Question

Tell how UV-vis spectrometry can be used to differentiate the following compounds. Calculate approximate ymax values for each compound when you can.

a) 1,4 hexadiene, 2,4-hexadiene, and 1,3 hexadiene

b) cyclohexanone, 2-cyclohexenone, and 2,4-cyclohexadienone

Answer 1

a) The structure of the three compounds are

1, 4 hexadiene is not conjugated so it will show UV vis spectrum of a standard olefin like ethylene around <217 nm.

2,4 hexadiene and 1,3 hexatriene are conjugated dienes

the max for 2,4 hexadiene will be the base value of buatdiene system (217 nm) + 2 x 2 alkyl substituent (2 x 5 nm) = 227 nm

the max for 1,2 hexadiene will be the base value of buatdiene system (217 nm) + 1 x 2 alkyl substituent (1 x 5 nm) = 222 nm.

b) The structure of these compounds are

cyclohexanone no conjugation ymax <215 nm

2-cyclohexenone - y max

6 membered cyclic ?, ? unsaturated ketones = 215 nm + Alkyl substituent or Ring residue in ? position = 12 nm

Total = 227 nm

2,4 cyclohexadienone

6 membered cyclic ?, ? unsaturated ketones = 215 nm +  Double bond extending conjugation = 30 nm +  Homodiene compound = 39 nm + 6. Exocyclic double bonds = 5 nm

Total = 289 nm