Addition of Halogen

Halogens may be added to alkenes at room temperature without a catalyst. One mole of alkene requires one mole of halogen. 

Addition of Br2 to alkenes is a useful reaction which shows us if a hydrocarbon is saturated or unsaturated. When an unknown hydrocarbon is added to a solution of Br2 in water, if the color of solution disappears, it is an unsaturated hydrocarbon (alkene or all e). If color doesn t change, it is an alkane. 

When an unsaturated hydrocarbon is added to a solution of bromine (tube on the left), it becomes colorless (tube on the right). 

Markovnikov was a Russian chemist. He studied at Kazan University under Butlerov and later came back to teach there. In 1869 while studying addition reactions, he noticed a trend in the structure of the favored product. 

This trend is now called the Markovnikov s Rule and it states that in the addition of HX to an alkene, hydrogen adds to the carbon with the most hydrogen atoms. This rule is a very useful aid for predicting experimental results. 

In the presence of a red phosphorous catalyst, chlorine and bromine react with carboxylic acids to produce a-halo acids. When excess amounts of halogen are used, the number of substituted hydrogens increases. 

Interpretations of stereochemistry have focused attention on the role played by cyclic halonium ions  

If the addition of Br to the alkene results in a bromonium ion, the anti stereochemistry can be readily explained. Nucleophilic ring opening by bromide ion would occur by back-side attack at carbon, with rupture of one of the C-Br bonds, giving overall anti addition  

On the other hand, a freely rotating open carbonium ion would be expected to give both syn and anti addition. If the principal intermediate were an ion pair that collapsed faster than rotation about the C-C bond, syn addition could predominate. 

Whether a bridged intermediate or a carbonium ion is involved in bromination depends primarily on the stability of the potential carbonium ion. Aliphatic systems normally go through the bridged intermediate but styrenes are a borderline case. When the phenyl ring has electron-releasing substituents there is sufficient stabilization to permit carbonium ion formation, while electron-attracting groups favor the 

The stereochemistry of chlorination can be explained in similar terms. Chlorine would be expected to be a somewhat poorer bridging group than bromine because 

Alkene chlorinations and brominations are among the most general of organic reactions and have therefore been the subject of much mechanistic study. Two of the principal points at issue in the description of the mechanism for a given reaction are  

The stereochemistry of chlorination can be explained in similar terms. Chlorine would be expected to be a somewhat poorer bridging group than bromine. Comparison of the data for cw- and trans-1-phenylpropenc in bromination and chlorination confirms that it is. Although anti addition is dominant for bromination, syn addition is preferred for chlorination. For nonconjugated alkenes, however, stereospecific anti addition is usually observed with both halogens. 

The reactivity of carbon-carbon double bonds toward acid-catalyzed addition of water is greatly increased by ERG substituents. The reaction of vinyl ethers with water in acidic solution is an example that has been carefully studied. With these reactants, the initial addition products are unstable hemiacetals that decompose to a ketone and alcohol. Nevertheless, the protonation step is rate determining, and the kinetic results pertain to this step. The mechanistic features are similar to those for hydration of simple alkenes. Proton transfer is rate determining, as demonstrated by general acid catalysis and solvent isotope effect data.  

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 initial step in bromination is the formation of a complex between the alkene and Br2. The existence of these relatively weak complexes has long been recognized. Their role as intermediates in the addition reaction has been established more recently. 

Reviews D. P. de la Mare and R. Bolton, in Electrophilic Additions to Unsaturated Systems, 2nd Edition, Elsevier, New York, 1982, pp. 136-197 G. H. Schmidt and D. G. Garratt, in The Chemistry of Double Bonded Functional Groups, Supplement A, Part 2, S. Patai, ed., Wiley-Interscience, New York, 1977, Chap. 9 M.-F. Ruasse, Adv. Phys. Org. Chem., 28, 207 (1993) M.-F. Ruasse, Industrial Chem. library, 7, 100 (1995) R. S. Brown, Industrial Chem. Library, 1, 113 (1995) G. Bellucci and R. Bianchini, Industrial Chem. Library, 7, 128 (1995) R. S. Brown, Acc. Chem. Res., 30, 131 (1997). 

The kinetics of bromination reactions are often complex, with at least three terms making contributions under given conditions. 

