Explain how you would use ir spectroscopy to distinguish between 1-bromo-3-methyl-2-butene and 2-bromo-3-methyl-2-butene.

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.

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