BackWe have studied the following reactions in previous chapters. For each, (i) indicate which reaction sheets they should appear on, (ii) show the best structure to use to represent them, and (iii) write the notes you could put in the margin so that the mechanism is implied.
(b) Radical halogenation of an alkane
Provide a mechanism for the chlorination of cyclohexane. Be sure to include initiation, propagation, and three possible termination steps.
For the following reaction, answer questions (a)–(d).
(a) Give an arrow-pushing mechanism, including the initiation step and two propagation steps.
Predict the major products of the following alkane halogenation reactions. [The number of products shown ignores the formation of racemic mixtures.]
(c)
Other molecules can be used as initiators in radical reactions. One such molecule is 2, 2'-azobisisobutyronitrile (AIBN). Show an arrow-pushing mechanism that rationalizes the formation of the following radical species. What are the driving forces of this reaction?
Cyclizations can be carried out under radical conditions involving Bu₃SnH and a catalytic amount of AIBN. Suggest a mechanism for this reaction. [Pay close attention to the bonds formed and broken in developing your mechanism. It may be helpful to number the carbons.]
What alkane, with molecular formula C5H12, forms only one monochlorinated product when it is heated with Cl2?
Explain why the rate of bromination of methane decreases if HBr is added to the reaction mixture.
How many alkyl halides are obtained from monochlorination of the alkanes in Problem 4 if stereoisomers are included?
c.
How many monochlorination products would be obtained if all stereoisomers are included?
What is the major product obtained from treating an excess of each of the following compounds with Cl2 in the presence of ultraviolet light at room temperature? Disregard stereoisomers.
a.
In the presence of an acid catalyst, acetaldehyde forms a trimer known as paraldehyde. Because it induces sleep when it is administered to animals in large doses, paraldehyde is used as a sedative or hypnotic. Propose a mechanism for the formation of paraldehyde.
Propose a mechanism for the following reaction:
How many alkyl halides are obtained from monochlorination of the alkanes in Problem 4 if stereoisomers are included?
d.
How many monochlorination products can be obtained from the radical chlorination of methylcyclohexane? Disregard stereoisomers.