Compare the dipole moments of the two substituted azulenes (a) and (b). Is the dipole moment of (a) greater than, less than or equal to that of (b) ?
(a)
(b)

Question

Compare the dipole moments of the two substituted azulenes (a) and (b). Is the dipole moment of (a) greater than, less than or equal to that of (b) ? (a) (b)

TestHub · Accepted Answer

Azulene is polarised so that the seven-membered ring is the positive end and the five-membered ring is the negative end. In (a) the electron-donating OMe is on the electron-poor seven-membered ring and the electron-withdrawing CN is on the electron-rich five-membered ring, so both groups strengthen this polarisation. In (b) they sit on the opposite rings and work against it. So a > b.

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