Page 266 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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212 Physical Organic Chemistry

PHYSICAL ORGANIC CHEMISTRY undertakes the A covalent bond represents a stabilization of the
investigation of the phenomena of organic chemistry by molecule relative to the separated atoms. Through sharing
quantitative and mathematical methods. This was the orig- of electrons, each atom achieves an “octet configuration”
inal approach, as presented in L. P. Hammett’s influen- analogous to that of the noble gases. For example, each of
tial book, “Physical Organic Chemistry: Reaction Rates, the hydrogen atoms in 1 is associated with two electrons,
Equilibria, and Mechanisms,” published in 1940. Prior to just as in a helium atom. Thus the hydrogens achieve the
Hammett’s studies, organic chemistry had largely been inertness or unreactivity of a noble gas. Hydrogen and he-
viewed as a collection of empirical observations, without lium are unique in that only two electrons are necessary
any underlying sense. Hammett’s contribution was to take to complete a filled shell. For the next rows of the peri-
the methodology of physical chemistry, apply it to organic odic table, eight valence electrons are necessary, hence
chemistry, and derive regularities that placed organic re- the designation “octet.” For example, carbon in methane
activity on a quantitative basis. (2) achieves its octet by sharing its four valence electrons
Since then, physical organic chemistry has broadened with four hydrogens, each contributing another electron,
its focus to the structure, properties, and reactions of or- and each of the carbons in ethane (3) shares a pair of elec-
ganic molecules, and especially to the relationship be- trons with each of three hydrogens and also with each
tween structure and reactivity. Physical organic chem- other. In ethene (4) each of two carbons achieves its octet
istry asks how chemical reactivity depends on molecular by sharing two pairs of electrons with each other and also
structure. Part of the answer comes from detailed and two additional electron pairs with two hydrogens. The four
quantitative studies of some reactions. Part comes from electrons that are shared between the two carbons form a
recognizing analogies among classes of reactions. This double bond, symbolized by two connecting lines. Simi-
latter is a particularly powerful method since it permits larly, ethyne (5) has a triple bond, with six electrons shared
the extension of understanding from one well studied between the two carbons, each of which achieves an octet.
class of reactions to another, less well understood one.
The underlying principle is that small perturbations of
molecular structure are unlikely to lead to major changes
in reactivity. This principle does not always hold, but
it holds often enough to provide a broad general the-
ory of chemical reactivity. In recent years physical or-
A system of graphic symbols, called Lewis structures,
ganic chemists have been successful in codifying the
permits the illustration of molecular structures. Accord-
underlying principles of chemical structure and reactiv-
ing to this system, every valence electron must be shown.
ity. The results have provided not only a deep under-
Some valence electrons are in covalent bonds, whether
standing of organic reactions, but also applications to
single, double, or triple. Some valence electrons are un-
organic synthesis, biochemical processes, and materials
shared and are symbolized as dots on the atoms to which
science.
they belong. Sometimes these are paired, as in ammonia
(6) or water (7), where the unshared electrons are called
lone pairs. Sometimes an unshared electron is unpaired,
I. MOLECULAR STRUCTURE: BONDING as in methyl radical (8 ), where it is called an odd electron.
This last is unusual in that the carbon lacks an octet. In
A. Covalent Bonding nearly all stable molecules every atom has its octet, and
Since structure determines chemical reactivity, it is neces- nonoctet atoms are usually quite reactive (NO and NO 2
are exceptions).
sary to have a thorough understanding of molecular struc-
ture. Two atoms are held together by the sharing of valence
electrons between them. The interaction that arises from
sharing a pair of electrons is called a covalent bond. In
early illustrations the two electrons were shown as dots.
Now the covalent bond is symbolized by a line connecting Exceptions to the octet rule are possible. Metallic ele-
the two atoms. The simplest case is the hydrogen molecule ments, at the left of the periodic table, often lack an octet.
(1), formed from two hydrogen atoms, each with one va- For example, the boron in BF 3 has only six electrons. More
lence electron. The line symbolizes the two electrons that common exceptions are with atoms in the second full row
are shared. of the periodic table, such as the phosphorus in PCl 5 or
the sulfurs in dimethyl sulfoxide (9) and sulfuric acid (10),
which have 10, 10, and 12 electrons, respectively.