The yields in this reaction are often relatively low because of the strongly acidic conditions. In addition, the reaction conditions are very favorable for carbocation rearrangements as shown in the following example  

For these reasons, acid-catalyzed hydration is often not the method of choice for preparing alcohols from alkenes. Another reaction that accomplishes this same transformation, often in higher yield, is described in Section 11.6. 

Show all of the steps in the mechanism for the addition of water to propene catalyzed by sulfuric acid. Explain whether propene or phenylethene (PhCH=CH2) has a faster rate in this reaction. 

Both Cl2 and Br2 add to carbon-carbon double bonds to produce dihalides as illustrated in the following examples. The other halogens are not commonly used—F2 because it is too reactive and I2 because it is not reactive enough. These reactions are usually run in an inert solvent such as CC14, CHC13, or CH2C12. 

CHAPTER I I ADDITIONS TO CARBON-CARBON DOUBLE AND TRIPLE BONDS 

Chlorination of molten maleic anhydride yields a,)3-dichlorosuccinic anhydride 55. Hydrolysis of the latter can yield the di-acid 56. The addition reaction is usually carried out under pressure.  

Alternately, when a mixture of fumaric acid, water, and chlorine are subjected to sunlight for 4 days, a quantitative yield of the meso dichlorosuc-cinic acid 57 is obtained/ Maleic acid on the other hand gives racemic (dl) acid 58. Thus, under these reaction conditions, trans addition is said to occur. However, if chlorine is added to soluble neutral salts of the acids in water in the presence of excess chloride ion, cis addition occurs. 

Addition of bromine has been the subject of a number of studies. When a neutral salt of maleic or fumaric acid (A = COO ) is brominated in water, the meso derivative is the predominant product. Bell and Pring also report that the meso product is obtained on bromination of the diethyl esters of fumaric and maleic acids. Terry and Eicheberger observed 78% meso product from disodium maleate, a cis addition product which is unexpected. The prevalent bromonium ion mechanism proposed by Roberts and Kim-ball could easily explain the trans addition as follows  

normal halogen addition occurs with fumaric acid. The difficulty is with maleic derivatives. It had been argued by Roberts and Kimball that 

Recently, this explanation has been questioned. Weiss has reexamined the reaction of maleate anion with bromine. He argues that if the Roberts-Kimball mechanism is correct, it could be tested against the following conclusions  

This is supported by the fact that the reaction with fumaric acid gives meso-dibromosuccinic acid, whereas maleic acid gives the chiral dibromosuccinic acid, of course as a racemic mixture. Note that these reactions are stereospecific. 

The addition of Cl2 to C=C double bonds is trans-selective only when it takes place via three-membered chloronium ions. But it can also take place without stereocontrol, namely, when carbenium ion intermediates appear instead of chloronium ions. This is observed in Cl2 additions that can occur via benzyl or ferf-alkyl cations. 

In general, an addition of I2 to C=C double bonds is thermodynamically impossible, although an iodonium ion can still form. 

Strained alkenes show enhanced reactivity toward acid-catalyzed hydration. trans-Cyclooctene is about 2500 times as reactive as the cis isomer. This reflects the higher ground state energy of the strained alkene. 

Reviews D. P. de la Mare and R. Bolton, in Electrophilic Additions to Unsaturated Systems, second edition, Elsevier, New York, 1982, pp. 136-197 G. H. Schmidt and D. G. Garratt, in The Chemistry of Double Bonded Functional Groups, Supplement A, Part 2, S. Patai (ed.), Wiley-Interscience, New York, 1977, Chapter 9. 

There is direct evidence for the existence of bromonium ions. The bromonium ion related to propene can be observed by NMR spectroscopy when l-bromo-2-fluoropropane is subjected to superacid conditions. The terminal bromine adopts a bridging position in the resulting cation. 

We saw earlier that halogens add across the double bond of an alkene to give l,2-dihalo compounds. In this section, we will consider the addition of chlorine, bromine, and bromine in water, a reaction that produces a halohydrin that is, a compound with a halogen on one carbon and an hydroxyl group on the adjacent carbon. 

The reaction of an alkene with Br or Cl occurs rapidly at room temperature. The reaction is usually carried out in carbon tetrachloride or methylene chloride as solvent. The product is called a vicinal (Latin, vicinalis, neighboring) dihalide. Only one product forms. 

Chlorine also adds to a carbon-carbon double bond, but iodine is not sufficiently reactive to give a good yield of addition product. The reaction of alkenes with fluorine is too reactive to control, and several competing reactions also occur if fluorine is used. 

