Halogens in alkyl halides

Exchange of halogen in alkyl halides (see also Makosza). 

Several examples of the use of potassium fluoride in displacing halogens in alkyl halides are given in Table 2. 

Azaphosphoranium salts 39 have been used as phase-transfer catalysts for the Finkelstein and Kolbe reactions (substitution of halogen in alkyl halides by iodide and cyanide, respectively) (78PS145). Representatives of... 

Just as aromatically bonded hydrogen is more acidic than aliphatically bonded hydrogen, it is observed that aromatically bonded halogen is more cationic than the halogen in alkyl halides. Thus aryl bromides react with alkyllithium compounds to form aryllithium derivatives and alkyl bromides ... 

Halogens on alkyl side chains generally resemble halogens in alkyl halides. As with alkyl halides, preparation is typically carried out by (1) substitution of side-chain hydroxyl groups or (2) addition of HX or X2 to side-chain olefins. Free-radical reaction with halogen molecules, X2, yields a mixture of products and works best for chlorination or bromination. 

Carbon-oxygen and carbon-halogen bonds are polar covalent bonds and carbon bears a partial positive charge in alcohols ( " C—0 ) and in alkyl halides ( " C—X ) Alcohols and alkyl halides are polar molecules The dipole moments of methanol and chloromethane are very similar to each other and to water... 

The carbon that bears the functional group is 5/r -hybridized in alcohols and alkyl halides. Figure 4.1 illustrates bonding in methanol. The bond angles at carbon are approximately tetrahedral, as is the C—O—H angle. A similar- orbital hybridization model applies to alkyl halides, with the halogen connected to 5/r -hybridized carbon by a a bond. Carbon-halogen bond distances in alkyl halides increase in the order C—F (140 pm) < C—Cl (179 pm) < C—Br (197 pm) < C—I (216 pm). 

In alkyl halides the leaving group is the halogen substituent — it leaves as a halide ion. 

The dynamics of carbon-halogen bond reductive cleavage in alkyl halides was studied by MP3 ab initio calculations, using pseudopotentials for the halogens and semidiffuse functions for the heavy atoms [104], The effect of solvent was treated by means of the ellipsoidal cavity dielectric continuum model. Both a concerted (i.e., a one-step) and a stepwise mechanism (in which an anion radical is formed at first) were... 

This reaction may be the origin of at least some of the halogen found in polymers produced in alkyl halide solvents. 

An alkyl halide (also known as a haloalkane) is an alkane in which one or more hydrogen atoms have been replaced with halogen atoms, such as F, Cl, Br, or I. The functional group of alkyl halides is R—X, where X represents a halogen atom. Alkyl halides are similar in structure, polarity, and reactivity to alcohols. To name an alkyl halide, first name the parent hydrocarbon. Then use the prefix fluoro-, chloro-, bromo-, or iodo-, with a position number, to indicate the presence of a fluorine atom, chlorine atom, bromine atom, or iodine atom. The following Sample Problem shows how to name an alkyl halide. 

In alkyl halides, the halogen atom is bonded to an alkyl group (R). 

Experimental data (Figure 4.2) for the dissociative electron transfer between radical anions and the carbon-halogen bond in alkyl halides indicates a linear relationship between log(k ) and Ed over a wide range of reaction rates [5, 9]. Very fast reactions become controlled by the rate of diffusion of two species towards each other, when every close encounter gives rise to electron transfer. A parabolic... 

Replacement of Halogens by Fluorine in Alkyl Halides and x-Haloalkenes... 

The halogen of an a-halo aldehyde or an a-halo ketone is exceptionally unreactive in SN1-displacement reactions, but is exceptionally reactive in Sn2 displacements, compared with the halogen of alkyl halides having comparable potential steric effects. Similar behavior is observed with a-halo carboxylic acids and is discussed further in Chapter 18. 

The well-known action of silver(i) salts on nucleophilic substitution in alkyl halides is another commonplace example of this effect. The silver ion interacts with the halide, thus weakening the carbon-halogen bond and enhancing the leaving ability of the halide... 

