Discussion
Why is a carbon-carbon double bond in a symmetric alkene weak or absent?
Answer: A
In a symmetrically substituted alkene the two ends are equivalent, so stretching produces almost no change in dipole moment and the band is very weak. The force constant and frequency are perfectly ordinary, near 1650, and masking by other bands is not the reason. That the absence of an expected band is itself evidence - here, of symmetry - is a useful habit, and it recurs throughout structural spectroscopy. Checking the units of each quantity catches a wrong formula before the arithmetic begins.
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