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SolutionsEpoxidation

1. Epoxide or osmate

Predict the result of the following reactions:

An isopropylmethylcyclohexene with 1) MCPBA and 2) H₂O, H₂SO₄, and with 1) OsO₄ and 2) H₂O
MCPBA forms the epoxide on the face opposite the isopropyl group and its acid opening gives the trans diol; OsO₄ forms the cyclic osmate on that same face and its hydrolysis gives the cis diol

Both reagents attack from the less hindered face, but opening of the epoxide is an SN2 (anti) whereas hydrolysis of the osmate keeps both C–O bonds (syn).

2. Which double bond?

Which double bond of limonene is epoxidized faster with 1 mol of MCPBA? Remember that limonene has a trisubstituted double bond in the ring and a disubstituted (terminal) one in the isopropenyl group.

The trisubstituted one in the ring. Peracids are electrophiles and react faster the richer in electrons the double bond is, that is, the more substituted it is.

3. Stereospecificity

Which stereoisomer of 2,3-epoxybutane is obtained from cis-but-2-ene with MCPBA? Is it chiral?

cis-2,3-Epoxybutane. Epoxidation is a syn addition and keeps the geometry of the alkene. The product has a plane of symmetry: it is a meso form, achiral.

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