Question

Which statement(s) is/are wrong about the interconversion of trans- and cis-isomeric alkenes by ultraviolet radiation with a suitable photosensitiser ?

• A. U.V. radiation provides energy to excite $\pi \overline{e}$ into an anti-bonding $\pi$ molecular orbital $\left({\pi }^{\ast }ABMO\right)$ momentarily breaking the $\pi$-bond.
• B. Rotation about the (C-C) single bond of either excited cis - or trans-molecules gives a common non-planar intermediate. When the $\overline{e}$ returns to the ground state, the $\pi$-bond reforms and the same mixture of the two geometric isomers results from either starting isomer.
• C. Because the rate of return from the common intermediate to the cis-ground state is faster, the cis-predominates even though it has the higher energy. The product ratio is kinetically controlled and not thermodynamically controlled.
• D. Because the rate of return from the common intermediate to the trans-ground state is faster, the transpredominates and has lesser energy. The product ratio is thermodynamically controlled and not kinetically controlled.

Answer 1

Answer: (D)

Because the rate of return from the common intermediate to the trans-ground state is faster, the transpredominates and has lesser energy. The product ratio is thermodynamically controlled and not kinetically controlled.

The statement (D) is wrong about the inter-conversion of trans- and cis-isomeric alkenes by ultraviolet radiation with a suitable photosensitiser.
Because the rate of return from the common intermediate to the trans-ground state is faster, the cis alkene predominates and has lesser energy. The product ratio is kinetically controlled and not thermodynamicaly controlled.