Cyclohexene bromine reaction

The products of allylic bromination reactions are useful for conversion into dienes by dehydrohalogenation with base. Cyclohexene can be converted into 1,3-cyclohexadiene, for example. 

Cyclohexene, bromination of, 47, 32 reaction with 1-butyl perbenzoate and cuprous bromide, 48,18 2-Cyclohexenol, 46, 32 2-Cyclohexen-I-ol, benzoate, 48, 18 2-Cyclohexenone, 45, 32 Cydohexyl allophanamide, 45, 72 Cyclohexylamine, 45, 85 reaction with 1-butyl hypochlorite, 46, 16... 

The rate of cyclohexene bromination, too (in carbon tetrachloride), is lowered by the presence of NBS (or of other hydrogen bromide scavengers)74,102 probably, the reaction is complicated by the presence of further complexes, such as the tribromide of the cyclohexene bromonium ion. 

There are some instances where the chlorination or bromination reaction can be used to good effect, however. Alkenes that have allylic hydrogens can sometimes be halogenated specifically at an allylic position in a process called allylic halogenation, another free radical chain reaction. For example, when low concentrations of bromine are photolyzed in the presence of the complicated cyclohexene shown in Figure 11.51, the product is exclusively brominated in one allylic position. 

The reaction of cyclohexene with bromine is potentially rather complicated. We know that alkenes react with bromine by an electrophilic addition mechanism. Might not this reaction occur in competition with the aUylic bromination reaction at low concentrations of bromine The answer is no because the free-radical chain reaction is much faster than the addition reaction if the concentration of bromine is low. The free-radical chain reaction for reaction of cyclohexene with bromine has the foUowing steps. 

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

The radical addition of halogen to an alkene has been referred to briefly in Section 9.3.2. We saw an example of bromination of the double bond in cyclohexene as an unwanted side-reaction in some allylic substitution reactions. The mechanism is quite straightforward, and follows a sequence we should now be able to predict. 

Kennedy and Stock reported the first use of Oxone for many common oxidation reactions such as formation of benzoic acid from toluene and of benzaldehyde, of ben-zophenone from diphenyhnethane, of frawi-cyclohexanediol Ifom cyclohexene, of acetone from 2-propanol, of hydroquinone from phenol, of e-caprolactone from cyclohexanone, of pyrocatechol from salicylaldehyde, of p-dinitrosobenzene from p-phenylenediamine, of phenylacetic acid from 2-phenethylamine, of dodecylsulfonic acid from dodecyl mercaptan, of diphenyl sulfone from diphenyl sulfide, of triphenylphosphine oxide from triphenylphosphine, of iodoxy benzene from iodobenzene, of benzyl chloride from toluene using NaCl and Oxone and bromination of 2-octene using KBr and Oxone . Thus, they... 

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