Page 189 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN011G-539 July 14, 2001 21:48

436 Organic Chemical Systems, Theory

observations made by chemists working with organic H
molecules. It is believed that the behavior of molecules H
H H C
can be understood, in principle, in terms of a few basic H
laws of physics and that it is only the mathematical com- H C C C C H Et
C 2 H 5
plexity of the resulting equations that limits the accuracy 1c
H H H H
with which the behavior of organic molecules can be pre- 1a 1b
dicted a priori. Despite the largely approximate nature of H
the theoretical treatments applicable to large molecules, H H
the theory has made substantial contributions, primarily H C
by providing the language through which the various ob-
H C C O O (CH 3 ) 3 C O O t-BuO 2
served phenomena can be interrelated. It permits the ra-
H 2b 2c
tionalization of trends and at times of individual observa- C
tions concerning the reactivity and properties of organic H H
H
molecules, and in some instances it has provided useful
2a
predictions.
The number of bonds formed by an atom (its “cova-
lency”) is dictated by the rules of valence. These state that
I. CLASSICAL BONDING THEORY
in order for an organic molecule to have reasonable stabil-
ity under ordinary conditions rather than to appear only as
Although the theory of organic chemistry has now been
atransientreactionintermediate,ifatall,thevalenceshells
cast in terms of quantum theory, most of the older quali-
of all atoms in the structure have to contain a certain num-
tative concepts of the classical bonding theory remain
ber of electrons: 2 for hydrogen, 8 for other main-group
useful. The classical theory thus represents a suitable in-
elements, and 18 for transition metal elements. The group
troductory level for the subject at hand.
of 8 electrons in the valence shell of an atom is often
referred to as a valence octet. In order to determine the
A. Structural Formulas
number of electrons in the valence shell of an atom, one
In the classical description an organic molecule is rep- counts all the unpaired electrons or electrons present in
resented by a structural formula. This is a collection of lone pairs on that atom, plus two electrons for each single
atomic symbols (C, H, O, N, etc.) representing atoms bond in which the atom is participating (four for a double
(including their inner-shell electrons but excluding their bond, six for a triple bond). In structures 1a through 1c all
valence-shell electrons; the d electrons of a transition atoms satisfy the rules of valence; in structures 2a through
metal atom are included, although they may participate in 2c the terminal oxygen atom does not.
bonding). At least one, but usually many, of these atoms Each type of bond is associated with a contribution to
must be carbon in order for the molecule to qualify as the total energy of the molecule, and these contributions
organic. The atomic symbols are connected by a network are approximately additive. Typical bond strengths are
of single, double, and triple lines, which stand for single, presented in Table I. These are to be taken only as a rough
double, and triple covalent bonds, respectively. These are guide since the immediate environment of the bond, steric
formed by the sharing of electron pairs between atoms, strain (Section I.B), and resonance (see Section I.C) can
with one bond representing one pair. The number of near- have significant effects.
est neighbors to which an atom is attached is called its Atoms with valence shells that contain fewer electrons
coordination number. than demanded by the valence rules are said to be co-
In addition, short lines (see 2a) or pairs of dots can ordinatively unsaturated and usually are carriers of high
be used to represent unshared (lone) electron pairs on an chemical reactivity (terminal oxygen in 2, the central car-
atom, but these are frequently omitted. Single dots rep- bon in 3). Atoms with valence shells that contain a larger
resent odd (unpaired) electrons if such are present (see number of electrons than dictated by the rules are said to
2a–2c). be hypervalent. This situation is rare for atoms of the ele-
The symbols C and H for the carbon atom and the hy- ments of the second row of the periodic table (presumably
drogens attached to it, respectively, are also frequently due to their small size and the resulting steric crowding)
omitted. Commonly occurring groups of atoms of well- but fairly common for those of the third and lower rows,
known internal structure are often indicated by giving the where the number of valence-shell electrons can be 10, 12,
kind and number of atoms involved (e.g., C 2 H 5 for ethyl) or even higher. Molecules containing hypervalent atoms
or by an abbreviation (in this case, Et). A few examples are often stable, particularly if the hypervalent atom is
are given in 1a through 2c: of lower electronegativity than its neighbors (e.g., the tin