Actually, in a book I found that 2-methylpropene is more stable than trans-but-2-ene. But, my teacher and also I am convinced that trans-but-2-ene is more stable than 2-methylpropene. My reasoning is that 2-methylpropene is less stable due to the steric repulsion of the -CH3 on one sp2 hybridised carbon. Also, 2-methylpropene is more reactive for the electrophilic addition reactions involving carbocation intermediate as a 3° carbocation will form and in case of trans-but-2-ene, a 2° carbocation is formed. Please say if I am correct and if not, please explain the correct comparative analysis.
Since these two compounds are constitutional isomers, their heats of formation may be compared directly. 2-Methylpropene (isobutene) has a heat of formation = -4.0 kcal/mol. Trans 2-butene's heat of formation is -2.7 kcal/mol. Isobutene liberates 1.3 kcal/mol fewer kcal/mol than trans-butene (E-2-butene) upon combustion. The more negative the heat of formation, the more stable. The book gets an A.