Addition of Halomethanes

One of the older preparative free-radical reactions is the addition of polyhalomethanes to alkenes. Examples of addition of carbon tetrabromide, carbon tetrachloride, and bromoform have been recorded. The reactions are chain processes that depend on facile abstraction of halogen or hydrogen from the halomethane  

In- + CBr4 —- InBr + -CBf3 -CBrs + CH2=CHR —- Br3CCH2CHR Br3CCH2CHR + CBr4 —- Br3CCH2CHR + -CBr3 

In- + CHBrj —- InH + -CBrj -CBrj + CH2=CHR —- Br3CCH2CHR Br3CCH2CHR + HCBf3 —- Br3CCH2CH2R + -CBr3 

Bromotrichloromethane can also be used effectively in the addition reaction. Because of the preferential abstraction of bromine, a trichloromethyl unit is added to the less substituted carbon atom of the alkene  

The efficiency of the halomethane addition process depends on the relative rate of halogen-atom abstraction versus that of addition to the alkene  

Sosnovsky, Free Radical Reactions in Preparative Organic Chemistry, Macmillan, New York, 1964, Chapter 2. 

One of the older preparative free-radical reactions is the addition of poly-halomethanes to alkenes. Examples of addition of carbon tetrabromide, carbon 

The addition of bromotrichloromethane to cyclohexene gives a nearly 1 1 mixture of the two possible stereoisomers.  

This result indicates that the initially added trichloromethyl group has little influence on the stereochemistry of the subsequent bromine atom abstraction. The intermediate 2-(trichloromethyl)cyclohexyl radical presumably relaxes to the equatorial conformation faster than the occurrence of bromine atom abstraction, and there is little preference for axial or equatorial approach. In contrast, with A -octahydronaphthalene, the addition is exclusively franx-diaxial. 

---

For n = 1, this is the standard synthesis of [l.l.l]propellane 40a.20,21 Also for n = 3, this method makes [3.1.1]propellane 40c accessible with reasonable effort.22 Although [2.1.1]propellane 40b was detected by its IR spectrum at liquid nitrogen temperature,23 it could not be obtained as a stable compound via this route.22 The addition of halomethanes across the central bond by a radical chain mechanism is common to 40a and 40c. The addition of carbon tetrabromide to 40c afforded a 45% yield of 41.18 Likewise, a number of bicy-clo[l.l.l]pentane derivatives 42 were obtained by reaction of the corresponding halomethanes with 40a.17,24... 

The addition of halomethanes across the central bond of [l.l.l]propellane 78 leads to l-halo-7-(n-halomethyl)tricyclo[4.2.0.02,7]octanes 79, which are suitable precursors for the generation of type 80 carbenes.26... 

The effects of a- vs. P-fluonnation imply that fluoroolefins normally react regioselectively with electrophiles to minimize the number of fluorines p to the electron-deficient carbon in the transition state. Diverse types of electrophilic additions follow this rule (equations 6-8, for example), although there are exceptions, especially for ionic additions of halomethanes to fluoroolefins [724]... 

O Paleta, Ionic Addition Reactions of Halomethanes with Fluoroolefins... 

Ionic Addition RedcCioni of Halomethanes with Ruoroolcfins... 

The utility of this method depends on the reaction conditions, on the type of alkyllithium, i.e. methyl- or butyllithium, and on the halomethane used for the synthesis of methylli-thium" (for a detailed discussion see Houben-Weyl, Vol. 4/3, pp 225-228 and Vol. E19b, pp 1505-1506). Chlorocyclopropanes 1 were prepared via addition of dichloromethyllithium (from dichloromethane and butyllithium or lithium diisopropylamide) to fulvenes followed by the cyclization of the adducts. ... 

Volatile Organic Compounds (VOCs) constitute a very important class of water pollutants because of their persistence in addition, many of them are suspected of being carcinogenic. There are about 60 VOCs, including benzene, toluene, ethylbenzene, and xylenes ( BTEX compounds ), halomethanes, and haloethanes. The presence of some of them in water is due to anthropic activities, for example, the use of chlorinated solvents in industries and laundries, and the formation of halomethanes as by-products of water disinfectants. With respect to Italian law DL 31/01, the maximum allowable concentration (threshold) for the sum of trichloroethylene and tetrachloroethylene concentrations in drinking water is 10 ppb, whereas the minimum account for the sum of a set of four halogenated compounds, namely chloroform, bromoform, bromodichloromethane, and chlorodibromomethane must be as low as possible and must not exceed 30 ppb. Note that 30 ppb is equivalent to 30 Tg L-i. 

UF and MF membranes alone do not remove total organic carbon (TOC) and tri-halomethane (THM) precursors [13]. Hence, pre-treatment entails addition of chemicals such as coagulants (FeCl3 or FeS04), alum or polyaluminium chloride to increase the size of suspended solids and colloidal particles and thus prevent or minimise colloidal, organic, and/or biological fouling. In the case of seawater desalination, membrane filtration has proven to be superior to multimedia filtration [14,15]. 

---