How does $\ce{H3O+}$ cause the formation of an aldehyde from the following epoxide?

Actual product

I thought that the epoxide would undergo opening to form a 1,2-diol, as illustrated below. Am I wrong?

My thoughts


1 Answer 1


You are correct if this epoxide is a alkyl peroxide. However, this is a 2-phenyloxirane (styrene oxide or phenylethylene oxide), which can stabilize a protonated intermediate by resonance, specifically with 2-hydroxyphenyl group. Thus, it can isomerize to phenylacetaldehyde through 1,2-hydride shift (similar to pinacol-pinacolone rearrangement). The following diagram illustrate how styrene oxide converted to phenylacetaldehyde (Ref.1):

Isomerization of styrene oxide

Note that styrene oxide is not stabilized by extra hydroxy group here. That's probably the reason the authors have used slica-supported phosphotungstic heteropoly acid in cyclohexane (no other nucleophile there).

The identical isomerization has been used in the synthesis of 2-phenylnaphthalenes from styrene oxides (Ref.2).


  1. Vinícius V. Costa, Kelly A. da Silva Rocha, Ivan V. Kozhevnikov, Elena V. Gusevskaya, "Isomerization of styrene oxide to phenylacetaldehyde over supported phosphotungstic heteropoly acid," Applied Catalysis A: General 2010, 383(1–2), 217-220 (https://doi.org/10.1016/j.apcata.2010.06.005).
  2. Kishor V. Wagh, Bhalchandra M. Bhanage, "Synthesis of 2-phenylnaphthalenes from styrene oxides using a recyclable Brønsted acidic $\ce{[HNMP]+HSO4−}$ ionic liquid," Green Chemistry 2015, 17(8), 4446-4451 (DOI: 10.1039/C5GC01170A).
  • $\begingroup$ In the first line, is it peroxide or epoxide? $\endgroup$ Commented Oct 4, 2023 at 15:50

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