Addition of Halogens to Double Bonds

To explain this reaction, consider the fact that, due to the overlapping p orbitals, double bonds are electron rich. This property allows olefins, under certain conditions, to act as nucleophiles. In the case of a double bond reacting with molecular bromine, the result is formation of a three-membered ring containing a positively charged bromine atom. This three-membered ring is known as a bridged bromonium ion. Concurrent to formation 

Arrow Pushing in Organic Chemistry An Easy Approach to Understanding Reaction Mechanisms. By Daniel E. Levy 

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The introduction of halogen into organic molecules can be carried out by a variety of addition or substitution reactions. The classical methods for the addition of halogen to double bonds or the substitution of halogen for hydroxyl by hydrohalic acids are too well known to bear repetition here. Discussed below, then, are methods that are of interest because of their stereospecific outcome or because they may be used on sensitive substrates. 

Unlike the addition of halogens across double bonds, addition of acids results in formation of asymmetrical products. Specifically, a different group is added to each side of the double bond. Thus, if this reaction is applied to asymmetrical olefins such as propene, multiple products might be expected to form as illustrated in Scheme 7.8. In fact, while a mixture of products is formed, there is an overwhelming presence of the secondary substituted product compared to that with substitution at the primary position. This preference of reaction products resulting from addition of protic acids across double bonds is governed by Markovnikov s rule. 

Addition of chlorine, bromine, and iodine to double bonds is a standard procedure in organic chemistry. Addition of fluorine to double bonds is used less often, as fluorine has a high tendency for radical reactions. However, a dilute stream of fluorine in nitrogen at low temperature and in polar solvents can suppress fluorine radical formation and encourage polar processes"2. The preferred syn addition of fluorine is opposite to the reaction of other halogens. 

Electrophilic addition reactions are another dass of reactions that have been extensively studied in organic solvents from a mechanistic point of view and bromine addition is one of the most investigated addition reactions. Chiappe et al. have used ionic liquids to synthesize vidnal dihaloalkanes and dihaloalkenes by dectrophilic addition of halogens to double and triple bonds (Scheme 5.1-15) [48-50]. Recently, dibromides have also been synthesized [51] in ionic liquids using electrogenerated bromine, whereas bromohydrins have been obtained [52] under two-phase conditions (water/IL) through a vanadium (V) catalyzed oxidation of bromide ions by hydrogen peroxide. 

Scheme 5.1-15 Synthesis of vicinal dihaloalkanes and dihaloalkenes by electrophilic addition of halogens to double and triple bonds in various ionic liquids.

It is also possible to start from chloroacetaldehyde derivatives such as 1,2-dihalogeno ethyl acetate yields can reach 90% (356). These compounds can be easily obtained by addition of halogen to the double bond of vinylacetate at 0 to 10°C. 

Halonium ion (Section 6 16) A species that incorporates apos itively charged halogen Bridged halonium 10ns are inter mediates in the addition of halogens to the double bond of an alkene... 

The addition of N204 to compds with conjugated double bonds takes place in a way similar to the addition of halogens to these compds. . . ... 

Addition of halogen to the double bond takes place when propene reacts with bromine or chlorine at low temperatures. 

Similar observations emerge from addition of halogens to butadiene. Thus, low-temperature bromina-tion gives predominantly the 1,2-adduct. At higher temperatures, the 1,4-adduct is the main product, and the mixture from the lower temperature reaction equilibrates to the same product ratio. The 1,4-product is the thermodynamically more stable it has the more-substituted double bond, and the two large bromine atoms are further apart in this isomer. Mechanisms for formation and equilibration of the products can be written as shown, using bromonium cation intermediates. It is perhaps less easy to see why the 1,2-adduct should be the kinetically controlled product, until... 

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. 

Unlike the addition of other halogens to double bonds, where trans addition occurs, Rozen suggested that the syn addition of fluorine procedes by way of a tight ion pair which collapses before any rotation about the C-C bond takes place (Fig. 73) [175]. 

Addition of Fluorine and Halogens to Double Bonds with Potassium... 

Because the haloform reaction is fast, in some cases it can be used to prepare unsaturated acids from unsaturated ketones without serious complications caused by addition of halogen to the double bond ... 

Interactions between the C=C double bond and halogens deserve particular attention. The importance of these interactions arises from the fact they are the first step of the reactions of addition of halogens to alkenes (and, in general, of the electrophilic addition to the carbon-carbon double bond68), which is a very useful functionalization reaction of the alkene double bond. 

Strong differences in the reactivity of the aromatic C=C double bond compared to the reactivity of the C=C double bond of olefins are observed olefinic electrophilic additions are faster than aromatic electrophilic substitutions. For instance, the addition of molecular bromine to cyclohexene (in acetic acid) is about 1014 times faster than the formation of bromobenzene from benzene and bromine in acetic acid113,114. Nevertheless, the addition of halogens to olefins parallels the Wheland intermediate formation in the halogenation of aromatic substrates. 

It can be concluded from these calculations that the addition of halogen to a double bond to form the symmetrical dihalide (a) is much easier than the substitution reaction (c). The former requires activation energies of the order of 25,000 calories while the latter requires energies of the order of 50,000 calories. The reaction rate (a) is approximately the same, as already discussed for all the halogens, chlorine, bromine, and iodine. It is evident that the activation energies are approximately the same for the atom-catalyzed chain reaction and for the bimolecular reaction. Ap-... 

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