Question

The UV-vis spec of acetophenone shows a peak at around 242nm while trans-p-anisalacetophenone shows two peaks at 340 and 245nm. I just want to know how these peaks came to be, relating to orbitals and energy gaps.

Answer 1

General Information:

Valence electrons can generally be found in one of three types of electron orbital: 1 single, or σ, bonding orbitals; 2 double or triple bonds (π bonding orbitals); and 3 non-bonding orbitals (lone pair electrons). Sigma bonding orbitals tend to be lower in energy than π bonding orbitals, which in turn are lower in energy than non-bonding orbitals. When electromagnetic radiation of the correct frequency is absorbed, a transition occurs from one of these orbitals to an empty orbital, usually an antibonding orbital, σ* or π* .

In general electronic transicions are:

sigma-sigma transitions are commun in all organics compounds (140-160nm)

pi-pi transitions are commun for molecules with double bonds (until 200nm)

n-sigma and n-pi are commun for molecules with oxigeno (carbonyl groups) ans heteroatoms (240-300nm). In our case, molecules bands of ketones, are due to n-pi and n-sigma transitions.

1) ACETOPHENONE

Acetophenone have sigma bonds, pi aromatic bonds, pi carbonyl bond, and n orbital for electron pair no bonding of oxygen. This molecule present transiciones sigma-sigma*, pi-pi* but energy of that transicions is high. Transicion to 242nm is due to n-pi* (see image).

2) TRANS-p-ANISALACETOPHENONE

This molecule presents two carbonyl groups. Blue carbonyl present transition n-pi* to 245nm. Transition n-pi* to 340nm corresponding to ketonic carbonyl (red) with shift to longer Wavelength or of less energy (bathochromic effect) because the conjugation with the adjacent olefin, gives greater stability to pi* antibonding orbital (see image), reducing the energy required for transition.