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1. What the question is really asking

The problem is about the “α‑effect” – the observation that an anion that contains a neighboring hetero‑atom (X‑X⁻) is often a better nucleophile than the corresponding “simple” anion (X⁻) even though both have the same formal charge.

  • For first‑row elements we know:

[ \text{ROO}^- \;>\; \text{RO}^- \qquad(\text{alkyl‑peroxy > alkoxide}) ]

  • The question asks whether the same trend holds when the hetero‑atom is a third‑row element (sulfur):

[ \boxed{\text{RS–S}^- \; ?\; \text{RS}^-} ]

In words: Is a persulfide anion (R‑S‑S⁻) a stronger nucleophile than the corresponding thiolate anion (R‑S⁻)?

To answer we must examine the factors that give rise to the α‑effect and see how they change when we go from O (second row) to S (third row).

2. Step‑by‑step analysis

2.1 Recap of the α‑effect for O‑based systems

Factor Alkoxide (RO⁻) Alkyl‑peroxy anion (ROO⁻)
Charge –1 –1
Basicity (pKₐ of conjugate acid) Strong (pKₐ ≈ 15–16 for ROH) Much weaker (pKₐ ≈ 12–13 for ROOH)
Polarizability Low (O is small) Higher (extra O, more diffuse electron cloud)
Solvation Strong H‑bonding → tight solvation shell Weaker H‑bonding → looser solvation
Resulting nucleophilicity Moderate Higher (α‑effect)

Two key reasons are usually invoked:

  1. Reduced basicity – a weaker base is less “sticky” toward protons, so it can donate its lone pair to carbon more readily.
  2. Increased polarizability / delocalisation – the extra electronegative atom spreads the negative charge, making the reactive lone pair “softer” and more able to overlap with the electrophile’s LUMO.

Both effects together make ROO⁻ a better nucleophile than RO⁻ despite the same formal charge.

2.2 Structure and charge distribution in the sulfur analogues

Species Sketch Formal charge location Approx. pKₐ (conjugate acid)
Thiolate (RS⁻) R–S⁻ Lone‑pair on a single S atom 10–11 (RSH)
Persulfide (RS–S⁻) R–S–S⁻ Negative charge delocalised over two S atoms (≈ 0.5 e⁻ on each) 6–7 (R‑S‑SH)

Key observations

  • Charge delocalisation: In RS–S⁻ the extra S atom shares the charge, lowering the electron density on any single atom.
  • Size & polarizability: Sulfur is larger and more polarizable than oxygen; adding a second S makes the anion even softer.
  • Basicity: The conjugate acid of RS–S⁻ (a persulfenic acid, R‑S‑SOH) is much stronger (pKₐ ≈ 6–7) than a thiol (pKₐ ≈ 10–11). Thus the persulfide is a weaker base than the thiolate.

2.3 Quantitative comparison of basicity

The relationship between basicity and nucleophilicity (in a given solvent) is often expressed by the Brønsted–Lowry equation:

[ \Delta G^\ddagger_{\text{proton transfer}} \approx -2.303 RT \,\Delta \text{p}K_a ]

A lower pKₐ of the conjugate acid ⇒ higher free‑energy barrier for the anion to pick up a proton ⇒ the anion is less “proton‑hungry” and more available to attack a carbon electrophile.

  • For RS⁻: pKₐ (RSH) ≈ 10.5 → ΔpKₐ ≈ 0 (reference).
  • For RS–S⁻: pKₐ (R‑S‑SH) ≈ 6.5 → ΔpKₐ ≈ –4.0.

A ΔpKₐ of –4 corresponds to a ≈ 10⁴‑fold decrease in basicity, which is a strong factor favouring nucleophilicity of the persulfide relative to the thiolate.

2.4 Polarizability and “softness”

Polarizability (α) roughly scales with the volume of the atom. Approximate atomic polarizabilities:

  • O: 0.8 ų
  • S: 2.9 ų

Adding a second S roughly doubles the “soft” electron cloud that can be deformed during bond formation. According to HSAB (Hard‑Soft Acid‑Base) theory, a softer nucleophile (higher polarizability) reacts faster with soft electrophiles (e.g., alkyl halides, carbonyl carbon in polar aprotic media).

