Chemistry - STEREO ISOMERISM Question with Solution | TestHub
This compound racemizes in base. Why is that ?
Options:
Answer:
Solution:
Ans. In base the pendant OH group attacks the lactone carbonyl intramolecularly (ester exchange), giving a symmetrical tetrahedral intermediate with a plane of symmetry; collapse of that intermediate with loss of either oxygen gives back either enantiomer of the lactone, so the compound racemizes.
Sol. Purpose of the problem : to draw your attention to the dangers in nearly symmetrical molecules and revision of ester exchange.
Ester exchange in base goes through a symmetrical tetrahedral intermediate with a plane of symmetry: the base deprotonates the side-chain OH and the resulting alkoxide adds to the lactone C=O, forming a bicyclic tetrahedral intermediate in which the two oxygen substituents on the former carbonyl carbon are equivalent (the intermediate has a plane of symmetry). Loss of the right-hand leaving group gives one enantiomer of the ester and loss of the left-hand leaving group gives the other, so the starting lactone is racemized.
