Page 33 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

12 This approach was extended by M. Sastry to a variety of organic compounds. 15 For
example, the charges calculated for the carbon atoms in CF CO C H are as shown
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CHAPTER 1
below. We see that the carbon of the methyl group carries a small negative charge,
Chemical Bonding whereas the carbons bound to more electronegative elements are positive.
and Molecular Structure
F O H H
F C C O C C H –0.030
.313
F H H
.198
0.016
The calculated charge on carbon for a number of organic molecules showed good
correlation with the core atomic binding energies, as measured by X-ray photoemission
spectroscopy. We discuss other methods of assigning charges to atoms based on
computational chemistry in Section 1.4. We also find that all these methods are in some
sense arbitrary divisions of molecules. The concept of electronegativity equalization
is important, however. It tells us that electron density shifts in response to bonding
between atoms of different electronegativity. This is the basis of polar substituent
effects. Furthermore, as the data above for ethyl trifluoroacetate suggest, a highly
electronegative substituent induces a net positive charge on carbon, as in the CF
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and C=O carbons in ethyl trifluoroacetate. Electronegativity differences are the origin
of polar bonds, but electronegativity equalization suggests that there will also be an
inductive effect, that is, the propagation of changes in electron distribution to adjacent
atoms.

1.1.5. Differential Electronegativity of Carbon Atoms
Although carbon is assigned a single numerical value in the Pauling electronega-
tivity scale, its effective electronegativity depends on its hybridization. The qualitative
relationship is that carbon electronegativity toward other bound atoms increases with
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2
the extent of s character in the bond, i.e., sp <sp <sp. Based on the atomic radii
approach, the carbon atoms in methane, benzene, ethene, and ethyne have electroneg-
ativity in the ratio 1:1.08:1.15:1.28. A scale based on bond polarity measures gives
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2
values of 2.14, 2.34, and 2.52 for sp , sp , and sp carbons, respectively. 16 A scale
based on NMR coupling constants gives values of 1.07 for methyl, 1.61 for ethenyl,
17
and 3.37 for ethynyl. If we use the density functional theory definition of electroneg-
ativity (see Topic 1.5.1) the values assigned to methyl, ethyl, ethenyl, and ethynyl
are 5.12, 4.42, 5.18, and 8.21, respectively. 18 Note that by this measure methyl is
significantly more electronegative than ethyl. With an atoms in molecules approach
(see Section 1.4.3), the numbers assigned are methyl 6.84; ethenyl 7.10, and ethynyl
19
8.23. Table 1.2 converts each of these scales to a relative scale with methyl equal to
1. Note that the various definitions do not reach a numerical consensus on the relative
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electronegativity of sp , sp , and sp carbon, although the order is consistent. We are
15 M. Sastry, J. Electron Spectros., 85, 167 (1997).
16
N. Inamoto and S. Masuda, Chem. Lett., 1003, 1007 (1982).
17
S. Marriott, W. F. Reynolds, R. W. Taft, and R. D. Topsom, J. Org. Chem., 49, 959 (1984).
18 F. De Proft, W. Langenaeker, and P. Geerlings, J. Phys. Chem., 97, 1826, (1995).
19
S. Hati and D. Datta, J. Comput. Chem., 13, 912 (1992).