Page 34 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Table 1.2. Ratio of Electronegativity for Carbon of Different 13
Hybridization
SECTION 1.1
Comparison Methyl Ethenyl Ethynyl
Description of Molecular
Atomic radii 1 0 1 15 1 28 Structure Using Valence
Bond Concepts
Bond polarity 1 0 1 09 1 18
NMR coupling 1 0 1 50 3 15
Density function theory 1 0 1 01 1 60
Atoms in molecules 1 0 1 04 1 20

not so concerned with the precise number, but rather with the trend of increasing
2
3
electronegativity sp <sp <sp. It is also important to note that the range of carbon
3
electronegativities is close to that of hydrogen. While sp carbon and hydrogen are
2
similar in electronegativity, sp and sp carbon are more electronegative than hydrogen.
If we compare the pK values of propanoic acid (4.87), propenoic acid (4.25),
a
and propynoic acid (1.84), we get some indication that the hybridization of carbon
does exert a substantial polar effect. The acidity increases with the electronegativity
of the carbon group.
CH CH CO H CH =CHCO H HC≡CCO H
2
2
2
2
3
2
Orbitals of different hybridization on the same carbon are also thought of as having
different electronegativities. For example, in strained hydrocarbons such cyclopropane
the C–H bonds are more acidic than normal. This is attributed to the additional s
character of the C–H bonds, which compensates for the added p character of the
strained C–C bonds. 20
It is also possible to assign electronegativity values to groups. For alkyl groups
the numbers are: CH , 2.52; C H , 2.46; CH CH, 2.41; CH C, 2.38; C H , 2.55,
2
3
5
6
3 3
3 2
5
CH =CH, 2.55; HC≡C, 2.79. 21 The order is in accord with the general trend that
2
more-substituted alkyl groups are slightly better electron donors than methyl groups.
2
The increased electronegativity of sp and sp carbons is also evident. These values are
based on bond energy data by a relationship first explored by Pauling and subsequently
developed by many other investigators. The original Pauling expression is
D A −B = 1/2 D A −A +D B−B +23
2 (1.8)
where
is the difference in electronegativity of A and B.
Thus the bond strength (in kcal/mol) can be approximated as the average of
the two corresponding homonuclear bonds and an increment that increases with the
difference in electronegativity. For any bond, application of this equation suggests a
“covalent” and “polar” term. The results for the series CH F, CH OH, CH NH , and
3
3
3
2
CH CH is shown below.
3 3
20 J. N. Shoolery, J. Chem. Phys., 31, 1427 (1959); N. Muller and D. E. Pritchard, J. Chem. Phys., 31,
1471 (1959); K. B. Wiberg, R. F. W. Bader, and C. D. H. Lau, J. Am. Chem. Soc., 109, 1001 (1987).
21
N. Matsunaga, D. W. Rogers, and A. A. Zavitsas, J. Org.,Chem., 68, 3158 (2003).