Fluorination, Chlorination, and Bromination Reactions

Organocatalysis has led to the development of new methods for the asymmetric a-halogenation of carbonyl compounds leading to the formation of stereogenic C-X centers. Hence, details of direct enantioselective fluorination, chlorination, and bromination reactions will be presented in the following sections. 

Many of the reactions of halogens can be considered as either oxidation or displacement reactions the redox potentials (Table 11.2) give a clear indication of their relative oxidising power in aqueous solution. Fluorine, chlorine and bromine have the ability to displace hydrogen from hydrocarbons, but in addition each halogen is able to displace other elements which are less electronegative than itself. Thus fluorine can displace all the other halogens from both ionic and covalent compounds, for example... 

Halogenation Chemical reaction of substances with a halogen-typically, fluorine, chlorine, and bromine. See chlorination. ... 

Howard CJ, Evenson KM. 1976. Rate constants for the reactions of OH with CH4and fluorine, chlorine, and bromine substituted methanes at 296K. J Chem Phys 64 197-202. 

The reaction is explosive with fluorine and occurs under all conditions. With chlorine and bromine reaction occurs rapidly when exposed to light, undergoing a photochemical chain reaction. With iodine, the reaction is very slow, even at elevated temperatures. Hydrogen is a strong reducing agent. At high temperatures, the gas reduces many metal oxides to lower oxides or metals ... 

Reactions with halogens, fluorine, chlorine and bromine occur with explosive violence. Thus, in contact with liquid bromine it explodes forming potassium bromide ... 

Despite continued interest in the halogenation of heteroaromatic systems [149-151], there are still relatively few reports concerning the direct fluorination of heterocycles compared to publications that describe analogous chlorination and bromination reactions. 

Halogenation. Fluorination, chlorination, and bromination of alkanes catalyzed by superacids have been reported.1,2 Reactions may be carried out in the liquid phase, or in the gas phase over solid superacids or supported noble metal catalysts. High selectivity and relatively mild reaction conditions are the main features of these transformations. 

Novel polyfluoroethyl Gngnard reagents containing fluorine, chlorine, and bromine are prepared through the metal-halogen exchange reaction [46 (equation 20)... 

Selenium hexafluoride, SeFg, the only clearly defined hexahalide, is formed by reaction of fluorine with molten selenium, It is more reactive than the corresponding sulfur compound, SFs, undergoing slow hydrolysis. Selenium forms tetrahalides with fluorine, chlorine, and bromine, and dihalides with chlorine and bromine. However, other halides can be found in complexes, e.g,. treatment of the pyridine complex of SeF/i in ether solution with HBr yields (py)2SeBrc Selenium tetrafluoride also forms complexes with metal fluorides, giving MSeF complexes with the alkali metals. 

Carbon-halogen groups are well known to exist for fluorine, chlorine and bromine. These groups can be attached by redox reactions involving the halogen elements cither in aqueous solution or in the gas phase. At elevated temperatures bulk reactions occur under intercalation and eventually under formation of bulk covalent carbon-halogen materials (CF1.1 solid, CCU... 

N. Sadeghi, I. Hikmet, I. Colomb and D. W. Setser, State-to-State Dynamics for the Reactions of Metastable Copper Atoms with Fluorine, Chlorine and Bromine, in Gas-Phase Metal Reactions, edited by A. Fontijn, pp. 363-392, North-Holland, Amsterdam, 1992. 

Setser et a/, 6°. 63.836.939 b ve discussed similar studies on fluorinated, chlorinated and brominated species. The reactions of chlorinated species have also been reported by other workers . ... 

Once released in the middle atmosphere, fluorine, chlorine and bromine atoms react rapidly with ozone to form FO, CIO and BrO. Further reactions, which will be presented in the following sections, lead to efficient catalytic destruction of odd oxygen via CIO and BrO. 

Sodium reacts with explosive violence when mixed with mineral acids, especially hydrofluoric, sulfuric, and hydrochloric acids. It bums spontaneously in fluorine, chlorine, and bromine and a luminous reaction occurs with iodine. Sodium added to liquid bromine can explode when subjected to shock (Mel-lor 1946). Violent explosions may result when a mixture of sodium and a metal halide is subjected to impact. Shock-sensitive mixtures are formed upon combination with halogenated hydrocarbons, such as methyl chloride, methylene chloride, or chloroform. It reacts with incandescence when combined with many metal oxides, such as lead oxide or mercurous oxide. 

Only fluorine, chlorine, and bromine react this way with alkanes. Iodine is essentially unreactive due to unfavorable reaction energetics. 

The transition state structure for the addition of molecular fluorine to ethylene VIIc was found by the 3-2IG calculations [17] to closely resemble that for the addition of HCl (IV). Of special interest are the computational results obtained on transition state structures Vlld, Vile for gas phase electrophilic addition reactions of molecular chlorine and bromine to ethylene. Whereas the four-centered structure VIIc for the fluorination of ethylene indicates concerted c/s-addition, which is in accord with experimental finding for this reaction [18], the transition state geometries for chlorination and bromination may be regarded as cyclic halonium ions backed by halogenide counter-ions. Noteworthy is that the calculations [17] predict the heterolysis to occur intrinsically without any assistance from polar solvents. The three-centered structures Vlld, Vile help to clarify the reason for trans-stereoselectivity of the chlorination and bromination reactions of ethylene [1, 19]. 

In these reactions a halogen atom replaces one or more of the hydrogen atoms of the alkane and the corresponding hydrogen halide is formed as a by-product. Only fluorine, chlorine, and bromine react this way with alkanes. Iodine is essentially unreactive, for a reason that we shall explain later. 

In Summary Fluorine, chlorine, and bromine react with methane to give halomethanes. All three reactions follow the radical chain mechanism described for chlorination. In these processes, the first propagation step is always the slower of the two. It becomes more exo-... 

Aryl chlorides and bromides are con veniently prepared by electrophilic aro matic substitution The reaction is lim ited to chlorination and bromination Fluorination is difficult to control lodi nation is too slow to be useful... 

The reaction with fluorine occurs spontaneously and explosively, even in the dark at low temperatures. This hydrogen—fluorine reaction is of interest in rocket propellant systems (99—102) (see Explosives and propellants, propellants). The reactions with chlorine and bromine are radical-chain reactions initiated by heat or radiation (103—105). The hydrogen-iodine reaction can be carried out thermally or catalyticaHy (106). 

The reactions are general for chlorine and bromine. Iodine does not react to form pentavalent phosphoms compounds. Fluorides are best formed with less active fluorinating agents. 

It reacts violently with halogens it combusts in fluorine, chlorine and gaseous bromine. With liquid bromine the reaction is more violent and can lead to a detonation as with iodine. 

Here, X and Y are halogens and R represents the remainder of the carbon skeleton. The reaction proceeds particularly well for the halogens chlorine and bromine, but much less so for fluorine, as the C—F bond is much stronger than either C—Cl or C—Br. However, the excited sodium halide, NaX1, is not the emitting species. Another collision between this molecule and another vapor-phase sodium atom is needed ... 

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