Page 38 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

According to this analysis, the C−X bond is successively both more polar and 17
harder in the order I < Br < Cl < F. This result is in agreement with both the properties
and reactivities of the methyl halides. When bonds are compared, reacting pairs of SECTION 1.1
greater hardness result in a larger net charge transfer, which adds an increment to the Description of Molecular
Structure Using Valence
exothermicity of bond formation. That is, bonds formed between two hard atoms or Bond Concepts
27
groups are stronger than those between two soft atoms or groups. This is an example
of a general relationship that recognizes that there is an increment to bond strength
resulting from added ionic character. 28
Polarizability measures the response of an ion or molecule to an electric field and
3
3
is expressed in units of volume, typically 10 −24 cm or Å . Polarizability increases with
atomic or ionic radius; it depends on the effectiveness of nuclear screening and increases
as each valence shell is filled. Table 1.4 gives the polarizability values for the second-
row atoms and some ions, molecules, and hydrocarbons. Methane is the least polar-
izable hydrocarbon and polarity increases with size. Polarizability is also affected by
hybridization, with ethane > ethene > ethyne and propane > propene > propyne.
It should be noted that polarizability is directional, as illustrated in Scheme 1.2
for the methyl halides and halogenated benzenes.
Polarizability is related to the refractive index n of organic molecules, which
was one of the first physical properties to be carefully studied and related to molecular
structure. 29 As early as the 1880s, it was recognized that the value of the refractive
index can be calculated as the sum of atomic components. Values for various groups
were established and revised. 30 It was noted that some compounds, in particular
compounds with conjugated bonds, had higher (“exalted”) polarizability. Polarizability
is also directly related to the dipole moment induced by an electric field. The greater
the polarizability of a molecule, the larger the induced dipole.
Table 1.4. Polarizability of Some Atoms, Ions, and Molecules a

Atoms Ions Molecules Hydrocarbons
H 0 67 H 2 O 1 45 CH 4 2 59
Li 24 3 N 2 1 74 C 2 H 6 4 47
Be 5 6 CO 1 95 CH 2 =CH 2 4 25
B 3 0 NH 3 2 81 HC≡CH 3 93
C 1 8 CO 2 2 91 C 3 H 8 6 29
N 1 1 BF 3 3 31 CH 3 CH=CH 2 6 26
O 0 8 CH 3 C≡CH 6 18
F 0 06 F − 1 2 n-C 4 H 10 8 20
Ne 1 4 Na + 0 9 i-C 4 H 10 8 14
Cl 2 2 Cl − 3 n-C 5 H 12 9 99
Ar 3 6 K + 2 3 Neopentane 10 20
Br 3 1 Br − 4 5 n-C 6 H 14 11 9
Kr 4 8 Cyclohexane 10 9
I 5 3 I − 7 C 6 H 6 10 3
Xe 6 9
a. T. M. Miller, in Handbook of Chemistry and Physics, 83rd Edition,
pp. 10-163–10-177, 2002.
b. A. Dalgano, Adv. Phys., 11, 281 (1962), as quoted by R. J. W. Le Fevre, Adv.
Phys. Org. Chem., 3, 1 (1965).

27 P. K. Chattaraj, A. Cedillo, R. G. Parr, and E.M. Arnett, J. Org. Chem., 60, 4707 (1995).
28 P. R. Reddy, T. V. R. Rao, and R. Viswanath, J. Am. Chem. Soc., 111, 2914 (1989).
29
R. J. W. Le Fevre, Adv. Phys. Org. Chem., 3, 1 (1965).
30 K. von Auwers, Chem. Ber., 68, 1635 (1935); A. I. Vogel, J. Chem. Soc., 1842 (1948); J. W. Brühl,
Liebigs Ann.Chem., 235, 1 (1986); J. W. Brühl, Liebigs Ann.Chem., 236, 233 (1986).