Preparing Alkyl Halides from Alkanes Radical Halogenation

Give lUPAC names for the following alkyl halides  

Draw structures corresponding to the following lUPAC names  

2 Preparing Alkyl Halides from Alkanes Radical Halogenation 

Recall from Section 6.3 that radical substitution reactions require three kinds of steps initiation, propagation, and termination. Once an initiation step has started the process by producing radicals, the reaction continues in a self-sustaining cycle. The cycle requires two repeating propagation steps in which a 

Although interesting from a mechanistic point of view, alkane halogenation is a poor synthetic method for preparing alkyl halides because mixtures of products invariably result. For example, chlorination of methane does not stop cleanly at the monochlorinated stage but continues to give a mixture of dichloro, trichloro, and even tetrachloro products. 

Structurally simple alkyl halides can sometimes be prepared by reaction of an alkane with Ch or Bi 2 through a radical chain-reaction pathway (Section 5.3). Although inert to most reagents, alkanes react readily with Cb or in the presence of light to give alkyl halide substitution products. T he reaction occurs by the radical mechanism shown in l- igurc 10.1 for chlorinatioii. 

The situation is even worse for chlorination of alkanes that have more than one sort of hydrogen. For example, chlorination of butane gives two mono-chlorinated products in addition to diclilorobutane, trichlorobutane, and so on. Thirty percent of the moiiochloro product is 1-chlorobutane, and seventy percent is 2-chlorobutane. 

As another example, 2-methylpropane yields 2-chloro-2-methylpropane and l-chloro-2-methylpropane in the ratio 35 65, along with more highly chlorinated products. 

Halontethane Bond length (pm) Bond strength (kJ/mol) (kcal/mol) Dipole moment (D) 

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Preparing Alkyl Halides from Alkanes Radical Halogenation 335 Table 10.1 A Comparison of the Halomethanes... 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

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