Final answer:
The structure of an alkene that reacts with HBr to give a certain product can be deduced by looking at the product's bromine placement and considering Markovnikov's rule. Alkenes react with HBr through a hydrohalogenation reaction, which provides the alkane with a bromine substituent.
Step-by-step explanation:
Alkene Structure Related to HBr Reaction
When an alkene reacts with hydrogen bromide (HBr), a hydrohalogenation reaction occurs, resulting in the addition of a hydrogen atom (H) and a bromine atom (Br) across the double bond. This will generate an alkane with the same carbon skeleton as the original alkene and with bromine substituted at one of the carbons that was originally part of the double bond. To draw the structure of the original alkene based on the product, you need to consider Markovnikov's rule, which states that in the addition reactions of HX (X being a halogen) to alkenes, the halogen will attach to the more substituted carbon (the carbon with the more carbon atoms attached).
For example, if the product is butyl bromide with the bromine on the second carbon (2-bromobutane), then the original alkene would be but-1-ene or but-2-ene, depending on the regioselectivity of the reaction.