Alkenes halogen electrophilic addition

In contrast to the free radical substitution observed when halogens react with alkanes halogens normally react with alkenes by electrophilic addition... 

The reactions of halogens and hydrogen halides with alkenes are electrophilic addition reactions. This means that the initial attack on the organic molecule is by an electron-deficient species that accepts a lone pair of electrons to form a covalent bond. This species is called an electrophile. In the case of the reaction with hydrogen bromide, the mechanism for the reaction is as shown. 

Electrophilic addition of hydrogen bromide to alkenes follows Markovnikov s rule, leading to the product with halogen on the more-substituted position. However, trace amounts of hydroperoxides (among other impurities ) may initiate a reaction that gives rise to the anti-Markovnikov product, with bromine in the less-substituted position. 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

Halonium ion (Section 7.2) A species containing a positively charged, divalent halogen. Three-membered-ring bromonium ions are implicated as intermediates in the electrophilic addition of Br2 to alkenes. 

Although the reaction of ketones and other carbonyl compounds with electrophiles such as bromine leads to substitution rather than addition, the mechanism of the reaction is closely related to electrophilic additions to alkenes. An enol, enolate, or enolate equivalent derived from the carbonyl compound is the nucleophile, and the electrophilic attack by the halogen is analogous to that on alkenes. The reaction is completed by restoration of the carbonyl bond, rather than by addition of a nucleophile. The acid- and base-catalyzed halogenation of ketones, which is discussed briefly in Section 6.4 of Part A, provide the most-studied examples of the reaction from a mechanistic perspective. 

The addition of halogens and halogen acids to alkenes has been shown to be predominantly trans and where the results do not agree, explanations have been given in terms of steric factors. Dewar has proposed that in all electrophilic addition reactions where a classical carbocation is formed, cis addition is the rule and where there is the preponderance of the trans product, the effect is due to steric factors. 

The mechanism for the electrophilic addition of a halogen to an alkene proceeds via a cyclic ion intermediate. 

It is important to be able to look at a molecular structure and deduce the possible reactions it can undergo. Take an alkene, for example. It has a 7t bond that makes it electron-rich and able to attack electrophiles such as water, halogens and hydrogen halides in electrophilic addition reactions. Haloalkanes, on the other hand, contain polar carbon-halogen bonds because the halogen is more electronegative than carbon. This makes them susceptible to attack by nucleophiles, such as hydroxide, cyanide and alkoxide ions, in nucleophilic substitution reactions. 

Predicting the outcome of electrophilic additions to alkynes from an extension of alkene reactivity usually works well, and can be applied to halogenations and hydrations. Hydration of an alkyne has a subtle twist, however the product is a ketone This can still be rationalized quite readily, though. 

Other methods for the preparation of alkyl halides are electrophilic addition of hydrogen halides (HX) to alkenes (see Section 5.3.1) and free radical halogenation of alkanes (see Section 5.2). 

We shall give particular attention here to the addition of bromine to alkenes because this reaction is carried out very conveniently in the laboratory and illustrates a number of important points about electrophilic addition reactions. Much of what follows applies to addition of the other halogens, except fluorine. 

Electrophilic addition of the halogens and related X—Y reagents to alkenes and alkynes has been a standard procedure since the beginning of modem organic chemistry.1 Anti electrophilic bromination of such simple compounds as cyclohexene and ( )- and (Z)-2-butene, and variants of this reaction when water or methanol are solvents (formation of halohydrin or their methyl ethers, respectively), are frequently employed as prototype examples of stereospecific reactions in elementary courses in organic chemistry. A simple test for unsaturation involves addition of a dilute solution of bromine in CCU to the... 

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