Alkenes, reaction with aqueous bromine

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

The two tests employed for the detection of unsaturation are decolourisation of a dilute solution of bromine in dichloromethane, and reaction with dilute aqueous potassium permanganate. It is essential to apply both tests since some symmetrically substituted alkenes (e.g. stilbene, C6H5,CH=CH-C6H5) react only slowly under the conditions of the bromine test. With dilute potassium permanganate solution the double bond is readily attacked, probably through the intermediate formation of a ds-diol. 

In aqueous solution bromine reacts with alkenes to give bromohydrins. Bromine is the electrophile in this reaction and adds to the carbon that has the greater number of attached hydrogens. 

Halohydrins are easily generated by treating alkenes with aqueous solutions of halogens. Bromine water and chlorine water add across double bonds with Markovnikov orientation (Section 8-11). The following reaction shows cyclopentene reacting with chlorine water to give the chlorohydrin. Treatment of the chlorohydrin with aqueous sodium hydroxide gives the epoxide. 

Bromine (Br2) is brown, and one of the classic tests for alkenes is that they turn a brown aqueous solution of bromine colourless. Alkenes decolourize bromine water alkenes react with bromine. The product of the reaction is a dibromoalkane, and the reaction below shows what happens with the simplest alkene, ethylene (ethene). 

Another addition reaction involves reaction of alkenes with aqueous solutions of bromine or chlorine rather than the anhydrous conditions noted above. The reaction can proceed by formation of a cation under aqueous... 

In a related reaction, chlorine and bromine react with alkenes in aqueous solution to give vicinal halohydrins, compounds that have a halogen and a hydroxyl group on adjacent carbons. 

Now let s draw the forward scheme. Ozonolysis converts the starting alkene into a ketone (with loss of a carbon atom). The resulting ketone is then treated with molecular bromine (Br2) and sodium hydroxide, followed by aqueous acid, to give a carboxylic acid (via a haloform reaction). Finally, the carboxylic acid is converted into an acid chloride upon treatment with thionyl chloride. 

Before discussing examples of the reactions, it should be pointed out that most types of vinylic halides used in this reaction are easily available. The UV-catalyzed addition of HBr to terminal acetylenes forms the 1-bromo-l-alkenes. The cis isomer is formed almost exclusively if the addition is carried out at dry ice temperature. The 2-bromo or 2-iodo-l-alkenes are obtained from reacting aqueous hydrogen halides with alkynes. The 2-substituted-1-bromo-l-alkenes are available by the bromination-base dehydrobromination reactions. 

In aqueous solution a bromonium ion is formed as the intermediate when bromine reacts with an alkene. This intermediate bromonium ion can react with a molecule of bromine to form a dibromide, and it can react with water to form a bromohydrin. Both reactions give a mixture of products ... 

Bromination of methylenecyclopropane with A -bromosuccinimide in dimethyl sulfoxide in the presence of water afforded different products depending on the ratio methylenecyclopropane/ Wbromosuccinimide and the amount of water available. Two equivalents of iV-bromosuc-cinimide predominantly gave l-bromo-l-(bromomethyl)cyclopropane (80%) in addition to some cyclobutanone (20%) derived from 1-(bromomethyl)cyclopropanol (Table 1, entry 7). With equimolar amounts of the alkene and AT-bromosuccinimide in aqueous solution (Table 1, entry 8), the bromohydrin, 1-bromo-l-hydroxymethylcyclopropane, was formed as the main product (70%).The reaction conditions suggest the addition of a positively charged bromine rather than a radical mechanism. 

Alkenes react rapidly with bromine at room temperature to form colourless dibromoalkanes. For example, if ethene gas is bubbled into bromine water (an aqueous solution of bromine) the colour of the solution turns from reddish-brown to colourless. This reaction is often used to test for unsaturation in a hydrocarbon ... 

In the preceding section, diatomic bromine, chlorine, or iodine reacted with an alkene in a nonaqueous solvent such as carbon tetrachloride. Exploring a reaction usually requires experimenting with reaction conditions, including changing the solvent. A reasonable experiment with an alkene that involves chlorine might use an aqueous solution saturated with chlorine gas. 

It is known that dissolving chlorine in water leads to a solution that contains hypochlorous acid (HOCl) and bromine dissolved in water contains hypobromous acid (HOBr). In one experiment, 1-pentene is mixed with chlorine and water (HOCl in aqueous media) and the major product is l-chloro-2-pentanol (48), in 43% isolated yield. ° In the previous section, chlorine reacted with 1-pentene in a nonaqueous solvent such as carbon tetrachloride to give a dichloride. To ascertain why this reaction is different, the first useful observation is that HOCl is in solution rather than Cl-Cl. The polarization of HOCl is HO -Cl, where chlorine is the electrophilic atom. The 7t-bond of an alkene should react with the positive chlorine atom, and cleavage of the Cl-H bond will give hydroxide ion, which is a... 

Because vinyl azides are electron-rich alkenes, treatment of these substrates with electrophiles was studied in detail. In most cases, the attack of the a-nitrogen atom or the j3-carbon of the olefinic part at the electrophile led to final products without an intact azido group such as amides or ketones. Only a few exceptions were reported, for instance, the reaction of the terminal enazides 40 or 54 with bromine in methanol yielding mainly the -azido ethers 227 (Scheme 5.28). Already in 1910, Forster and Newman investigated the conversion of the parent compound 1 with bromine in aqueous solution.Due to the explosion-like course of the reaction and the high sensitivity of the addition product to hydrolysis generating bromoacetaldehyde, however, the desired substance 228 could not be characterized. Quite recently, is has been shown that the transformation 1 228... 

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