Alkyl halides radicals

Incorporation of triethylamine into the reaction medium produced more reduction product presumably due to electron transfer from the triethylamine to the excited alkyl halide. This results in a weakly-bound amine-alkyl halide pair [57]. The alkyl halide radical anion releases X- (Scheme 17). In a related example, it is known that solutions of aliphatic amines in CC14 are unstable to light quickly forming white crystalline precipitates [60]. The initial reaction is formation of a singlet radical pair via excitation of a ground state charge-transfer complex. 

In his paper dealing with the separation of radical and anion cyclizations Garst also comments on the apparent halogen effects on product distributions in both sodium naphthalenide and sodium mirror reactions 96 While there are other possible explanations, the possibility that alkyl halide radical anions could be intermediates that undergo reactions other than fragmentation to alkyl radicals and halide ions should be kept in mind. ... 

The alkyl halide radical anion is very unstable and short-lived, decomposing to an alkyl radical and it halide ion. In fact, the electron transfer very-likely occurs dissocialively. forming the radical directly with the transfer of the electron (eq. 2.15). T his is supported theoretically 43. and is consistent with the fact (see below) that the ease of reduction increases with the stability oflhc radical formed. Low temperature LRR studies indicate only weak polarization interaction between the radical and the halide ion 44. Nevertheless, it has been suggested that even this weak interaction might affect the partition of radical species during the lifetime of a radical pair 45. lit contrast. 

Sevonlh. for typical alkyl halides, radical nap pine dala loi both IX Pl) and PMPO- conform to the D model with the value ol - < item III s ilieme 7.1X). The eomplieations thal may nival idate IX Pl h 1)1 trapping results lor eyci, fiogv i hi.iniiiles mav not occur vv ill ly phal aikv I li.ilules... 

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. 

Other related nickel-catalyzed Negishi, Hiyama, and Suzuki-Miyaura reactions of unactivated secondary alkyl halides, radical intermediates may be involved [140, 142-144],... 

Wuftz synthesis Alkyl halides react with sodium in dry ethereal solution to give hydrocarbons. If equimolecular amounts of two different halides are used, then a mixture of three hydrocarbons of the types R — R, R — R and R —R, where R and R represent the original radicals, will be formed. The yields are often poor owing to subsidiary reactions taking place. 

Two mechanisms have been proposed for the Wurtz reaction (compare Section III,7) and for the Wurtz-Fittig reaction. According to one, sodium reacts with the alkyl halide to produce a sodium halide and a free radical, which subsequently undergoes coupling, disproportionation, etc. ... 

Having gamed one electron the alkyl halide is now negatively chaiged and has an odd numbei of elections It is an anion radical The extra electron occupies an antibondmg oibital This anion radical fragments to an alkyl radical and a halide anion... 

One-electron oxidation of carboxylate ions generates acyloxy radicals, which undergo decarboxylation. Such electron-transfer reactions can be effected by strong one-electron oxidants, such as Mn(HI), Ag(II), Ce(IV), and Pb(IV) These metal ions are also capable of oxidizing the radical intermediate, so the products are those expected from carbocations. The oxidative decarboxylation by Pb(IV) in the presence of halide salts leads to alkyl halides. For example, oxidation of pentanoic acid with lead tetraacetate in the presence of lithium chloride gives 1-chlorobutane in 71% yield ... 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

Alkyl halide Lithium Anion radical Lithium cation... 

Grignard reagents are a very important class of organometallic compounds. For their preparation an alkyl halide or aryl halide 5 is reacted with magnesium metal. The formation of the organometallic species takes place at the metal surface by transfer of an electron from magnesium to a halide molecule, an alkyl or aryl radical species 6 respectively is formed. Whether the intermediate radical species stays adsorbed at the metal surface (the A-modelf, or desorbs into solution (the D-model), still is in debate ... 

Application of radical reactions to organic synthesis has recently received much atrendon, and various important reacdons have been discovered in this field. Alkyl halides, sulfides, seleniJes, and thiocarbonyl compounds have been used as precursors to alkyl radicals. Some examples are illustrated in Scheme 7.18. ... 

Preparing Alkyl Halides from Alkanes Radical Halogenation 33B... 

Structurally simple alJkyl halides can sometimes be prepared by reaction of an alkane with Cl2 or Br2 through a radical chain-reaction pathway (Section 5.3). Although inert to most reagents, alkanes react readily with Cl2 or Br2 in the presence of light to give alkyl halide substitution products. The reaction occurs by the radical mechanism shown in Figure 10.1 for chlorination. 

Recall from Section 5.3 that radical substitution reactions require three kinds of steps initiation, propagation, and termination. Once an initiation step has started the process by producing radicals, the reaction continues in a self-sustaining cycle. The cycle requires two repeating propagation steps in which a radical, the halogen, and the alkane yield alkyl halide product plus more radical to carry on the chain. The chain is occasionally terminated by the combination of two radicals. 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

Alkyl halides can be reduced to alkanes by a radical reaction with tributyltin hydride, (C4H9)3SnH, in the presence of light (hv). Propose a radical chain mechanism by which the reaction might occur. The initiation step is the light-induced homolytic cleavage of the Sn— H bond to yield a tributyltin radical. 

ATRP catalysts may be used to generate radicals and thus alkoxyamines can be produced from alkyl halides in high yield (Scheme 9.21).174 The alkoxyaminc 102 was obtained in 92% yield 174 whereas reaction of TEMPO with PMMA under ATRP conditions is reported to provide a macromonomer (Section 9.7.2.1). 

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