Radicals halogen containing alkyl

Metathetical reactions of mixed halogen-containing alkyl radicals... 

Because radical chlorination of an alkane can yield several different monosubstitution products as well as products that contain more than one chlorine atom, it is not the best method for synthesizing an alkyl halide. Addition of a hydrogen halide to an alkene (Section 4.1) or conversion of an alcohol to an alkyl halide (a reaction we will study in Chapter 12) is a much better way to make an alkyl halide. Radical halogenation of an alkane is nevertheless still a useful reaction because it is the only way to convert an inert alkane into a reactive compound. In Chapter 10, we will see that once the halogen is introduced into the alkane, it can be replaced by a variety of other substituents. 

Detailed studies of the chlorine-atom-sensitized oxidation of chloromethane, dichloromethane, 1,1,2-trichloroethane, CH2CC12, and C2C14148 have been reported. Reports on the photolysis of Freons of interest to upper-atmosphere chemistry are discussed in the last section of this Report, and laser enhancement of some halogen-containing molecular reactions is discussed in Section 11. A paper concerned with the mechanism of photodissociation of alkyl and aryl halides was discussed earlier.38 The photochemical chlorination of 1,2-dichloroethane149 and fluorination of carbonyl fluoride,150 reactions of 2 radicals,151 and the photochemical decomposition of FaO at elevated temperatures152 have been reported. 

The initiation step in ATRP involves homolytic cleavage of an activated halogen-containing compound and subsequent addition of this radical to the monomer as shown in Scheme 23. The fragment that is retained at the a-end of the polymer can be composed of functional groups that are tolerant to the ATRP catalyst and propagating chain end. Two main classes of functionalized initiators have been reported, activated alkyl halides and sulfonyl halides. 

Monomers which have been successfully polymerized using ATRP include styrenes, acrylates, methacrylates, and several other relatively reactive monomers such as acrylamides, vinylpyridine, and acrylonitrile, which contain groups (e.g., phenyl, carbonyl, nitrile) adjacent to the carbon radicals that stabilize the propagating chains and produce a suf cientiy large atom transfer equilibrium constant. The range of monomers polymerizable by ATRP is thus greater than that accessible by nitroxide-mediated polymerization, since it includes the entire family of methacrylates. However, acidic monomers (e.g., methacrylic acid) have not been successfully polymerized by ATRP and so also halogenated alkenes, alkyl-substimted ole ns, and vinyl esters because of then-very low intrinsic reactivity in radical polymerization and radical addition reactions (and hence, presumably, a very low ATRP equilibrium constant). 

Cationic Starches. The two general categories of commercial cationic starches are tertiary and quaternary aminoalkyl ethers. Tertiary aminoalkyl ethers are prepared by treating an alkaline starch dispersion with a tertiary amine containing a P-halogenated alkyl, 3-chloto-2-hydtoxyptopyl radical, or a 2,3-epoxypropyl group. Under these reaction conditions, starch ethers are formed that contain tertiary amine free bases. Treatment with acid easily produces the cationic form. Amines used in this reaction include 2-dimethylaminoethyl chloride, 2-diethylaminoethyl chloride, and A/-(2,3-epoxypropyl) diethylamine. Commercial preparation of low DS derivatives employ reaction times of 6—12 h at 40—45°C for complete reaction. The final product is filtered, washed, and dried. 

Primary alkyl chlorides are fairly stable to fluorine displacement. When fluorinated, 1-chloropropane is converted to 1-chloroheptafluoropropane and 1-chloto-2-methylbutane produces 39% l-chlorononafluoro-2-methylbutane and 19% perfluoro-2-methylbutane. Secondary and tertiary alkyl chlorides can undergo 1,2-chlorine shifts to afford perfluonnated primary alkyl chlorides 2-Chloro-2-methylpropane gives l-chlorononafluoro-2-methylpropane, and three products are obtained by the fluorination of 3-chloropentane [7] (equation 1). Aerosol fluorina-tion of dichloromethane produces dichlorodifluoromethane which is isolated in 98% purity [4 (equation 2). If the molecule contains only carbon and halogens, the picture is different. Molecular beam analysis has shown that the reaction of fluorine with carbon tetrachlonde, lodotrichloromethane, or bromotrichloromethane proceeds first by abstraction of halogen to form a trichloromethyl radical [5]... 

It is the purpose of the present article to consider the evidence that the rate parameters offer concerning the nature of the transition states involved in the various radical reactions and how these in turn are affected by the chemical nature of the species involved. Although our principal concern shall be with alkyl radical reactions, we shall also consider some molecular reactions which are closely related and finally the behavior of some systems containing oxygen and halogen atoms as well. 

The halogenation of alkanes in the presence of sulphur dioxide yields alkanesulphonyl chlorides (5.79), and these are made in large quantities for conversion to metal alkanesulphonates (used as emulsifiers in polymerizations) or to nitrogen-containing derivatives. The sulphur dioxide acts by trapping the alkyl radical it does not terminate the chain mechanism, and so quantum yields can be very high (—2000). 

Interactions of C02 with alkyl halogenides or with compounds containing halogen-nitrogen bonds take place as dissociative electron captures. As a result, free radicals are formed according to Scheme 1-88 ... 

Metal alkyl. A compound of a metal with directly linked aliphatic or aromatic hydrocarbon radicals, as in zinc dimethyl, Zn(CHs)2, or mercury diphenyl, Hg(C6H5)2. A normal alkyl has sufficient organic groups to satisfy the normal valence of the metal and therefore contains no halogen or other substituent in place of organic radicals. 

On an industrial scale alkyl halides—chiefly the chlorides because of the cheapness of chlorine—are most often prepared by direct halogenation of hydrocarbons at the high temperatures needed for these free-radical reactions (Secs. 3.19, 6.21, and 12.12-12.13). Even though mixtures containing isomers and compounds of different halogen content are generally obtained, these reactions are useful industrially since often a mixiure can be used as such or separated into its components by distillation. 

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