Question

Introduction

A common method for synthesizing alkenes is by dehydrating (removing the elements of water) an alcohol. The dehydration is accomplished by heating the alcohol with either phosphoric or sulfuric acid. In this experiment, the dehydration reaction is illustrated by the conversion of 2-methylcyclohexanol to a mixture of alkenes (remember the Saytzeff rule).

?? + H3PO4 mixture of alkenes - CH - H20

Information

Each step of this E1 elimination reaction is reversible and thus, the reaction may be driven to completion by removing one or more of the products, in this case, the alkenes. For this reason, the reaction can be done using the setup for a simple distillation.

Information

Reaction Procedure

5.0 ml of 2-methylcyclohexanol was placed in a 50 ml round bottom flask. Phosphoric acid (85%, 5 ml) was added cautiously, and the mixture swirled. Boiling stones were added to prevent bumping during heating. The mixture was heated to reflux temperature and refluxed for 15 minutes. The products were collected between 85-90 ?C, in a small round bottom flask, cooled in ice water.

Isolation Procedure

The distillate was washed with 2 x 5 ml saturated sodium chloride solution in a 125-ml separatory funnel. The organic layer was transferred to a 50 ml Erlenmeyer flask of known mass (40.12 g).

After collecting the product, you weighed the Erlenmeyer flask to find the combined mass (flask + product mixture) to be 43.54 g.

1) What are the correct steps in the mechanism for the reaction performed above?

Options to choose from:

A and D

E and D

B, C, and D

or

A, C and D

2) Why would you expect the products to have a lower boiling point that the reactants?

3) Another way to generate alkenes is by the reaction of an alkyl halide with a base to perform a dehydrohalogenation. Which of the following would be found in the product mixture of the following reaction of 2-bromo-3-methylbutane with potassium tert-butoxide? Also, what is the major product?

Answer 1

Correct option = A, C and D Due to pressure of H.bonding, alchds have higher Boiling points. This molecular H-bonding is absent is and thus they have low B.P. K^+ O^-C(CH_3) _3 = very strong base and weak nucleophile * Promotes E_2