Answer to Question #270088 in General Chemistry for AMa

Question

Ethanal, CH3CHO, produces two peaks in the UV spectrum. The peaks are at 180 and

290 nm. By using various type of electronic transition states, identify and explain

about two peaks appeared in the UV spectrum

Answer 1

Carbon-Oxygen double bond in ethanal, has:

 $a)\>\pi-$ electrons as a part of the double bond,

$b)\>$ lone pairs on the oxygen atom.

An electron can be excited from a $π$ bonding to $π$  anti-bonding orbital,

or an electron can be excited from an oxygen lone pair (a non-bonding orbital) into a $π$  anti-bonding orbital.

The non-bonding orbital has a higher energy than a $π$ bonding orbital.

Therefore the jump from an oxygen lone pair into a $π$ anti-bonding orbital needs less energy.

That means it absorbs light of a lower frequency and therefore a higher wavelength.

Ethanal can therefore absorb light of two different wavelengths:

• the $π$ bonding to $π$ anti-bonding absorption peaks at 180 nm.
• The non-bonding to $π$ anti-bonding absorption peaks at 290 nm.