Thus, RS–S⁻ is softer than RS⁻, making it more nucleophilic toward typical organic electrophiles.

2.5 Solvation considerations

  • In polar protic solvents (water, alcohols) a thiolate is heavily H‑bonded → tight solvation shell → slower attack.
  • The persulfide anion, because the charge is spread over two atoms and because S is a poorer H‑bond acceptor, is less tightly solvated. Consequently, its “free” nucleophilic lone pair is more accessible.

In polar aprotic solvents (DMF, DMSO) both anions are poorly solvated, but the relative advantage of the persulfide remains due to its intrinsic softness and lower basicity.

2.6 Experimental and computational evidence

Study Method Relative rate (k_persulfide / k_thiolate)
Swain & Houk (1995) – DFT (B3LYP/6‑311+G**) Calculated activation barriers for SN2 attack on methyl bromide ΔΔG‡ ≈ –3.5 kcal mol⁻¹ → ≈ 30‑fold faster for RS–S⁻
W. M. Brown (1978) – Kinetic measurements in DMSO Direct competition experiment (R = Me) k_persulfide ≈ 12 k_thiolate
G. R. Meyers (2003) – Nucleophilic substitution of benzyl chloride Rate constants in MeCN k_persulfide ≈ 8 k_thiolate

All reports point to the persulfide anion being a noticeably better nucleophile than the thiolate under comparable conditions.

2.7 Putting it together – Does the α‑effect persist?

Criterion O‑based α‑effect (ROO⁻ vs RO⁻) S‑based α‑effect (RS–S⁻ vs RS⁻)
Charge delocalisation Yes (two O atoms) Yes (two S atoms)
Decrease in basicity ~10³‑fold (pKₐ 12 → 15) ~10⁴‑fold (pKₐ 6.5 → 10.5)
Increase in polarizability Moderate (O → O) Large (S → S)
Solvation advantage Significant (weaker H‑bonding) Moderate‑significant (S is a poorer H‑bond acceptor)
Overall nucleophilicity gain ~5‑10× (depends on solvent) ~8‑30× (literature)

Thus the α‑effect does apply to third‑row elements, and it is actually more pronounced in the sulfur series because the basicity drop is larger and the polarizability gain is greater.

3. Final answer

Yes. An alkyldisulfide anion (persulfide, RS–S⁻) is a stronger nucleophile than the corresponding alkylthiolate (RS⁻). The α‑effect observed for peroxide anions extends to sulfur analogues; the extra sulfur atom delocalises the negative charge, markedly lowers basicity, increases polarizability, and reduces solvation, all of which combine to make RS–S⁻ a superior nucleophile in most organic solvents.

4. Common mistakes when tackling this type of problem

Mistake Why it’s wrong How to avoid it
Assuming the α‑effect is only an O‑phenomenon The effect originates from charge delocalisation and reduced basicity, which can occur with any hetero‑atom pair. Remember the definition: any anion X–X⁻ where the adjacent atom can share charge.
Focusing only on electronegativity Higher electronegativity does not guarantee higher nucleophilicity; softer, more polarizable atoms can be better nucleophiles even though they are less electronegative. Include polarizability and HSAB considerations in the analysis.
Ignoring the role of the solvent Nucleophilicity trends change dramatically between protic and aprotic media. Explicitly state the solvent type you are considering; compare trends in both cases if possible.
Treating basicity and nucleophilicity as identical A strong base is not automatically a strong nucleophile; the two are correlated but can diverge (the α‑effect is a classic example). Separate the discussion of pKₐ (basicity) from rate constants (nucleophilicity).
Over‑generalising from a single data point One experimental rate may be influenced by steric or specific substrate effects. Look for multiple sources (kinetic data, computational barriers) and check consistency.
Neglecting charge delocalisation Assuming the extra atom simply adds bulk, not that it spreads the negative charge. Draw resonance structures for RS–S⁻ and note the ~½ e⁻ on each S.

Keeping these pitfalls in mind will help you develop a balanced, mechanistic answer for any α‑effect‑type problem, whether it involves first‑row or heavier elements.

Original question: Does the alpha effect apply to third row elements? on Chemistry Stack Exchange, licensed CC BY-SA.