When (3E)-1,3 -pentadiene is reacted with one equivalent of HBr at 40^ C , a major structural product is formed under thermodynamic control. The total number of stereoisomers of this major product is.........

Step-by-step solution

Protonation of 1,3 -pentadiene ( CH ₂= CH - CH = CH - CH ₃ ) occurs at C1 to form the more stable secondary allylic carbocation, which is resonance stabilized: CH ₃- C ^+ H - CH = CH - CH ₃ CH ₃- CH = CH - C ^+ H - CH ₃ At 40^ C , the reaction is under thermodynamic control, yielding the most stable alkene (most substituted). Attack of the bromide ion at either C2 or C4 of the resonance-stabilized carbocation yields the same disubstituted alkene: 4 -bromo- 2 -pentene ( CH ₃- CH = CH - CH ( Br )- CH ₃ ). The molecul