HBr is added to 1,3-butadiene at −78 °C (kinetic conditions). Which product predominates and why?
AThe 1,4-addition product, because it is thermodynamically more stable
BThe 1,2-addition product, because nucleophilic attack at the closer allylic carbon is faster
CA 50/50 mixture of 1,2 and 1,4 products, because the allylic cation is fully symmetric
DNo reaction occurs at low temperature; heat is required to generate the allylic cation
At low temperatures, kinetic control operates: the reaction favors the product that forms fastest, which is 1,2-addition. The allylic carbocation intermediate has charge at both C2 and C4, but the nucleophile (Br⁻) attacks C2 faster because it is the closer site. At higher temperatures or longer reaction times (thermodynamic control), the more stable 1,4-product — which has a more substituted double bond — accumulates because the system reaches equilibrium.
Question 2 Multiple Choice
A student claims: 'Raising the reaction temperature causes the 1,2-addition product of HBr with 1,3-butadiene to become more thermodynamically stable.' What is wrong with this claim?
ANothing — higher temperature does increase the stability of the 1,2-product relative to the 1,4-product
BTemperature changes the kinetics of product formation but does not change the relative thermodynamic stability of the products
CAt higher temperature only 1,4-addition occurs because the 1,2-pathway is completely blocked
DThe 1,2-product is actually always more stable; the 1,4-product is the kinetic product
The thermodynamic stability of the products (determined by the position of the double bond and degree of substitution) is a property of the molecules themselves and does not change with temperature. Higher temperature allows the reversible reaction to reach equilibrium, shifting the ratio toward the more stable 1,4-product — but this happens because more energy is available to interconvert products, not because their stability ranking has changed. The 1,2-product is always kinetically favored (forms faster) and the 1,4-product is always thermodynamically favored (lower energy).
Question 3 True / False
In the electrophilic addition of HBr to 1,3-butadiene, the intermediate allylic carbocation has positive charge delocalized over two carbon positions.
TTrue
FFalse
Answer: True
When H⁺ adds to C1 of 1,3-butadiene, the resulting cation has charge distributed between C2 and C4, as shown by two resonance structures. This delocalization is what makes both 1,2-addition (nucleophile attacks C2) and 1,4-addition (nucleophile attacks C4) possible. A localized carbocation on a single carbon, as in simple alkene addition, would give only one product.
Question 4 True / False
The s-cis and s-trans conformers of 1,3-butadiene are geometric isomers — they cannot interconvert at room temperature because they have different configurations about a double bond.
TTrue
FFalse
Answer: False
s-cis and s-trans are conformational isomers (rotamers) that interconvert freely by rotation about the central C2–C3 single bond, not geometric isomers. The 's' stands for 'single bond,' distinguishing them from cis/trans isomerism about a double bond. Because rotation about a single bond has a low energy barrier, both conformers are accessible at room temperature. The s-trans conformer is more stable (less steric strain), but the s-cis conformer is essential for pericyclic reactions like the Diels-Alder reaction.
Question 5 Short Answer
Why does the 1,4-addition product accumulate under thermodynamic control even though the 1,2-product forms faster?
Think about your answer, then reveal below.
Model answer: Under thermodynamic control (high temperature or long reaction time), the reaction is reversible. Both products can re-form the allylic cation intermediate, but the more stable 1,4-product, once formed, is less likely to revert because it lies in a deeper energy well. At equilibrium, the product distribution reflects relative stability, not formation rate. The 1,4-product is more stable because its double bond is more substituted (and more stabilized by hyperconjugation). The 1,2-product predominates only under kinetic control, where the reaction is quenched before equilibrium is reached.
The key distinction is between rate of formation (kinetics) and equilibrium position (thermodynamics). Kinetic control locks in the faster-forming product; thermodynamic control lets the system find the lowest energy distribution. In conjugated diene chemistry, these two controls give different products precisely because the faster pathway (1,2) doesn't lead to the most stable outcome.