Chapter 17, Problem 51PP

Interpretation Introduction

Interpretation:

The compound having longer ${\text{λ}}_{\text{max}}$ in the given compounds should be identified.

Concept introduction:

• The π-systems of conjugated dienes absorb UV or visible light (200nm – 800nm).
• The wavelength of maximum absorption of a compound is called ${\text{λ}}_{\text{max}}$.
• The value of ${\text{λ}}_{\text{max}}$ is highly depending on the extent of conjugation (number of conjugated double bonds).                                                                                                       Conjugated double bonds are the $\text{C}\text{=}\text{C}$ bonds, which are separated by one σ-bond.
• According to Woodward-Fieser rule, the portion of the molecule which absorbs UV-visible light is called chromophore and the groups attached to the chromophore are auxochromes. Woodward-Fieser rule gives the estimate value for ${\text{λ}}_{\text{max}}$.
• The estimate value for ${\text{λ}}_{\text{max}}$ are given in below table,

FEATURES	${\text{λ}}_{\text{max}}$- value	Example	Calculation
Conjugated- diene	Base value     = 217		Base:217nm
Each additional double bond	+30		Base: 217 $+2\times 30$
Each auxochromic alkyl group	+5		Base: 217                   $+3\times 5$
Each exocyclic double bond	+5		Base: 217      + 5
Homoannular diene (both double bonds are situated in a ring)	+ 39		Base: 217     +39