Concise Physical Chemistry

c21 JWBS043-Rogers September 13, 2010 11:30 Printer Name: Yet to Come

352 PHOTOCHEMISTRY AND THE THEORY OF CHEMICAL REACTIONS

state to products. There are two distinct factors influencing the reaction: (1) the

thermodynamic factor, which tells where the equilibrium is going and where it will

be after infinite time, and (2) thekinetic factor, indicating the time scale necessary

to reach equilibrium—that is, whether “infinite time” is a matter of microseconds,

hours, weeks, eons, and so on. We have seen that the thermodynamic factor is

largely determined by the difference between the enthalpy levels of the reactants and

products. The kinetic factor is largely determined by the height of the enthalpy barrier

between them.

21.7.1 Optical Inversion

An example of the Eyring mechanism is the explanation of certain geometric changes

that take place during organic reactions. If a carbon atom is attached to four different

groups, it isoptically activeand it rotates the plane of polarized light (Fig. 21.5).

The angle of optical rotation can be measured experimentally. Such a substance can

be geometrically changed by a chemical reaction involving an attacking ion. If the

attacking ion is identical to group D, a substance is produced that is identical to the

original compound except that it rotates light in exactly the opposite direction. This

inversionof optical rotation is readily explained using the concept of the activated

complex.

A

B C

D

FIGURE 21.5 An optically active species.

The incoming D brings about a geometric inversion of groups A, B, and C

attached to the central carbon. The geometric inversion causes a reversal in the

angle of optical rotation. The enthalpy barrier to inversion, which is the control-

ling factor in therateof inversion, is the 5-bonded carbon atom at the center

of Fig. 21.6. This is clearly an unstable species, hence it represents an enthalpy

maximum.

D + D D D + D

FIGURE 21.6 Inversion of optical activity.