Page 190 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Organic Chemical Systems, Theory 437

TABLE I Typical Bond Energies in Organic Molecules a The sum total of formal charges on atoms in a molecule
is equal to its net charge, and this is always indicated.
X X H X C X N X O X C X C
Negativelychargedmoleculesarecalledanions,positively
C 100 81 69 84 148 194 charged ones cations. The electrostatic force of attrac-
N 93 69 38 43 148 213 tion between two oppositely charged ions is sometimes
O 110 84 43 33 172 referred to as an ionic bond.
F 135 105 65 50 Typical bonding situations in which atoms of elements
Si 72 69 103 that are most commonly found in organic molecules find
P 77 63 themselves in molecular structures are listed in Table II.
S 83 65 128 Analogous bonding situations are found throughout each
Cl 103 79 48 50 column of the periodic table, except that atoms of second-
Br 88 67 53 row elements resist hypervalency.
I 71 57 57 The hydrogen atom does not suffer from steric con-
N N 38 N N 100 N N 226 straints in its ordinary univalent state, in which it makes
O O 33 N O 145 only one bond. It can enter into a special kind of weak hy-
S S 54 O O 96 pervalent interaction known as the hydrogen bond, which
attaches it to a lone-pair-carrying second atom. The hy-
a
In kilocalories per mole; 1 kcal = 4.184 kJ.
drogen bond is indicated by a dotted line. As usual for
hypervalent interactions, hydrogen bonding is particularly
important if the neighbors of the hydrogen atom are highly
atom in 4; the electronegativity of an element increases as
electronegative.
one moves up and to the right in the periodic table).
Those molecules that contain an odd number of elec-
trons cannot satisfy the rules for all of their atoms and are B. Molecular Geometries
known as free radicals (e.g., 2).
Classical structural formulas imply molecular geometries.
In addition to atomic symbols and symbols for bonds,
These are determined by bond lengths, valence angles, and
lone pairs, and unpaired electrons, the classical struc-
dihedral angles and describe the average nuclear positions
tural formulas of organic chemistry also indicate atomic
when the molecule is at equilibrium.
charges (e.g., 3–5):
In real molecules, at least some vibrational and inter-
CH 3 H H nal rotational motion is always present. In many organic
CH 3 H 3 C O
CH 3 C Sn CH 3 H N C C molecules, this can be neglected in the first approxima-
H 3 C tion, and the molecules can be considered rigid. Some,
CH 3 H H O
CH 3 particularly those lacking rings and multiple bonds, are
3 4 5 definitely floppy at room temperature due to nearly free
rotation around single bonds but can be viewed as rigid at
The way to determine the charge on an atom is to count sufficiently low temperatures.
the valence-electron ownership of an atom in the molecule
and compare it with the number of valence electrons on a 1. Bond Lengths
neutral isolated atom of the same element. If the two agree,
the formal charge is zero. If there is one more electron on Bond lengths are generally determined by the nature of
the atom in the molecule than on an isolated atom, the the two atoms bonded, with minor variations depending
formal charge is minus one and so on. on the environment. Each kind of atom can be associated
In order to determine the valence-electron ownership of with the value of its “covalent radius.” A bond length is
an atom in a molecular structure, one counts all electrons approximately equal to the sum of the covalent radii of the
indicated as lone pairs as well as unpaired electrons on the participating atoms. Typical lengths of the most common
atom, plus one for each bond in which the atom partici- bonds in organic molecules are listed in Table III.
pates. Thus, one assumes that the two electrons of a bond
are shared equally between the two atoms that it joins. To
2. Valence Angles
indicate that this is unrealistic when the two atoms differ
in their electronegativity, the charges are referred to as for- Valence angles are the angles between two bonds on the
mal. For molecules containing transition metal elements, same atom, generally dictated by the coordination number
the individual formal charges are frequently not indicated of the atom. However, if lone pairs are present on an atom,
at all. each of these counts for yet another neighbor. On the other