Identify the Products of a Reaction Under Kinetic Control

Answer

Two of the five options describe the products of a kinetically controlled reaction: (b) and (d). They are two ways of stating one criterion.

Why (b) and (d) give the same answer

The product that forms fastest is the one whose pathway crosses the lowest free-energy barrier. “Smallest free energy of activation” and “formed at the fastest rate” therefore pick out the same species; they differ only in whether the criterion is read off an energy diagram or off a rate measurement.

Why the other options do not qualify

What kinetic control looks like in a real reaction

The textbook example is the addition of one equivalent of $\mathrm{HBr}$ to 1,3-butadiene:

$\mathrm{CH_2{=}CH{-}CH{=}CH_2 + HBr \longrightarrow \text{products}}$

Protonation at a terminal carbon gives a resonance-stabilised allylic carbocation, and bromide can then bond at either end of the allyl system, so two adducts compete:

At 0 °C the addition is effectively irreversible, so bromide simply attacks the nearer end of the allyl cation and the faster-forming 1,2-adduct accumulates even though it is the less stable of the two. At 40 °C the addition becomes reversible; the mixture equilibrates, and the more stable 1,4-adduct — the one with the more substituted internal double bond — takes over. Heating an isolated 0 °C mixture to 40 °C shifts the ratio towards the 40 °C value, which is the direct evidence that the two adducts interconvert through the shared cation.

The general rule

• Cold and effectively irreversible: the outcome is set by relative rates, so the major products are the kinetic products.
• Hot enough to be reversible: the outcome is set by relative stability, so the major product is the thermodynamic product.
• The two criteria disagree only when the faster-forming product is also the less stable one. When a single product is both the fastest to form and the most stable, changing the conditions changes nothing.