SolutionsAldehydes and ketones
Exercise 1
What product does the Baeyer–Villiger oxidation give from (a) cyclopentanone, (b) 3-methylbutan-2-one and (c) benzaldehyde?
(a) δ-Valerolactone (tetrahydropyran-2-one). (b) Isopropyl acetate, CH3COOCH(CH3)2: the secondary group migrates, not the methyl. (c) Benzoic acid: in aldehydes hydrogen migrates (although electron-rich aryl groups can migrate to give formates, the Dakin reaction).
Exercise 2
Ethyl 4-oxopentanoate must be converted into (a) ethyl 4-hydroxypentanoate and (b) ethyl pentanoate. Which reagents would you use? Would the Clemmensen or the Wolff–Kishner reduction be suitable for (b)?
(a) NaBH4 in ethanol at low temperature: it reduces the ketone and not the ester (the hydroxy ester tends to cyclize to γ-valerolactone). (b) Wolff–Kishner conditions (KOH, 200 °C) would hydrolyse the ester, and Clemmensen conditions (hot concentrated HCl) may also hydrolyse it. It is better to form the dithioacetal with ethane-1,2-dithiol and BF3 and desulfurize it with Raney nickel, under neutral conditions.