Question

Problem Set for “Cyclohexene”

1. Why do we use H₃PO₄ instead of H₂SO₄ as a catalyst for the synthesis of cyclohexene

2. Why do we use H₃PO₄ instead of HCl as a catalyst for the synthesis of cyclohexene?

3. What alkene(s) would be produced on dehydration of each of the following alcohols? If more than one product is possible, use Zaitsev’s rule to predict which product would be formed in greater amounts.

   (a) 2-methylcyclohexanol

   (b) 2,2-dimethylcyclohexanol

   (c) 1,2-cyclohexanediol

4.    In the process of the purification of cyclohexene, we choose the saturated NaCl solution to wash the crude product instead of the same volume of water. Explain why.

5.    A common observation test to confirm the presence of an alkene (or unsaturation, in general) is the bromine test.

   (a) What was the observation in the bromine test that allowed you to conclude an alkene was present?

   (b) Write the equation for the reaction between the major alkene produced and bromine. Be sure to show the correct stereochemistry in the product.

6. A sample of 9.0 mL of cyclohexanol was mixed with 2.0 mL of 85% phosphoric acid and heated. The reaction yielded 4.0 mL of cyclohexene. Calculate percent yield for this reaction.

7. Provide a detailed mechanism for the reaction 2,2-dimethylcyclohexanol with phosphoric acid. Give the major product and do not forget the charges and arrows.

Answer 1

1. H2SO4 is a strong oxidizing agent so on reaction with alcohol to form alkene it will oxidise some of the alkene to CO2 and itself get reduced to sulfur dioxide. It also reacts with alcholo to form mass of carbon. H3PO4 on the other hand is safer and facilitates a less complex reaction.

2. HCl can not be used in cyclohexene synthesis this is because it has a good nuclophile Cl- so it can end up into formation of chlorocyclohexane. That is why acid which is having a poor nucleophile is used during the reaction.

3.

According to zaitsev's rule alkene having more substitued double bond will be more stable.

5. (a) Bromine water decolourises the alkene solution.

(b)

7.