Structures of 1-bromo-3-methyl-2-butene and 2-bromo-3-methyl-2-butene are shown below.
It can be seen that 1-bromo-3-methyl-2-butene is containing a C-H bond in which carbon is sp² hybridized (i.e. =C-H) while such bonding is absent in 2-bromo-3-methyl-2-butene.
So, as we know that the peak of C-H stretching of alkenes is found in the region of 3010-3100 cm⁻¹ with medium intensity. Therefore, 1-bromo-3-methyl-2-butene will show this peak and 2-bromo-3-methyl-2-butene will lack this peak in IR spectrum.
