The structures for all of the stereoisomers of 1 , 2 , 3 , 4 , 5 , 6 -hexachlorocyclohexane are to be written, with meso forms and pair of enantiomers labeled. Also, the stereoisomer that undergoes slow E2 reactions is to be identified along with the reason is to be explained. Concept Introduction: Stereoisomers are isomers that have the same atoms but are arranged differently, that is, the same molecular formula but different structural formulae. Enantiomers are the stereoisomers having non-superimposable mirror images of each other. Enantiomers have same physical and chemical properties. A meso compound is a molecule that has a mirror image, and also has a plane of symmetry. E2 elimination reaction is one of the elimination processes which proceeds via one step mechanism, resulting in two products, also called bimolecular reaction. The order of the reaction is 2 and the rate depends on both the substrate and the base. In E2 elimination reaction an alkene is formed. In E2 reactions the two elimination reactions should be anti periplanar to each other.

Question

(a) When cis-1-bromo-2-methylcyclohexane undergoes an E2 reaction, two products (cycloalkenes) are formed. What are these two cycloalkenes, and which would you expect to be the major product? Write conformational structures showing how each is formed. (b) When rans-1-bromo-2-methylcyclohexane reacts in an E2 reaction, only one cyclo- alkene is formed. What is this product? Write conformational structures showing why it is the only product.

Answer 1

Question

Chapter FRP, Problem 29P

Interpretation Introduction

Interpretation:

The structures for all of the stereoisomers of 1,2,3,4,5,6-hexachlorocyclohexane are to be written, with meso forms and pair of enantiomers labeled. Also, the stereoisomer that undergoes slow E2 reactions is to be identified along with the reason is to be explained.

Concept Introduction:

Stereoisomers are isomers that have the same atoms but are arranged differently, that is, the same molecular formula but different structural formulae.

Enantiomers are the stereoisomers having non-superimposable mirror images of each other. Enantiomers have same physical and chemical properties.

A meso compound is a molecule that has a mirror image, and also has a plane of symmetry.

E2 elimination reaction is one of the elimination processes which proceeds via one step mechanism, resulting in two products, also called bimolecular reaction. The order of the reaction is 2 and the rate depends on both the substrate and the base. In E2 elimination reaction an alkene is formed. In E2 reactions the two elimination reactions should be anti periplanar to each other.

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(a) When cis-1-bromo-2-methylcyclohexane undergoes an E2 reaction, two products (cycloalkenes) are formed. What are these two cycloalkenes, and which would you expect to be the major product? Write conformational structures showing how each is formed. (b) When rans-1-bromo-2-methylcyclohexane reacts in an E2 reaction, only one cyclo- alkene is formed. What is this product? Write conformational structures showing why it is the only product.

The alkene shown undergoes bromination. (a) Draw the product(s) of bromination of this compound, including all expected stereoisomers (if any). Use wedge‑and‑dash bonds to designate the stereochemistry at any chirality centers, and make sure to draw an explicit hydrogen if a chirality center has one. (b) Characterize the starting alkene as having the E or Z configuration. (c) characterize the product(s).

Draw an approximate reaction-energy diagram showing the curves for the two possible pathways for ionic addition of HBr to 1-methylcyclohexene. (a) Formation of the major product, 1-bromo-1-methylcyclohexane, and (b) formation of the minor product, 1-bromo-2-methylcyclohexane. Point out how these curves show that 1-bromo-1-methylcyclohexane should be formed faster