The reaction between singlet oxygen and limonene (below) is often used to characterize the behaviour of singlet oxygen as an electrophile rather than an initiator for a free radical mechanism.

(a) Draw a molecular energy level diagram that shows the electronic arrangement for the lowest energy singlet state of dioxygen(3 marks) (b) Since singlet oxygen is a high energy state, the formation of a diradical seems plausible: Propose a sequence of steps for this diradical reacting with limonene and explain why the major product, which is obtained (Scheme 1), does not support this reaction