Question

(a) Tetramethyloxirane is too hindered to undergo nucleophilic substitution by the hindered alkoxide, potassium tert-butoxide. Instead, the product is the allylic alcohol shown. Propose a mechanism to explain this reaction. What type of mechanism does it follow?

(b) Under mild acid catalysis, 1,1-diphenyloxirane undergoes a smooth conversion to diphenylethanal (diphenylacetaldehyde). Propose a mechanism for this reaction. (Hint: Think Pinacol.)

Short answer

a)

b)

Step-by-step solution

Explanation of part (a):

Tetramethyloxirane undergoes E2 elimination reaction with strong base tert.butoxide , as tert.butoxide is bulky base, thus firstly, it abstracts the proton from less hindered carbon of the epoxide which further facilitates the ring opening reaction as three-membered cyclic epoxide is strained due to angle strain. The epoxide oxygen is the leaving group in this elimination reaction. Then, oxygen of epoxide acquires negative charge and becomes good base and abstracts the proton from tert.butyl alcohol and forms required product. The product has new pi-bond formation, thus it confirms it is a elimination reaction and in particular, E2 elimination reaction.

Formation of allylic alcohol via E2 mechanism

Explanation of part (b):

1,1-diphenyloxirane in acidic medium undergoes epoxide ring opening as three membered epoxide ring is strained due to angle strain and also when oxygen of epoxide acquires proton from medium, then it gains positive charge on itself, thus to neutralize it, ring opening becomes necessary. A carbocation is formed which undergoes hydride shift to form carbon-oxygen double bond as it is stable. Then, further on charge neutralization on oxygen, we get our required product, that is, diphenylethanal.

Formation of diphenylethanal from 1,1-diphenyloxirane