Engineering Chemistry S Datta

518 ENGINEERING CHEMISTRY
Here, the base value is 217 mµ
So, calculated value of λ max
is 217 mµ
Observed λ max
value is 217 mµ.
Conjugated dienes in which the double bonds are contained within rings absorb somewhat
differently. If the conjugated double bonds are contained in separate, but fused,
six-membered rings (a heteroannular diene), a base value of 214 mµ is used; if the conjugated
double bonds are contained in the same ring (a homoannular diene), a base value of 253 mµ is
used. For each alkyl substituent group on the diene system or, for each ring to which a carboncarbon
double bond is exocylic, 5 mµ are added to the calculated value.
Homoannular diene Heteroannular diene
For cyclic conjugated dienes:
Example 1.
Homoannular diene (I)
Calculated λ max
Here, the base value is 253 mµ
For 3-ring residules + 5 × 3 mµ
For one ring to which
the carbon-carbon double bond is exocylic + 5 mµ
Calculated: λ max
value is 273 mµ.
Observed: λ max
value is 275 mµ.
Example 2.
Heteroannular diene (II)
(II)
Here, the base value is 214 mµ
For 3-ring residues + 15 mµ
For one ring to which the
Carbon-carbon double bond + 5 mµ is exocylic.
Calculated: λ max
value is 234 mµ
Observed: λ max
value is 235 mµ.