Final answer:
The major product expected when 1,3-butadiene is treated with 1 equivalent of HBr, under thermodynamic control, is the 1,4-addition product, where the bromine attaches to the last carbon in the butadiene.
Step-by-step explanation:
The student asks about the reaction product when 1,3-butadiene is treated with a single equivalent of hydrobromic acid (HBr). When an alkene like 1,3-butadiene reacts with HBr, a hydrohalogenation reaction occurs. This type of reaction typically follows Markovnikov's rule, where the hydrogen atom from HBr attaches to the carbon with the greatest number of hydrogen atoms, and the bromine attaches to the carbon with fewer hydrogen atoms.
However, in the case of conjugated dienes like 1,3-butadiene, two possible products can form: the 1,2-addition product and the 1,4-addition product. The 1,2-product results from the addition of HBr across the first double bond, whereas the 1,4-product is formed by the addition across the entire diene system. Under thermodynamic control (typically with higher temperatures and longer reaction times), the 1,4-addition product is the major product since it leads to a more stable, conjugated system.
If the reaction conditions favor kinetic control (low temperature and short reaction time), then the 1,2-addition product may be favored due to its faster formation rate. Nevertheless, the question specifies only one major product, suggesting that we are considering conditions under which the thermodynamically stable product is favored, so the expected major product from treating 1,3-butadiene with 1 equivalent of HBr is the 1,4-addition product, where the bromine attaches to the last carbon in the butadiene.