1 (46)

Prévia do material em texto

Chapter 7 Suggested solutions for Chapter 7 43 Purpose of the problem Further thoughts on the relationship between conjugation and light absorption plus revision of Chapter 1. Suggested solution One of the dyestuffs in the shaving foam, Fast Green FCF, has a conjugated system very similar to those we have just been discussing in Problem 6, though it has extra solubilizing substituents on the atom. Fast Green FCF core conjugated system to give visible colour Et Et N N extra nonconjugating extra nonconjugating substituent to give substituent to give solubility in water solubility in water SO₂O extra conjugating substituent to "tune" colour and oH give solubility in water Quinoline yellow is quite different. It has two conjugated ring systems linked by a single bond. However, they are also linked by conjugation from the OH group in one ring to the nitrogen atom the other. In fact, the whole molecule is conjugated. Because the conjugation is more limited, this molecule absorbs violet light and so appears yellow. It has the usual sulfonate solubilizing groups. The 'quinoline' part of the name refers to the left-hand ring system. Quinoline yellow 0 0 N N N quinoline HOO₂S HOO₂S SO₂OH The compounds in the 'spectrum of molecules' are much simpler and you may well have been surprised after reading Chapter 7 that they are coloured at all. 0 N 0 CI CN F N 0 CH N N F MeO CI F CN 0 NO red orange yellow green blue purple You probably found 'red' easiest to deal with as there is an obvious extended conjugation throughout the whole molecule. The conjugation in 'blue' is less obvious but clearly more effective blue compound absorbs longer wavelength light than a red compound) and relies on a redistribution of electrons that makes both rings aromatic. This compound is called azulene (an azure alkene) as a blue hydrocarbon is very remarkable.