Cyclohexene dehydrohalogenation

Cyclohexenone has been prepared by dehydrohalogenation of 2-bromocyclohexanone, by the hydrolysis and oxidation of 3-chlorocyclohexene, by the dehydration of a-hydroxycyclohexa- ione, by the oxidation of cyclohexene with chromic acid or hydrogen peroxide in the presence of a vanadium catalyst, by I lie addition of acroleiti to ethyl acetoacctate followed by cycliza-lion, hydroly.sis, and decar])oxylation, by the reduction of N,N-dimelliyliiniline with sodium and ethanol itt liquid ammonia... 

Cyclohexyl bromide, for exfflnple, is converted to cyclohexene by sodium ethoxide in ethanol over 60 times faster than cyclohexyl chloride. Iodide is the best leaving group in a dehydrohalogenation reaction, fluoride the poorest. Fluoride is such a poor leaving group that alkyl fluorides are rarely used as starting materials in the preparation of alkenes. 

The two most common elimination reactions arc dehydroUalogenalion—the loss of HX from an alkyl halide—and dehydration—(he loss of water from an alcohol. Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base such as potassium hydroxide. For example, bromocvclohexane yields cyclohexene when treated with KOH in ethanol solution. 

The alkylation reaction is limited to the use of primary alkyl bromides and alkyl iodides because acetylide ions are sufficiently strong bases to cause dehydrohalogenation instead of substitution when they react with secondary and tertiary alkyl halides. For example, reaction of bromocyclohexane with propyne anion yields the elimination product cyclohexene rather than the substitution product 1-propynylcyclohexane. 

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. 

The preparation of cyclohexylbenzene from cyclohexene and benzene was described in text Section 12.6. Cyclohexylbenzene is converted to 1-phenylcyclohexene by benzylic bromination, followed by dehydrohalogenation. 

More recently, it has been shown that acetylation of cyclohexene with acetic anhydride in the presence of stannic chloride is less troublesome and does not necessitate dehydrohalogenation. ... 

Dehydrohalogenation. Use of the reagent for this purpose is illustrated by a procedure for the preparation of 1,3-cyclohexadiene. The chlorination is done in excess refluxing cyclohexene with t-butyl hypochlorite and dibenzoyl peroxide as... 

In the case of a still less reactive halide, or one with a tendency to undergo dehydrohalogenation, it is advantageous to add a reactive halide such as 1-bromo-naphthalene or n-butyl bromide for entrainment. Thus the reaction of 0.05 mole of eyclohexyl chloride and 0.05 mole of 1-bromonaphthalene with 0.33 g. atom of magnesium and isopropanol (0.3 mole) in 50 + 20 ml. of decalin afforded a mixture of 83% of cyclohexane and 10% of cyclohexene (removable with sulfuric acid). By this procedure cyclohexyl fluoride gives cyclohexane (33%) and benzotrifluoride gives toluene (10%). Fluorobenzene is inert. 

Cyclohexene can be synthesized by partial hydrogenation of benzene, by partial dehydrogenation of cyclohexane, or by dehydrohalogenation of cyclohexyl halides. The hydrogenation of benzene is the most viable route in the Asahi process, cyclohexene is obtained with a 60% yield and 80% selectivity, with the remainder being converted into cyclohexane. Asahi has developed a process for the addition of water to cyclohexene to produce cyclohexanol that can then be oxidized to AA using the conventional nitric acid oxidation. 

Modified Darzens reaction. The original Darzens synthesis of u./3-unxaturated ketones3 involved the addition of acid chlorides to cyclohexenes in the presence of aluminum chloride or stannic chloride followed by dehydrohalogenation. For example, the addition of acetyl chloride to cyclohexene affords l-acetyl-2-chlorocyclohexane, which on dehydrohalogenation (dimethylaniline) gives methyl cyclohexenyl ketone. 

Pyrolysis of hexafluoropropylene oxide generates difluorocarbene for cyclo-propanation reactions, as does the action of heat on chlorodifluoromethane. Halogenocarbenes formed in the thermal decomposition of polyhalogenomethanes at 500—650°C can be trapped by cyclopentene and cyclohexene. The primary adducts are unstable under these conditions, suffering dehydrohalogenation to halogeno-benzenes and cycloheptadienes, respectively. ... 

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