Transition states epoxide ring-opening

Figure 8.3 Transition state of the epoxide ring-opening step (P = growing polymer chain) for (a) dizinc catalyst (b) dimagnesium catalyst.

Whereas the hydrolysis of l-chloro-2-butene gives comparable yields of products from attack of solvent at both primary and secondary carbons, very little product from the acid-catalyzed hydrolysis of butadiene oxide 10 is formed from the attack of solvent at the primary carbon. The transition state for acid-catalyzed epoxide ring opening has a relatively reactant-like geometry, and therefore positive charge de-localization into the adjacent double bond at the transition state is expected to be less than that for allyl chloride hydrolysis. 

Because there is some carbocation character developed in the transition state for an add-catalyzed epoxide ring opening, attack of the nudeophile on imsymmetrical epoxides occurs preferentially at the carbon better able to bear a partial positive charge. 

Fig. 10.3 Structure of transition states of epoxide ring opening of methyloxirane (2) with hydroxyl anion (MP2/6-31G level of theory [7]) and formiate (B3LYP/6-31G [35]) level of theory)...

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