Alkenes, chlorinated/brominated

Chlorinated, Brominated, and Iodinated Alkanes, Alkenes, and Alkanoates... 

Cyclic alkenes with alkali alkoxides give almost exclusively products of nucleophilic substitution of one or more vinylic or allylic fluorine or halogen atoms. The preference of the attack is expressed by the sequence fluorine > chlorine > bromine.68... 

Iodine, less reactive than bromine, is best added to alkenes by use of IC1 and IBr (see Section 1.8.3.3). Iodine itself adds rapidly but reversibly to alkenes forming diiodides by mechanisms that can be either ionic or radical. The position of the equilibrium depends upon the structure of the alkene, the solvent and the temperature. Simple vicinal diiodides survive distillation in the dark, but are unstable toward iodine or radicals. In the presence of functions containing free OH groups, such as alumina, HI generated from I2 adds to alkenes irreversibly with the result that the HI adduct, rather than the I2 adduct, is the exclusive product.86 A comparison of the reaction of 1,5-cyclooctadiene with chlorine, bromine and iodine in CH2CI2 reveals that chlorine gas at-50 C gives a 93 7 mixture of trans- and cis-5,6-dichlorocyclooc-... 

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. 

In reactions with HX, alkenes behave more or less as Brpnsted-Lowry bases. Alkenes can also function as Lewis bases in reactions with electrophilic halogens. Symmetrical reagents such as bromine or chlorine are not usually polarized, but when they are brought into proximity with a n bond, a dipole is induced in the X—X bond [C=C- X+X ]. This type of structure is sometimes called a charge transfer complex. It is important to note that this occurs only with reagents such as diatomic chlorine, bromine, or iodine that are polarizable. Molecules that are not polarizable, such as diatomic hydrogen, do not react in this way, and actually require... 

If boron of an alkylborane could be replaced with a halogen, the product would be an alkyl halide. However, reaction of alkylboranes (neat) with chlorine, bromine, or iodine is very difficult. a when halogenation is done with bromine or iodine dissolved in dichloromethane, however, the reaction is fast and is synthetically useful.A simple example is the reaction of alkenes with boranes followed by addition of bromine, which leads to the alkyl bromide. An example is taken from the synthesis of 2-bromobutane (70) from 2-butene in 88% yield. 0 jhe bromination occurs by a free radical mechanism. Initial reaction with bromine generates a... 

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 ... 

What about diatomic fluorine, F-F For many years, elemental fluorine was thought to be too reactive and too dangerous for reaction with alkenes. To void such problems, fluorine is typically mixed with an inert gas such as nitrogen or argon. Diluted in this manner, fluorine does react with alkenes, but the yields are often poor and, in some cases, solvents for the alkene, such as methanol, participate in the reaction. 1-Phenylpropene (PhCH=CH2), for example, reacted with fluorine in methanol to give 51% of the corresponding difluoride, along with 49% of 2-fluoro-l-methoxy-l-phenylpropane. The problems associated with fluorine lead to a simpliflcation. In this chapter, alkene reactions are reported only for chlorine, bromine, or iodine but not fluorine. 

Chlorine, bromine, and iodine react with alkenes to give the three-membered ring halonium ion, which reacts with the halide nucleophile to give trans dichlorides, dibromides, or diiodides. Alkynes react to give vinylhalonium ions that lead to vinyl dihalides 14,15,16,17,18,19, 20, 21, 22, 64, 73, 74, 75, 82, 83, 98,108. HOCl and HOBr react with alkenes to give halohydrins 23, 24, 75, 82, 83. 

Reaction is initiated by interaction of the tt electrons of the alkene with bromine (or chlorine, as the case may be) that acts likes an electrophile to form an intermediate in which bromine bears a positive charge. A bromine atom bearing a positive charge is called a bromonium ion, and the cyclic structure of which it is a part is called a bridged bromonium ion. 

Consider a regression for a set of 66 halogenides and their boiling points [116]. The molecules considered include branched and cyclic halo-alkanes and several halo-alkenes involved as heteroatoms chlorine, bromine, and iodide. All the molecules and their boiling points, which cover the range from -24°C (chloromethane) to... 

So, a modification of our earlier addition mechanism suffices to explain the reaction. In contrast to the addition to aikenes we saw in Chapter 9 (HX, carbocations, and boranes), in alkene halogenation (bromination and chlorination) a three-membered ring is produced as an intermediate. In some, very spedalized cases the bromonium ion can even be isolated, and it seems its intermediacy in most brominations is assured. 

Li, X.A., Johnson, K.E. and Treble, R.G., Alkane cracking, alkene polymerization, and Friedel-Crafts alkylation in liquids containing the acidic anions HX2, XH(AIX4), XH(Al2X7), and A12X7 (X = chlorine, bromine), J. Mol. Catal. A Chem. 214,121-127 (2004). 

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