Bond distances carbon-chlorine

The carbon-bromine bond is longer than the carbon-chlorine bond therefore although the charge e in the dipole moment expression p, = e d k smaller for the bromine than for the chlo nne compound the distance d is greater... 

Interatomic Distances and Bond Type for Carbon-Chlorine Bonds... 

Fig. 2.—The relation between bond angle and carbon-chlorine distance for phosgene and the chloroethylenes.

Waals radii. In carbon tetrachloride the chlorine atoms are only 2.87 A apart, and yet the properties of the substance indicate that there is no great strain resulting from the repulsion that should correspond to the van der Waals diameter 3.6 A. Even in methylene chloride and chloroform, where the strain might be relieved bv increasing the bond angle, the chlorine-chlorine distance is only 2.92 A. We conclude that the nonbonded radius of an atom in directions close to the bond direction (within 35°) is about 0.5 A less than the van der Waals radius a unicovalent atom can be considered as a sphere that is whittled down on the side of the bond. 

In order to study this phenomenon, which may be described as involving the conjugation of an unshared pair of electrons on the chlorine atom with the double bond or aromatic nucleus, and to determine the amount of double-bond character in carbon-chlorine bonds of this type, values of the carbon-chlorine distance in chloroethylenes and chloro-... 

The carbon-halogen bond distances in acyl halides increase in the direction F < Cl < Br < I, and are similar, but slightly larger than, those of the alkyl halides (Table 7). Nuclear quadrupole resonance frequencies of halogen compounds suggest that the charge density on the chlorine atom of an acyl chloride is greater than that on an alkyl chloride (Table 8). 

Figure 4-7 Models showing the degree of atomic compression required to bring a chlorine molecule to within bonding distance of carbon and hydrogen of methane...

It s possible to check the accuracy of atomic radii by making sure that the assigned values are additive. For instance, since the atomic radius of Cl is 99 pm and the atomic radius of C is 77 pm, the distance between Cl and C nuclei when those two atoms are bonded together ought to be roughly 99 pm + 77 pm, or 176 pm. In fact, the measured distance between chlorine and carbon in the chloromethane molecule (CH3CI) is 178 pm, remarkably close to the expected value. 

It s relatively easy to measure dipole moments experimentally, and values for some common substances are given in Table 10.1. Once the dipole moment is known, it s then possible to get an idea of the amount of charge separation in a molecule. In chloromethane, for example, the experimentally measured dipole moment is /x = 1.87 D. If we assume that the contributions of the nonpolar C-H bonds are small, then most of the chloromethane dipole moment is due to the C-Cl bond. Since the C-Cl bond distance is 178 pm, we can calculate that the dipole moment of chloromethane would be 1.78 X 4.80 D = 8.54 D if the C-Cl bond were ionic (that is, if a full negative charge on chlorine were separated from a full positive charge on carbon by a distance of 178 pm). But because the measured dipole moment of chloromethane is only 1.87 D, we can conclude that the C-Cl bond is only about (1.87/8.54)(100%) = 22% ionic. Thus, the chlorine atom in chloromethane has an excess of about 0.2 electron, and the carbon atom has a deficiency of about 0.2 electron. 

Dipole moment is the product of charge and distance. Although the electron distribution in the carbon-chlorine bond is more polarized than that in the carbon-bromine bond, this effect is counterbalanced by the longer carbon-bromine bond distance. 

C(sp)—Cl bond distances in various acetylenes (Table 28) have a remarkably constant value of ca 163.5 pm and variations due to electron-donating (Me, t-Bu, SiH3) or electron-withdrawing substituents (F, Cl, CN) at the opposite carbon are smaller than the experimental uncertainties. ED and MW for chlorobromoacetylene result in rather different ra and r0 values for the C—Cl bond length and this discrepancy may be due to large-amplitude bending vibration of this linear molecule. A similar, but smaller difference between ra and rs values occurs for chlorocyanoacetylene. The rs value for chlorine cyanide is also in line with the results for the acetylenes. 

The number of compounds which contain carbon-bromine bonds and which have been studied in the gas phase is again smaller than the number of corresponding chlorine compounds. Sufficient data, however, are available for compounds with only one bromine bonded to carbon, to discuss the effects of various substituents on the C—Br bond distance. Only very limited structural results exist for compounds with more than one bromine atom bonded to carbon, i.e. for compounds with CBr2 or CBr3 groups. 

Reaction of a stable carbene with sulfuric chloride results in abstraction of the chloride cation to give the adduct 48 (Scheme 27).57 The C2 carbon resonates at 133.05 ppm in the 13C NMR spectrum and the solid state structure exhibits a chlorine-NHC bond distance of 1.696(9) A. The fluor-ine-NHC analogue was prepared by Kuhn and co-workers by reaction of the stable carbene with S02F2.50 The solid state structure exhibits a fluorine-NHC bond distance of 1.291(14) A. 

These qualitative arguments do not take into consideration the possible/likely roles of the nearby halogen. While this may be justified in the case of CO2-HCI, since the initial carbon-chlorine distance is quite large (i.e., approximately 4.5 A compared to an equilibrium C—Cl bond length of 1.75 A in chloroformic acid), it is far less justified for the case of C02-HBr. 

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