Bonding in alkyl halides

Aryl halides can also be reduced by tin hydrides76,77, although these reactions always require initiators because the stronger C—X bonds in aryl halides are less reactive than the C—X bonds in alkyl halides. In fact, a series of meta- and para-substituted bromobenzenes, where X is either meta- or para-CH3O-, C=N, Cl, F, CF3, CH3, Bu-f or 2,6-dichloro, have been reduced by tributyltin deuteride (equation 60). It is worth noting that the more reactive bromide is reduced selectively in the presence of the less reactive chloride and fluoride groups (equation 61). 

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... 

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. 

The carbon-halogen bond in alkyl halide is polar. When a stronger nucleophile approaches an alkyl halide, the halogen atom along with its bonding electron pair is replaced by the incoming nucleophile, which forms a new bond with the carbon atom of the alkyl group. 

An alkyl halide (eg methyl chloride) readily undergoes nucleophilic substitution, on boiling with alcoholic AgNOj and gives a white precipitate of silver chloride. The C - X bond in alkyl halide is polar and can be readily cleaved hence alkyl halides undergo substitution readily. 

The carbon atom of C - X bond in alkyl halides is a good target for attack by nucleophiles and gives rise to nucleophilic substitution reactions. 

In monomolecular substitution (8 1) the dissociation of the C—X bond in alkyl halide takes place first with the formation of a carbonium ion, followed by its rapid reaction with the nucleophilic agent. Tertiary alkyl halides always react in this way. 

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-halogen bonds of aryl halides are both shorter and stronger than the carbon-halogen bonds of alkyl halides In this respect as well as m their chemical behavior they resemble vinyl halides more than alkyl halides A hybridization effect seems to be responsible because as the data m Table 23 1 indicate similar patterns are seen for both carbon-hydrogen bonds and carbon-halogen bonds An increase m s... 

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). 

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... 

He did not think of it then, and when he did, he showed that apparently the formation of a metal-carbon bond was unlikely on thermochemical grounds [68] in other words, he was taken in by his own propaganda but in a Note added in Proof in that same work he indicated that electrochemical factors (solvation and Coulombic energies) could make this initiation exo-energetic. However, at that time, 1960-1970, such a suggestion would have been no more plausible than when it was made by others. It was only the painstaking and detailed exploration of the nature of the solutions of A1X3 in alkyl halides [104, 112] that provided the basis of fact which was required to make the theory plausible. 

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

The polarity of carbon-halogen bond of alkyl halides is responsible for their nucleophilic substitution, elimination and their reaction with metal atoms to form organometallic compounds. Nucleophilic substitution reactions are categorised into and on the basis of their kinetic properties. Chirality has a profound role in understanding the reaction mechanisms of Sj l and Sj 2 reactions. Sj 2 reactions of chiral all l halides are characterised by the inversion of configuration while Sj l reactions are characterised by racemisation. 

You have read (Unit 10, Class Xll) that the carbon - halogen bond In alkyl or benzyl haUdes can be easily cleaved by a nucleophile. Hence, an allqrl or ben l haUde on reaction with an ethanollc solution of ammonia undergoes nucleophilic substitution reaction m which the halogen atom Is replaced by an amino (-NHJ group. This process of cleavage of the C-X bond by ammonia molecule Is known as ammonolysis. The reaction Is carried out In a sealed tube at 373 K. The primary amine thus obtained behaves as a nucleophile and can further react with allqrl halide to form secondary and tertiary amines, and finally quaternary ammonium salt. 

Reactions with Acyl Halides. The palladium-carbon bond in alkyl VII and acyl VIII complexes also can be broken by acyl and alkyl halides, respectively, to give ketones (55). The reaction of the benzyl complex Vila with acetyl chloride in the presence of 2,6-lutidine provided good yield of methyl benzyl ketone, XVII. In the absence of the base, lower yields (26% ) were obtained. Surprisingly, a similar reaction between the benzyl complex and an excess of phenylacetyl chloride afforded a 156% yield of dibenzyl ketone, XVIII, based on the starting benzyl complex (see bottom of p. 105). 

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. 

The insertion reactions of 16 into the C-Hal bond of alkyl halides were found to be the most convenient model for CIDNP studies. Since in accordance with Scheme 10 the photolysis of 17 leads to the formation of triplet germylene 16, we start with the first evidence of the reactions of triplet excited 1676. 

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. 

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