When 3,4-dimethyl-2,4-hexadiene reacts with HBr what is the major product?
Conjugated dienes can undergo two types of additions with HX, they can undergo 1,2-additions and 1,4-additions.  A 1,2-addition occurs when HX adds to only one of the double bonds, not interacting with the other.  a 1,4-addition occurs when HX adds X to one of the double bonds, and because the system is conjugated, the hydrogen will be added three carbons away from X.  The 1,2-adduct and 1,4-adduct are either thermodynamically or kinetically stable.  The kinetic product will form quicker, but is less stable, it will dominate at low temperatures and short reaction times.  The thermodynamic product is more stable but takes more time to form.  It will be the major product at high temperature and long reaction times.  In this case, the kinetic product would be 2-bromo-3,4-dimethyl-3-hexene, the thermodynamic product would be 4-bromo-3,4-dimethyl-2-hexene, because the thermodynamic product has the bromine on the most substituted carbon.  Depending on the reaction conditions, either could form.  At low temps. and short reaction times, 2-bromo-3,4-dimethyl-3-hexene will form, at high temps. and long reaction times, 4-bromo-3,4-dimethyl-2-hexene will form.