Question 5 of 13: Solvent Effects on Three Reactions (Hughes–Ingold Rules)

Question 5: Solvent Effects on Three Reactions (Hughes–Ingold Rules)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Governing principle (Hughes–Ingold). A change to a more polar solvent accelerates a reaction whose transition state is more charge-separated (more polar, more ionic) than its ground-state reactants, because the polar solvent preferentially stabilises the more-charged species; conversely it retards a reaction whose transition state is less charge-separated than the starting material, because the polar solvent preferentially stabilises the (already more polar) ground state, raising the effective barrier. Each part below is answered by comparing the polarity/charge of the reactant(s) to that of the transition state.

1. (i) Ph₃P + Br₂ → Ph₃P⁺Br Br⁻: ACCELERATED by a polar solvent.
   Ph3P (neutral)

   Ph3PBr+ Br- (ion pair)
   Two neutral reactants combine to generate a fully charge-separated ion pair; the transition state, part-way along the reaction coordinate, already carries substantial developing positive charge on P and negative charge on the departing Br. This is the textbook case of charge creation: a polar solvent stabilises that developing charge far better than it stabilises the neutral starting materials, lowering the barrier and speeding the reaction.
2. (ii) Betaine → CO₂ + NMe₃: RETARDED by a polar solvent.
   carbamate betaine — already charge-separated starting material
   Here the situation is reversed: the starting material itself is already a fully charge-separated zwitterion (a discrete + and − charge held within one molecule), while the products (CO₂, NMe₃) are both entirely neutral. The reaction therefore destroys charge separation on the way to the transition state/products. A polar solvent stabilises the already-polar starting betaine more than it stabilises the far-less-polar transition state, which raises the effective activation barrier — the reverse of case (i), and the classic example used to illustrate that a more polar solvent does not universally speed every polar reaction; it only speeds reactions that are becoming more polar.
3. (iii) Ester + NH₃ → amide: mildly ACCELERATED by a polar solvent.
   ester (model, R=Me)

   amide product
   Both reactants are neutral, and both products (amide + MeOH) are neutral overall, so this looks superficially like case (ii) — but the mechanism proceeds through a tetrahedral addition intermediate/transition state in which ammonia's nitrogen has begun bonding to the carbonyl carbon while significant negative charge has begun developing on the (former carbonyl) oxygen — i.e., the transition state carries meaningfully more charge separation than either the neutral ester or the neutral amide product. A polar solvent stabilises that developing dipolar character, giving a real but smaller rate enhancement than in case (i) (where the transition state develops full, permanent ionic character, not merely a transient partial charge separation that collapses again once the tetrahedral intermediate expels methoxide).