Halogen-bond

As described in Section 6.4.3, the Lewis acid I2 is used to catalog Lewis basicity via determination of the equilibrium constants K ) associated with adduct formation. 

The experimental challenge is determining the I2 and I2—Lewis base adduct concentrations these are often obtained via UV-Visible spectroscopy by examining charge transfer and donor-acceptor transitions (Section 6.4.2). 

An extensive compilation of values is available for a variety of main group Lewis bases, with most determined in heptane. There is interest in developing an I2 affinity scale defined similarly as the BF3 affinity scale (based on A//° of adduct formation). Many A f ° values have been determined for reactions of I2 with Lewis bases, but reconciliation of data obtained via different experimental conditions for reliable I2 affinity comparisons remains a challenge. 

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The halogens F Cl Br and I do not differ much in their preference for the equatorial position As the atomic radius increases in the order F < Cl < Br < I so does the carbon-halogen bond dis tance and the two effects tend to cancel... 

The carbon that bears the functional group is sp hybridized m alcohols and alkyl halides Figure 4 1 illustrates bonding m 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 sp hybridized carbon by a ct bond Carbon-halogen bond distances m alkyl halides increase m the order C—F (140 pm) < C—Cl (179 pm) < C—Br (197 pm) < C—I (216 pm)... 

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... 

Relatively simple notions of attractive forces between opposite charges are suffi cient to account for many of the properties of chemical substances You will find it help ful to keep the polarity of carbon-oxygen and carbon-halogen bonds m mind as we develop the properties of alcohols and alkyl halides m later sections... 

The rate of elimination depends on the halogen the reactivity of alkyl halides increasing with decreasing strength of the carbon-halogen bond... 

What are the implications of the effects of the various halide leaving groups Because the halogen with the weakest bond to carbon reacts fastest Ingold concluded that the carbon-halogen bond breaks m the rate determining step The weaker the carbon-halogen bond the easier it breaks... 

The E2 mechanism is a concerted process m which the carbon-hydrogen and carbon-halogen bonds both break m the same elementary step What if these bonds break m separate steps s... 

Step (1) Alkyl halide dissociates by heterolytic cleavage of carbon-halogen bond (Ionization step)... 

One possibility is the two step mechanism of Figure 5 12 m which the carbon-halogen bond breaks first to give a carbocation intermediate followed by depro tonation of the carbocation m a second step... 

The alkyl halide m this case 2 bromo 2 methylbutane ionizes to a carbocation and a halide anion by a heterolytic cleavage of the carbon-halogen bond Like the dissoci ation of an aUcyloxonmm ion to a carbocation this step is rate determining Because the rate determining step is ummolecular—it involves only the alkyl halide and not the base—It is a type of El mechanism... 

Because the carbon-halogen bond breaks m the slow step the rate of the reaction depends on the leaving group Alkyl iodides have the weakest carbon-halogen bond and are the most reactive alkyl fluorides have the strongest carbon-halogen bond and are the least reactive... 

The carbon-halogen bond in an alkyl halide is polar... 

Secondary alkyl halides react by a similar mechanism involving attack on benzene by a secondary carbocation Methyl and ethyl halides do not form carbocations when treated with aluminum chloride but do alkylate benzene under Friedel-Crafts conditions The aluminum chloride complexes of methyl and ethyl halides contain highly polarized carbon-halogen bonds and these complexes are the electrophilic species that react with benzene... 

The value of alkyl halides as starting materials for the preparation of a variety of organic functional groups has been stressed many times In our earlier discussions we noted that aryl halides are normally much less reactive than alkyl halides m reactions that involve carbon-halogen bond cleavage In the present chapter you will see that aryl halides can exhibit their own patterns of chemical reactivity and that these reac tions are novel useful and mechanistically interesting... 

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 strength of their carbon-halogen bonds causes aryl halides to react very slowly in reactions in which carbon-halogen bond cleavage is rate determining as m nude ophilic substitution for example Later m this chapter we will see examples of such reactions that do take place at reasonable rates but proceed by mechanisms distinctly dif ferent from the classical S l and 8 2 pathways... 

Aryl halides are compounds of the type Ar—X where X = F Cl Br or I The carbon-halogen bond is stronger m ArX than m an alkyl halide (RX)... 

Ketene Insertions. Ketenes insert into strongly polarized or polarizable single bonds, such as reactive carbon—halogen bonds, giving acid hahdes (7) and into active acid haUdes giving haUdes of p-ketoacids (8) (46). Phosgene [77-44-5] (47) and thiophosgene [463-71-8] (48) also react with ketenes. 

Silicon halides are typically tetrahedral compounds. The siUcone—halogen bond is very polar thus the siUcon is susceptible to nucleophilic attack, which in part accounts for the broad range of reactivity with various chemicals. Furthermore, reactivity generally increases with the atomic weight of the halogen atom. 

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