Question

List all possible electronic transitions and describe why they are important and what instrument is used to identify these electronic transitions.

(i) CH₄

(ii) CH₃Br

(iii) CH₃COCH₃

and

(iv) CH₃CHO

Answer 1

UV-visible spectroscopy is used to identify these electronic transitions.

A) CH4:-

σ→σ* transition :
These transitions can occur in such compounds in which all the electrons are involved in single bonds and there are no lone pair of electrons.
Energy required for σ→σ* transition is very large so the absorption band occurs in the far UV region. So this transition cant normally be observed.

B) CH3Br :-

n→σ* transition
Saturated compounds with lone pair of electrons undergo n→σ* transition in addition to σ→σ* transition. Corresponding absorption bands appear at longer wavelengths in near UV region.

C) CH3COCH3 and CH3CHO :-

R- Band :- In conjugated systems the energy separation between the ground and excited states is reduced and the system then absorbs at longer wavelengths and with a greatly increased intensity (k-band is intense and at longer wavelength). Moreover, due to the lessening of the energy gap, the n→ π* transition due to the presence of the heteroatom and lone pair i.e. the r-band also undergoes a red shift with little change in intensity.