Answer to Question #283828 in Organic Chemistry for Reens

Question

a) using fmo model explain why in thermal condition disrotatory motion would necessary for cyclization of 1,3,5- hexatriene and conrotatory of 2,4- hexadiene

b) discuss the visibility of [1,3] and [1,5] hydrogen shifts on the basis of orbital symmetry and geometry consideration

Answer 1

A [1,5] shift involves the shift of 1 substituent (hydride, alkyl, or aryl) down 5 atoms of a π system. Hydrogen has been shown to shift in both cyclic and open-chain compounds at temperatures at or above 200 ˚C. These reactions are predicted to proceed suprafacially, via a Hückel-topology transition state.