Discussion
Assertion (A): An axial-axial coupling in a chair cyclohexane is much larger than an axial-equatorial one.
Reason (R): The axial-axial pair has a dihedral angle near 180 degrees while the axial-equatorial pair is near 60.
Select the correct answer from the options below.
Answer: C
Both hold, and the second is the reason. By the Karplus relation a dihedral angle near one hundred and eighty gives a coupling of ten to thirteen hertz, whereas sixty degrees gives only two to five. Measuring the coupling therefore tells you whether a substituent is axial or equatorial, which is a routine conformational assignment in sugar and steroid chemistry. The argument works only because the ring holds the angle fixed; free rotation would average the two values into a single intermediate figure.
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