Question

For the following SN2 reaction, draw the organic and inorganic products and select the correct (R) or (S) designation around the stereocenter carbon in the organic substrate and organic product. Include wedge/dash bonds and H on a stereocenter.

Answer 1

Concepts and reason

Two compounds are given which are undergoing ${{\rm{S}}_{\rm{N}}}2$ reaction whose product needs to identify as organic and inorganic. Also, the stereocentre present in the reactant molecule and that formed in product molecule needs to be assigned as R and S. Descriptor R and S can be assigned to the centers by following the CIP rule which is based on certain criteria of giving priorities to the groups.

In ${{\rm{S}}_{\rm{N}}}2$ reaction nucleophile attacks the haloalkane and halogen leaves as a leaving group. ${{\rm{S}}_{\rm{N}}}2$ reaction is stereospecific in nature. Thus, write the product accordingly.

Fundamentals

${{\rm{S}}_{\rm{N}}}2$ reaction is bimolecular nucleophilic substitution reaction which takes place between a haloalkane and a nucleophile. The reaction takes place in one step through a transition state and the reaction is stereospecific in nature. Consider the following scheme of a ${{\rm{S}}_{\rm{N}}}2$ reaction:

In the reaction, the nucleophile attacks from the back side of the reaction as the front side is hindered by the leaving group.

The priority to the groups attached to the stereocentre are decided by CIP sequence rules, that is, Cahn-Ingold-Prelog rules which are as follows:

1.Firstly, locate the stereocentre in the given compound and the groups attached to it.

2.Then, compare the atomic number of the atom attached to the stereocentre. Atom having the highest atomic number assigned as the highest priority and the atom having the lowest atomic number assigned as the least priority.

3.If the first atom attached on both the sides of the stereocentre is same, then the priority is assigned to the substituent based on the next atom attached to the substituent.

4.Lowest priority group should be below the plane or away from the viewer, if is it present above the plane then change the descriptor so that the actual configuration of the molecule should not change.

For instance, if the configuration of the molecule comes out as R and lowest priority group is present above the plane then change the configuration from R to S and vice versa.

5.Then, rotation in the direction of priority order, that is, from 1 to 2 to 3 is carried out.

6.If the rotation appears as clockwise then, the stereoisomer is R. If the rotation appears as anticlockwise then, the stereoisomer is S.

Write the product of the given reaction by ${{\rm{S}}_{\rm{N}}}{\rm{2}}$ mechanism and mark them as organic product and inorganic product of the reaction as shown below:

Assign the priority to the groups attached to the chiral center resent in the organic substrate as well as in organic product of the reaction as shown below:

Hence, the descriptor for the organic substrate is ‘S’ and for organic product is ‘R’.

Ans:

The organic and inorganic product of the reaction is marked as follows:

Hence, the correct R and S designation to the given organic substrate and the product formed is shown below: