Electrophilic addition reactions halogenation

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. 

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. 

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. 

Dioxin and 1,4-dithiin both undergo easy electrophilic addition reactions, e.g. of halogens to the double bonds. Alcohols under acid catalysis form ketal addition products. 

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. 

The reaction of either the enol or the enolate anion (Equations 17-2 or 17-3) with Br2 resembles the first step in the electrophilic addition of halogens to carbon-carbon multiple bonds (Section 10-3A). However, the second step,... 

When hydrogen abstraction or electrophilic addition reactions may be inhibited by multiple halogen substitutions or steric hindrance, a hydroxyl radical can be reduced to a hydroxide anion by an organic substrate shown in Equation (7.28) ... 

Electrophilic addition reactions involving halogen-containing... 

IZV118) and the formation of (31) is analogous to the reaction (197)->(98) via a four-membered 1,2-oxathietane 2,2-dioxide intermediate. Subsequent products derived from (31) by electrophilic addition reactions at the alkenic double bond have been described in Section 4.33.3.2.2 and the synthesis of 4,5-dichloro-l,3,2-dioxathiolane 2,2-dioxide (154) by chlorination of ethylene sulfate (18) is discussed in Section 4.33.3.5. Cyclic sulfites, on the other hand, cannot be halogenated without ring opening (cfSection 4.33.3.2.4). 

Strategy Reaction of cyclohexene with HC1 or HBr is an electrophilic addition reaction in which a halogen acid adds to a double bond to produce a haloalkane. 

For electrophilic additions of halogens to alkenes, not only is the reaction rate strongly solvent-dependent [79-81] [cf. Eq. (5-29) in Section 5.3.2), but the stereochemical course may also be affected by the polarity of the medium [79, 386-388], For example, the stereoselectivity of bromine addition to cis- and trans -stilbene according to Eq. (5-140) has been found to be solvent-dependent, as shown in Table 5-23 [79, 386],... 

A reaction pathway involving heterolytic cleavage of the B—B bond might be expected to yield trans addition products if intermediates such as a coordinated boronium ion (VI) or a coordinatively stabilized carbonium ion (VII) were involved or nonspecific addition if a carbonium ion intermediate such as (VIII) were produced. The formally similar electrophilic addition of halogen to simple olefins is, of course, well known to lead to... 

Then we shall examine the stereochemistry of several reactions we have already studied—free-radical halogenation of alkanes, and electrophilic addition of halogens to alkenes- and see how stereochemistry can be used to get information about reaction mechanisms. In doing this, we shall take up ... 

Alkene chlorinations and brominations are very general reactions, and mechanistic study of these reactions provides additional insight into the electrophilic addition reactions of alkenes. Most of the studies have involved brominations, but chlorinations have also been examined. Much less detail is known about fluorination and iodination. The order of reactivity is F2 > CI2 > Br2 > I2. The differences between chlorination and bromination indicate the trends for all the halogens, but these differences are much more pronounced for fluorination and iodination. Fluorination is strongly exothermic and difficult to control, whereas for iodine the reaction is easily reversible. 

The halogens Br2 and CI2 add to alkenes. This may be surprising because it is not immediately apparent that an electrophile—which is necessary to start an electrophilic addition reaction—is present. 

However, the bond joining the two halogen atoms is relatively weak (see the bond dissociation energies listed in Table 3.1) and, therefore, easily broken. When the tt electrons of the alkene approach a molecule of Br2 or CI2, one of the halogen atoms accepts the electrons and releases the shared electrons to the other halogen atom. Therefore, in an electrophilic addition reaction, Br2 behaves as if it were Br" " and Br , and CI2 behaves as if it were Cl and CF. 

F2 and I2 are halogens, but they are not used as reagents in electrophilic addition reactions. Fluorine reacts explosively with alkenes, so the electrophilic addition of F2 is not a synthetically useful reaction. The addition of I2 to an alkene is a thermodynamically unfavorable reaction The vicinal diiodides are unstable at room temperature, decomposing back to the alkene and I2. 

Benzene s aromaticity causes it to undergo electrophilic aromatic substitution reactions. The electrophilic addition reactions characteristic of alkenes and dienes would lead to much less stable nonaromatic addition products. The most common electrophilic aromatic substitution reactions are halogenation, nitration, sulfonation, and Friedel-Crafts acylation and alkylation. Once the electrophile is generated, all electrophilic aromatic substitution reactions take place by the same two-step mechanism (1) The aromatic compound reacts with an electrophile, forming a carbocation intermediate and (2) a base pulls off a proton from the carbon that... 

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