Question

a) Draw the major organic substitution product (ignoring stereochemistry) formed in the following reaction.  

b) If the reactant's configuration were (1 R,2R), then the configuration of the substitution product(s) will be (select all that apply):  

• 1R, 2R.  

• 1S, 2S.  

• 1R, 2S.  

• 1S, 2R.

Answer 1

Concepts and reason

The concept used to solve the part a is ${{\rm{S}}_{\rm{N}}}{\rm{1}}$ . The substrate contains a tertiary halide and the nucleophile is a weak. In ${{\rm{S}}_{\rm{N}}}{\rm{1}}$ reactions, rearrangement of carbocation takes place. Here, racemic mixture of products will be formed.

For part b, 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

${{\rm{S}}_{\rm{N}}}{\rm{1}}$ reaction is a two-step reaction. The rate of the reaction depends only on the concentration of substrate. It does not depend on the concentration of incoming nucleophile.

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.

Part a

The reaction is as follows:

Part b

The chiral center is labelled as follows:

The configurations of the reactants and products are assigned as follows:

Ans: Part a

The major organic substitution product is,