Question

Rank the following compounds in order of increasing${\mathbf{\lambda }}_{\max }$

Short answer

The value of ${\mathbf{\lambda }}_{\max }$increases as the number of double bonds increases. In a $\mathbf{\pi }\to {\mathbf{\pi }}^{*}$transition, the electron gets promoted from the HOMO to LUMO. Conjugation increases the energy of HOMO and decreases the energy of LUMO.

Step-by-step solution

Step-by-step-solutionStep 1: Effect of conjugation on λmax

The value of ${\mathbf{\lambda }}_{\max }$increases as the number of double bonds increases. In a $\mathbf{\pi }\to {\mathbf{\pi }}^{*}$transition, the electron gets promoted from the HOMO to LUMO. Conjugation increases the energy of HOMO and decreases the energy of LUMO.

Ranking the compounds in the order of increasing λmax

The compound C has a conjugated double bond when compared with other molecules like A, B and D. The compound with more number of conjugated double bonds will have a higher ${\text{λ}}_{\text{max}}$value. The carbon-carbon (-C=C-) transition from the HOMO to LUMO, raises the energy of HOMO and lowers the energy of LUMO. The${\text{λ}}_{\text{max}}$ value of the compounds in the increasing order can be given as:

role="math" localid="1652419263320" ${\text{λ}}_{\text{max}}$value of the compounds in the increasing order