Aryl halides are relatively unreactive toward nucleophilic substitution reactions. This lack of reactivity is due to several factors. Steric hindrance caused by the benzene ring of the aryl halide prevents SN2 reactions. Likewise, phenyl cations are unstable, thus making SN1 reactions impossible. In addition, the carbon-halogen bond is shorter and therefore stronger in aryl halides than in alkyl halides. The carbon-halogen bond is shortened in aryl halides for two reasons. First, the carbon atom in aryl halides is sp2 hybridized instead of sp3 hybridized as in alkyl halides. Second, the carbon-halogen bond has partial double bond characteristics because of resonance. 

Other examples of reported molecular complexes are tabulated in Refs. 202,506, and 507. However, it is necessary to note that the participation in these syntheses of acids, ammonium salts, halogens, and alkyl halides [506,507,551,562] provokes doubts in respect of determining the role of the electrochemical processes, since the reagents above can themselves dissolve elemental metals in various liquid media, forming coordination compounds (Sec. 3.4.3). 

The polar C-X bond present in alkyl halides has a substantial dipole moment. Alkyl halides are poorly soluble in water, but are soluble in organic solvents. They have boiling points that are similar to alkanes of comparable molecular weight. Due to polarity, the carbon is an electrophilic centre and the halogen is a nucleophilic centre. Halogens are extremely weak nucleophilic centres and therefore, alkyl halides are more likely to react as electrophiles at the carbon centre. 

A second important reaction of L-AuR species is the oxidative addition (see Oxidative Addition) of halogen or alkyl halides, which leads, at least in a first step, to organo(dihalo)gold(IIl) or diorgano(halo)gold(III) products. The structures are transformed from linear to square planar, and therefore cis and trans isomers are possible. The products may undergo secondary reactions and/or become subject to a reductive elimination (see Reductive Elimination) of other substituent combinations (equations 41 and 42). ... 

Much early work [21] involved the use of silver(I) fluoride, conveniently prepared from the oxide or carbonate with 40% hydrogen fluoride, for the exchange of single halogen atoms in alkyl halides [22] and other systems [23]. The use of calcium fluoride as a solid, inert support may increase the reactivity of silver fluoride [24] (Figure 3.1). 

It unites with both halogens in polymethylene halides (two equivalents of sodium amide are required) to form 1-phenyl cycloalkyl cyanides. The action of substituted alkyl halides on phenyl acetonitrile and its horaologs furnishes valuable intermediates for syntheses. ... 

Syntheses of alkyl phenyl ethers, C,H, OR, are carried out by refluxing aqueous or alcoholic solutions of alkali phenolates with alkyl halides the yields vary with the nature of the alkyl halides (40-80%). The reactive halogen in benzyl halides is easily replaced by an alkoxyl group (95%). ° The choice of a solvent is sometimes important. Thus, in the preparation of the alkyl ethers of o- and p-hydroxybiphenyl from a mixttire of the phenol, alkyl halide, and powdered potassium hydroxide, high yields are obtained using acetone as a solvent, whereas, with alcohol as solvent, only small yields are obtained. Triarylmethyl chlorides react with alcohols directly (97%). ... 

The carbon-halogen bond is stronger in vinyl halides than in alkyl halides, and Ni complexes with slightly lower reduction potentials have to be used to catalyze the reductive cyclization of vinyl halides on to alkene functions. Using conditions similar to those for the alkyl and aryl halides, 5-c.vo-cyclized products were obtained from 88 [237]. In a single case (methyl substitution at the position of S- .vc-attack), only the 6- /o-product was obtained [Eq. (53)] [237]. 

The alternative interpretation is simple. In alkyl halides the carbon holding halogen is 5/ -hybridized. In aryl and vinyl halides, carbon is j/ 2-hybridized the bond to halogen is shorter and stronger, and the molecule is more stable (see Sec. 5.4). 

Now, as we have seen (Sec. 5.2), a double bond is shorter than a single bond joining the same pair of atoms if the carbon-halogen bond in aryl and vinyl halides has double-bond character, it should be shorter than the carbon-halogen bond in alkyl halides. Alternatively, a bond formed by overlap of an sp orbital should be shorter than the corresponding bond involving an sp orbital. 

We saw in the preceding chapter that the carbon-halogen bond in alkyl halides is polar and that the carbon atom is electron-poor. Thus, alkyl halides are electrophiles, and much of their chemistry involves polar reactions with nucleophiles and bases. 

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