Progressive increase in conjugation moves the absorption to longer wavelength.
Explain the answer with special reference to UV-absorption by alkenes.
When alkenes are exposed to UV light of Energy ΔE (Energy Difference between HOMO and
LUMO), this light will be absorbed and one electron jumps from HOMO to LUMO, or we can
say that, π to π* transition occurs. Alkenes undergoes the π - π* electronic transition because ππ* energy gap is narrower than σ - σ* gaps.
This absorption is greatly affected by the conjugation of double bonds. As conjugation
increases absorption moves to longer wavelength. For example, ethene absorbs light at 165 nm
whereas for 1,3 butadiene the energy required for transition is less and absorbance occurs at
longer wavelength i.e. 217 nm as energy is inversely proportional to wavelength.
Explanation:
This can be explained on the basis of their orbitals diagrams. As we know in ethene each carbon
is sp​2 hybridized and there is one unhybridized 2p​z orbital on each carbon that make π bond
between two carbon as shown below .
These two 2p​z atomic orbitals interact with each other to form two molecular orbital one is
Bonding Molecular orbital (B.M.O.) and other is Anti bonding Molecular orbital (A.B.M.O)
According to Frontier molecular orbital theory the single B.M.O is HOMO( Highest Occupied
Molecular Orbital) and A.B.M.O is LUMO (Lower unoccupied molecular orbital) .
In ethene Energy gap ΔE between HOMO and LUMO is high i.e. 173 kcal/mol and it will
absorbs UV light at 165 nm.
In 1,3 butadiene each carbon is also sp​2 hybridized and there is one unhybridized p​z orbital on
each carbon so there are total 4 2p​z orbitals. These Four atomic orbitals interact with each other
to form four molecular orbitals. Two are Bonding Molecular orbital (B.M.O.) and two are Anti
bonding Molecular orbital (A.B.M.O).Here the energy gap ΔE between HOMO and LUMO
decreased as compared two ethene i.e. 132kcal/mol and absorbance of UV light will occur at
longer wavelength i.e., 217nm.
If we increase the conjugation system to three double bonds as in 1,3,5 hexatriene the energy
gap ΔE between HOMO and LUMO for π - π* transition decrease to 111 kcal/mol. So the
absorbance of UV light occurs at more longer wavelength i.e., 258nm.
Conclusion:
So from these examples we can conclude that progressive increase in conjugation moves the
absorption to longer wavelength because conjugation decreases the energy gap between HOMO
and LUMO. Hence less energy is required for electronic transitions, Therefore transitions occur
at longer wavelengths.