Page 207 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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454 Organic Chemical Systems, Theory

usually employed in the laboratory requires the knowledge occupied MOs of one reacting center with the unoccupied
and understanding of activation enthalpies and activation ones of the other and vice versa. Often a reaction can be
entropies for elementary reaction steps; that is, the shapes viewed as an interaction of a Lewis acid with a Lewis
of the relevant potential energy surfaces in the vicinity of base. Then the former kind of favorable interactions pre-
the local minimum in the reactant catchment basin and in vails in the reaction of a so-called hard acid with a hard
the vicinity of the transition state saddle point in the ridge base, while the latter kind prevails in the interactions of a
separating it from the product catchment basin. so-called soft acid with a soft base. Interactions of a hard
The reaction path of an elementary step is usually de- partner with a soft one are generally less favorable.
fined as the steepest descent path from the transition point In the frontier MO theory of organic reactivity, intro-
totheminimainthestartingandthefinalcatchmentbasins. duced by Fukui, attention is limited to the HOMO and
It describes the geometry of the approach of the reacting the LUMO of the reacting partners. Usually, their energy
molecules and, specifically, of the reaction centers toward separations are such that only one of the HOMO–LUMO
one another. This approach is frequently described in pic- interactions has to be considered, namely that between the
torial terms, such as “face-to-face” approach of an olefinto higher lying of the two HOMOs (that of the better donor
a diene in a Diels–Alder reaction or a “backside attack” by molecule) and the lower lying of the two LUMOs (that of
a nucleophile on a carbon atom carrying a leaving group. the better acceptor molecule). A favored orientation for
Frequently, the geometrical path is described in even more the reaction will be one in which the interaction of these
quantitative terms—for example, by giving an angle of ap- two MOs is maximized.
proach of a nucleophile attacking a carbonyl group. Very helpful tools for identifying favorable reaction
Note, however, that within the constraints of the transi- paths and recognizing superficially similar unfavorable
tion state theory only the geometry at the transition state ones are correlation diagrams. In their most useful form
and in its immediate vicinity matters, and the preceding these resemble crosscuts through potential energy hyper-
pathtakenbythemoleculestoapproachthispointisirreve- surfaces and indicate how electronic states of starting ma-
lant. Molecules do not react by following a well-described terial correlate with those of a product. Usually, the easiest
straight path of minimum energy toward the transition way to construct such state correlation diagrams is to pro-
state. Rather, they undergo random excursions in their ceed in several steps starting with an MO correlation dia-
shape, mutual orientation, and distance that have little to gram, proceeding to a configuration correlation diagram,
do with any particular path and are dictated by collisions and finally introducing configuration mixing to obtain the
with the surrounding medium. This chaotic motion contin- desired state correlation diagram.
ues until the molecules happen to acquire enough energy The use of molecular symmetry is helpful in the con-
and the appropriate direction of motion to reach the transi- structionofthesediagramsbecausethisfrequentlypermits
tion state, and then passage to the product catchment basin ready identification of the nodal patterns in the wave func-
(i.e., a chemical reaction) follows. tions involved, which determine the behavior of the energy
Althoughtheconceptofareactionpaththereforecannot along the reaction path. Since nodal patterns are not par-
betakenasliterallyrepresentingtheactualmovementsofa ticularly sensitive to minor perturbations, it is often possi-
reacting molecule, it is useful in that it helps to understand ble to ignore those secondary perturbations that lower the
the geometries at which transition states occur. In most symmetry of the reacting system and still obtain useful re-
reactions other than mere conformational changes, bonds sults. For example, the face-to-face interaction of ethylene
are broken and made (either directly or by means of elec- with propylene can be understood on the basis of consid-
tron transfer). If old bonds had to be broken completely ering the interaction of two ethylene molecules instead.
before new ones were made, the enthalpies of activation The use of correlation diagrams has been particularly
would be given by bond strengths. In reality, they are fre- helpful in the case of the so-called pericyclic reactions.
quently lower. The search for favorable energy paths and These are elementary reaction steps the transition state
low-energy transition states is thus equivalent to a search of which is characterized by a cyclic array of mutually
for ways in which the transition state can be stabilized by overlapping and interacting AOs on the reacting centers.
introducing new bonding interactions before the old ones It is not necessary and usually not possible for all the
have been completely lost. interactions along the periphery of the cycle to be equal
Two important ways of introducing favorable interac- in magnitude.
tions between the reacting centers can be visualized from Such transition states are isoelectronic with cyclic con-
the knowledge of the electronic wave functions of the re- jugated π systems and can therefore be classified as aro-
actants. One of these is to bring unlike charges on the two maticorantiaromatic.Notsurprisingly,otherfactorsbeing
partners close together and keep like charges well sepa- equal, the aromatic transition states are favored energeti-
rated. The other is to introduce favorable interactions of cally over the antiaromatic ones. The differences are the