Q.) Explain the change in stereochemistry observed between steps two and three in the mechanism.

A.) In step two of the reaction mechanism, an intramolecular S_N2 reaction takes place to form a seven- membered ring. The nucleophilic oxygen atom attacks the electrophilic tetrahedral carbon atom, causing the leaving group CH₂CH₃OH to leave. Since this is an S_N2 reaction, the stereochemistry of the electrophilic atom is inverted, as observed in the seven-membered ring product in the subsequent step of the mechanism.