Rearrangement reactions seldom occur when halogens add to alkenes. We recall that water adds to 3,3-dimethyl-1-butene to give a rearranged product as well as the expected product. In contrast, bromine reacts with this alkene to give a single product. This fact suggests that a carbocation similar to that formed in the addition reactions we discussed previously is not formed in the addition of bromine. 

The addition of bromine to cycloalkenes can potentially form two stereoisomeric vicinal dibromides. However, only the trans isomer forms. The reaction is stereospecific. The stereochemistry of the products indicates that the halogen atoms bond from opposite faces of the double bond. 

Usually the halogen is dissolved in some inert solvent such as tri- or tetrachloro-methane, and then this solution is added dropwise to the alkene. Reaction is nearly instantaneous, even at room temperature or below. No light or heat is required, as in the case of substitution reactions. 

PROBLEM 3.8 Write an equation for the reaction of bromine at room temperature with 

The addition of bromine can be used as a chemical test for the presence of unsaturation in an organic compound. Bromine solutions in tetrachloromethane are dark reddish-brown, and both the unsaturated compound and its bromine adduct are usually colorless. As the bromine solution is added to the unsaturated compound, the bromine color disappears. If the compound being tested is saturated, it will not react with bromine under these conditions, and the color will persist. 

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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. 

Many of the features of the generally accepted mechanism for the addition of halogens to alkenes can be introduced by referring to the reaction of ethylene with bromine... 

Addition of halogens (Sections 6 14-6 16) Bromine and chlorine add to alkenes to form vicinal dihalides A cy clic halonium ion is an intermediate Stereospecific anti addition is observed... 

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... 

Addition of halogens proceeds stepwise, sometimes accompanied by oxidation. Iodine forms 2,3-diiodo-2-butene-l,4-diol (53). Depending on conditions, bromine gives 2,3-dibromo-2-butene-l,4-diol, 2,2,3,3-tetrabromobutane-l,4-diol, mucobromic acid, or... 

Catalysis. Catalytic properties of the activated carbon surface are useful in both inorganic and organic synthesis. For example, the fumigant sulfuryl fluoride is made by reaction of sulfur dioxide with hydrogen fluoride and fluorine over activated carbon (114). Activated carbon also catalyzes the addition of halogens across a carbon—carbon double bond in the production of a variety of organic haUdes (85) and is used in the production of phosgene... 

The nature of the substituent introduced is determined by the anion present with HX opening, X may be F, Cl, Br, OH, or OMe among others. (See ref. 297, 298, 345, 346, 347, for example). Where X is halogen, the same overall result may sometimes be achieved by addition of halogen followed by elimination. ... 

Addition of Halogen Fluorides to Unsaturated Systems by Andrew E. Feiring... 

Table 2. Products of Addition of Halogen Fluorides to Norbomene...

Additions of halogen fluorides to the more electrophilic perfluonnated olefins generally require different conditions Reactions of iodine fluoride, generated in situ from iodine and iodine pentafluoride [62 102 103, /05] or iodine, hydrogen fluoride, and parapeiiodic aud [104], with fluormated olefins (equations 8-10) are especially well studied because the perfluoroalkyl iodide products are useful precursors of surfactants and other fluorochemicals Somewhat higher temperatures are required compared with reactions with hydrocarbon olefins Additions of bromine fluoride, from bromine and bromine trifluonde, to perfluonnated olefins are also known [lOti]... 

More general procedures for additions of halogen fluorides to highly fluori-nated olefins involve reactions with a source of nucleophilic fluoride ion, such as an alkali metal fluoride, in the presence of aposttive halogen donor [62 107, lOff, 109, 110, 111] (equations 11 and 12) These processes are likely to occur by the generation and capture of perfluorocarbamonic intermediates Tertiary fluormated carbanions can be isolated as cesium [112], silver [113], or tns(dimethylamino)sul-... 

Generally, additions of halogens to fluoroalkenes are less stereoselective than the analogous reactions with nonfluorinated systems. The stereochemical mode of addition can be either anti or syn Partitioning between these paths is determined... 

The addition of halogenated aliphatics to carbon-carbon double bonds is the most useful type of carbon-carbon bond forming synthetic method for highly halogenated substrates Numerous synthetic procedures have been developed for these types of reactions, particularly for the addition of perfluoroalkyl iodides to alkenes using thermal or photolytic initiators of free radical reactions such as organic peroxides and azo compounds [/]... 

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