What is action of m-CPBA on trans-but-2-ene and cis-but-2ene? Explain.

Action of m-CPBA on Trans-But-2-ene and Cis-But-2-ene

m-Chloroperbenzoic acid (m-CPBA) is a commonly used reagent in organic chemistry, particularly for the formation of epoxides from alkenes. It is a peracid, meaning it contains a peroxy group (-O-O-) that can act as an electrophile, reacting with the double bond of alkenes to form an epoxide.

1. Reaction of m-CPBA with trans-But-2-ene

Trans-but-2-ene has the following structure:

$$\text{CH}_3\text{-CH=CH}\text{-CH}_3$$

In this molecule, the two methyl groups (-CH₃) are on opposite sides of the double bond, making it a trans-alkene.

When m-CPBA is added to trans-but-2-ene, the reaction proceeds through an electrophilic addition mechanism:

• The peracid (m-CPBA) reacts with the π-electrons of the C=C double bond.
• A three-membered epoxide ring is formed by the attack of the peracid oxygen on one of the carbon atoms of the double bond. The oxygen atom of m-CPBA adds to the carbon atom that is less hindered.
• Because m-CPBA is a chiral molecule and the reaction occurs in a specific orientation, the epoxide formation is stereospecific.
• In the case of trans-but-2-ene, the result will be an epoxide with anti-stereochemistry. This means the two substituents (the methyl groups) will end up on opposite sides of the epoxide ring.

Thus, when m-CPBA reacts with trans-but-2-ene, it forms a trans-2,3-epoxybutane product, with the two methyl groups positioned on opposite sides of the epoxide ring.

2. Reaction of m-CPBA with cis-But-2-ene

Cis-but-2-ene has the structure:

$$\text{CH}_3\text{-CH=CH}\text{-CH}_3$$

In this molecule, the two methyl groups are on the same side of the double bond, making it a cis-alkene.

When m-CPBA is added to cis-but-2-ene, the reaction also proceeds through an electrophilic addition mechanism, but with a key difference in the resulting stereochemistry:

• Just like in the case of trans-but-2-ene, the peracid reacts with the π-electrons of the double bond to form a three-membered epoxide ring.
• However, because the two methyl groups are on the same side of the double bond, the resulting epoxide will have cis-stereochemistry.
• The two substituents (the methyl groups) will be on the same side of the epoxide ring.

Thus, when m-CPBA reacts with cis-but-2-ene, it forms cis-2,3-epoxybutane, with the two methyl groups on the same side of the epoxide ring.

Conclusion

The action of m-CPBA on trans-but-2-ene and cis-but-2-ene results in the formation of epoxides with anti stereochemistry. The key difference is that the stereochemistry of the epoxide ring is different due to the initial spatial arrangement of the substituents around the double bond:

• Trans-but-2-ene forms trans-2,3-epoxybutane.
• Cis-but-2-ene forms cis-2,3-epoxybutane.

Both reactions are examples of how m-CPBA can be used to synthesize epoxides from alkenes, with the stereochemistry of the products being determined by the initial orientation of the substituents on the alkene.

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