophile than iodide.

• T. True
• F. False

Answer: True

In polar aprotic solvents there are no hydrogen-bond donors to solvate anions, so the hydrogen-bond cage that buries F⁻'s electron density in water is absent. Without solvation, nucleophilicity tracks basicity more closely, and fluoride — the smallest, most electronegative halide with highest charge density — is the strongest nucleophile among the halides. The nucleophilicity order in aprotic solvents (F⁻ > Cl⁻ > Br⁻ > I⁻) is essentially the reverse of the protic-solvent order, illustrating how powerfully solvent can invert reactivity trends.

Question 4

Potassium tert-butoxide (t-BuO⁻ K⁺) is both a strong base and a strong nucleophile because it carries a full negative charge on oxygen.

• T. True
• F. False

Answer: False

t-BuO⁻ is indeed a strong base — protons are tiny and easily approached by the oxygen lone pairs. However, it is a poor nucleophile toward carbon electrophiles because the three methyl groups surrounding the oxygen create severe steric hindrance. Carbon electrophilic centers in alkyl halides are far more sterically demanding than a bare proton. This is the steric argument for basicity-nucleophilicity divergence: bulky bases can grab protons but cannot reach into crowded carbon centers. In reactions with secondary or tertiary alkyl halides, t-BuO⁻ favors E2 elimination over SN2 substitution precisely because it is a better base than nucleophile.

Question 5

Explain why basicity and nucleophilicity are related but distinct properties. Identify one factor that causes them to diverge and explain the mechanism of that divergence.

Model answer: Basicity is a thermodynamic property measuring the equilibrium extent to which a species accepts a proton (quantified by pKa of the conjugate acid). Nucleophilicity is a kinetic property measuring the rate at which a species attacks an electrophilic carbon center. Both involve electron donation, but the reaction partner, geometry, and conditions differ. A key divergence factor is solvent: in protic solvents, small, highly basic anions like F⁻ are tightly hydrogen-bonded, burying their electron density and slowing carbon attack. Large, polarizable anions like I⁻ are weakly solvated, so their electrons remain kinetically available — making them excellent nucleophiles despite being weak bases.

Steric bulk is another divergence factor: t-BuO⁻ is a strong base but poor nucleophile because bulky methyl groups block approach to sp³ carbon. Polarizability also matters: large atoms have diffuse electron clouds that begin overlapping with an electrophilic center at longer range, increasing nucleophilicity beyond what basicity predicts. The fundamental distinction is that basicity describes equilibrium with a tiny proton, while nucleophilicity describes a bimolecular rate constant toward a much larger, more sterically demanding carbon center.