Which cyclization(s) should occur with a decrease in π-electron energy ?
(I) CH$_{2}$=CH-CH$_{2}$^• $\to$ + H$_{2}$
(II) CH$_{2}$=CH-CH$_{2}$$^{-}$ $\to$ + H$_{2}$
(III) CH$_{2}$=CH-CH$_{2}$$^{+}$ $\to$ + H$_{2}$
(IV) CH$_{2}$=CH-CH=CH$_{2}$ $\to$ + H$_{2}$

Question

Which cyclization(s) should occur with a decrease in π-electron energy ?

(I) CH$_{2}$=CH-CH$_{2}$^• $\to$ structure + H$_{2}$

(II) CH$_{2}$=CH-CH$_{2}$$^{-}$ $\to$ structure + H$_{2}$

(III) CH$_{2}$=CH-CH$_{2}$$^{+}$ $\to$ structure + H$_{2}$

(IV) CH$_{2}$=CH-CH=CH$_{2}$ $\to$ structure + H$_{2}$

TestHub · Accepted Answer

A decrease in π-electron energy needs the cyclic product to be aromatic. (I) gives the cyclopropenyl radical (3π) : not aromatic. (II) gives the cyclopropenyl anion (4π) : anti-aromatic, energy rises. (III) gives the cyclopropenyl cation (2π, aromatic) : energy falls. (IV) gives cyclobutene, which is not aromatic. Only (III).

Chemistry - GOC Question with Solution | TestHub

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