Preparing Alkyl Halides from Alkenes Allylic Bromination

Give lUPAC names for the following alkyl halides  

Draw structures corresponding to the following lUPAC names  

We ve already seen several methods for preparing alkyl halides, including the reactions of HX and X2 with alkenes in electrophilic addition reactions (Sections 7.6 and 8.2). The hydrogen halides HCl, HBr, and HI react with alkenes by a polar mechanism to give the product of Markovnikov addition. Bromine 

Another laboratory method for preparing alkyl halides from alkenes is by reaction with iV-bromosuccinimide (abbreviated NBS) in the presence of light to give products resulting from substitution of hydrogen by bromine at the position next to the double bond—the allylic position (Section 8.13). Cyclohexene, for example, gives 3-bromocyclohexene. 

This allylic bromination with NBS is analogous to the methane chlorination reaction discussed in Section 6.3 and occurs hy a similar radical chain reaction mechanism. As in methane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br-radical, which cycles hack into the first step and carries on the chain. The Br2 results from reaction of NBS with the HBr formed in the first step. 

Problem 10.3 Draw and name all monochloro products you would expect to obtain from radical chlorination of 2-methylpentane. Which, if any, are chiral  

Problem 10.4 Taking the relative reactivities of 1°, 2°, and 3° hydrogen atoms into account, what product(s) would you expect to obtain from monochlorination of 2-methylbutane What would the approximate percentage of each product be (Don t forget to take into account the number of each sort of hydrogen.) 

Why does bromination with NBS occur exclusively at an allylic position rather than elsewhere in the molecule The answer, once again, is found by looking at bond dissociation energies to see the relative stabilities of various kinds of radicals. 

A similar calculation for the chlorination of 2-methylpropane indicates that each of the nine primary hydrogens accounts for 65% 9 = 7.2% of the product, while the single tertiary hydrogen (R3CH) accounts for 35% of the product. Thus, a tertiary hydrogen is 35 7.2 = 5 times as reactive as a primary hydrogen toward chlorination. 

The observed reactivity order of alkane hydrogens toward radical chlorination can be explained by looking at the bond dissociation energies given previously in Table 6.3 on page 204. The data show that a tertiary C-H bond (400 kj/ mol 96 kcal/mol) is weaker than a secondary C-H bond (410 kj/mol 98 kcal/ mol), which is in turn weaker than a primary C-H bond (421 kJ/mol 101 kcal/ mol). Since less energy is needed to break a tertiary C H bond than to break a primary or secondary C-H bond, the resultant tertiary radical is more stable than a primary or secondary radical. 

3 Preparing Alkyl Halides from Alkenes Allylic Bromination 

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PREPARING ALKYL HALIDES FROM ALKENES ALLYLIC BROMINATION... 

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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