Question

Draw the structure of the product formed in the following reaction of an epoxide with ethoxide ion. If more than one stereochemical product is possible, draw only one isomer.

Answer 1

Concepts and reason

The concept used to solve this question is cleavage of epoxide and nucleophilic substitution in the presence of acid catalyst.

Asymmetric carbon (chiral center) can be identified, when the carbon atom is bonded with four different groups or atoms. Assign the R and S configurations to the asymmetric carbon atoms.

Fundamentals

Under acidic conditions, the nucleophile preferentially attacks the more substituted ring carbon. Under basic or neutral conditions, the nucleophile preferentially attacks at the less substituted (or less sterically hindered ring carbon).

Chirality center: An atom (usually carbon) which is bonded to four different atoms or groups is known as chiral center. In general, it is represented with an asterisk (*).

R and S configuration can be assign to the compound by using the following CIP (Cahn-Ingold-Prelog) sequence rules:

Rule 1: Select the chiral center in the compound and assign priorities to the atoms based on its atomic number. The atom which is having highest atomic number gets the first priority and atom which is having least atomic number (usually hydrogen) gets fourth priority.

Rule 2: If the isotopes of same atom are attached to the chiral center, then then atom with the higher atomic mass receives higher priority.

Rule 3: If the first atom of each substituent is same then give priority to the second atom in the each substituent.

Rule 4: If the substituents have multiple bonds, then the multiple bonded atoms are considered as same number of single boned atoms.

If the rotation of the numbering is in clockwise direction, then the configuration of the isomer is R. Similarly, if the rotation of the numbering is in anticlockwise direction, then the configuration of the isomer is S.

The formation of product is as follows:

The configuration for the formed products are as follows:

Ans:

The structure of the product is as follows: