16. Conjugated Systems

What are the products of the following reactions, assuming that one equivalent of each reagent is used in each reaction? Disregard stereoisomers.

b.

Draw the products obtained from the reaction of one equivalent of HBr with one equivalent of 1,3,5-hexatriene.

a. Which product(s) will predominate if the reaction is under kinetic control?

b. Which product(s) will predominate if the reaction is under thermodynamic control?

a. How could each of the following compounds be prepared from a hydrocarbon in a single step?

b. What other organic compound would be obtained from each synthesis?

2.

What are the major 1,2- and 1,4-addition products of the following reaction? Indicate the kinetic and the thermodynamic products.

b.

What are the major 1,2- and 1,4-addition products of the following reaction? Indicate the kinetic and the thermodynamic products.

a.

What are the products of the following reactions, assuming that one equivalent of each reagent is used in each reaction?

b.

A student wanted to know whether the greater proximity of the nucleophile to the C-2 carbon in the transition state is what causes the 1,2-addition product to be formed faster when 1,3-butadiene reacts with HCl. Therefore, she decided to investigate the reaction of 2-methyl-1,3-cyclohexadiene with HCl. Her friend told her that she should use 1-methyl-1,3-cyclohexadiene instead. Should she follow her friend's advice?

The experiment shown next and discussed in Section 8.13 shows that the proximity of the chloride ion to C-2 in the transition state causes the 1,2-addition product to form more rapidly than the 1,4-addition product.

a. Why was it important for the investigators to know that the preceding reaction was being carried out under kinetic control?

Predict the products of the following reactions.

(e) buta-1,3-diene + bromine water

Predict the products of the following reactions.

(b) cyclopentadiene + anhydrous HCl

Predict the products of the following reactions.

(f) hexa-1,3,5-triene + bromine in CCl₄

The following diene gives the same product regardless of whether the reaction is run under conditions of kinetic (0 °C) or thermodynamic (100 °C) control. Predict the product and explain this observation.

Would you expect the following nucleophiles to do 1,2- or 1,4-addition?

(g)

(h)

(i)

What products would be obtained from the reaction of 1,3,5-hexatriene with one equivalent of HBr? Disregard stereoisomers.

Whereas stabilized enolates do 1,4-addition, unstabilized (normal) enolates can do both 1,2- and 1,4-addition depending on the situation. Why might this be?