Norman 1. Allinger
Wayne State University Detroit, Mich~gan
Energy Relations in Teaching Organic Chemistry
Up
until about 10 or 15 years ago elementary organic chcmistry was taught as a very practically oriented subject. There were really only two major goals, synthesis and structure determination. Since that time there has occurred a gradual change in viewpoint, and this change is now more or less complete. Instead of being asked to classify reactions and memorize them, the beginning student is now usually taught the theory of a reaction before he is taught the experimental facts. There are sound arguments in favor of the older method of teaching: the most compelling is that theories may change; experimental facts will not. The topic of this paper, "Energy Relations in Teaching Organic Chemistry," is not regarded by the author as an approach to organic chemistry, but rather a basis which will go along with any approach. The amount of knowledge available in the field of organic chemistry continues to increase, and the old approach is no longer considered as being very practical by most teachers of the subject. The old approach is obviously sound, since almost all of us that today teach the subject in fact ourselves learned by the old method. The old approach is simply too time consuming and inefficient to cover the amount of material we would like to cover in the time that we would like to allot to it. The goals of the elementary course have expanded, rather than changed, in the last several years. Structure determination and synthesis remain the principal occupation of a majority of practicing organic chemists, and they are useful and necessary tools to even the most theoretically oriented among us. Usually organic chemistry is taught before physical chemistry, and such an order is assumed here. If physical chemistry is taught and taught properly before organic, as it is in a few schools, then presumably one need not teach energy relations in the organic course at all. It can be assumed the student knows about them, and they can be utilized. My personal experience has been that even after studying a moderate amount of physical chemistry, many students still lack a real grasp of energetics, and I feel this is because many teachers of elementary physical chemistry dwell excessively on infinite heat reservoirs, and pistons moving in cylinders and the like. The average organic student is left cold by this approach, since he is really interested more in chemistry than in plumbing. The connection between the two tends to be obscure to the student a t this level. There is at least one excellent textbook, "Organic Chemistry" by Morrison and Boyd, which has a very
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firm basis of energetics throughout. Other texts may refer to energetics to a greater or lesser extent, or not a t all, but it is easy enough for the lecturer to add the material as he desires. Implications of Structure
We wish to apply energetics to both structures and to reactions. If, following Eyring, we think of the transition state in a reaction as a structure, which just happens to have a short life-time, then the study of reactions is reduced to a study of structures. It is necessary to emphasize to the beginning student that if the thermodynamics of a reaction are unfavorable, the reaction will not go, and further, that even if the thermodynamics are favorable, the reaction still may not go for kinetic reasons. Let us ask the question, what are the characteristics of reactions which proceed, as opposed to those which do not? These two reactions, for example, will seem very similar to the beginner, and he will R-OH R-OH
+ H-Br + NaBr
--
R-Br R-Br
+ HOH
+ NaOH
have trouble seeing off-hand why the first will go and the second will not. The student already knows that acids react with bases to form salts plus water. The first reaction has an acid going to water, which can be viewed as related to acid-base reactions. The second reaction has a salt going to a base, which one wouldn't expect, in fact even the beginning student might predict that the reaction should go in the reverse direction, which it does. Bond energies and their use can be conveniently introduced a t this point. Keeping in mind a few rules of thumb, such as water, carbon dioxide, and salts are very stable and reactions which yield them as products are usually favorable, the student may decide that these reactions will go:
Actually, the first reaction does go, a t least to an equilibrium point, but the second and third go only in the presence of a catalyst and the fourth only under extreme conditions. Use of Reaction Coordinate Diagrams
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lantic City, New Jersey, September, 1962
The hydrogenation of ethylene offers an example of a reaction which thermodynamics says should go, but Volume 40, Number 4, April 1963
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201
which in the absence of a catalyst fails. The reaction coordinate diagram conveniently introduces the concept of activation energy and shows the function of a catalyst.
I
Reaction Coordinate
+
Within a limited series, stability of the reaction product will frequently parallel the stability of the transition state, and this concept can be widely used as long as the hazards involved are emphasized. Consider the example of the hydrolysis by an S1, path of a primary versus tertiary halide.
The reasons for the greater stability of the tertiary carboninm ion can be discussed, and then the reaction coordinate diagram can be considered (right column). Inspection of the reaction coordinate diagram shows that the tertiary halide is slightly more stable than the primary bromide, while the tertiary carboninm ion is quite a bit more stable than the primary ion. The energy change for the first step of the reaction (formation of the ion) is consequently greater for the normal case (AEn > AE'), and from the curves it is easy to see that the difference in activation energies parallels the difference in the energy of reaction, thus Enn> Eat.
different mono insertion alkanes, and these arise in strict proportion to the numbers of identical hydrogens, thus the numbers of identical hydrogens are as indicated, and the ratios of products are given by the ratios of these numbers.
Finally a more complicated case, say free radical chlorination, can be examined, in which both entropy and enthalpy must be considered. Here a tertiary carbon is more reactive than a secondary, which is more reactive than a primary, the relative rates of a single hydrogen being 4.4, 3.2, and 1 respectively under a specific set of conditions. The ratio of products will then, of course, be the product of the probability and energy terms, so that the chlorides isolated from the chlorination of isopentane will be in the ratio of 1 X 6:4.4 X 1:3.2 X 2 : l X 3 (for reaction a t the carbons from left to right in the structure shown).
-
Reaction Coordinate
Entropy Is Important
The differencebetween enthalpy and free energy can be pointed out, a t least in a rudimentary way. The Boltzmann distribution and its temperature dependence can be examined, and these factors can then be combined to yield a rate equation. For a simple one step B C, reaction is dependent on the process of A molecules: (1) colliding, (2) having sufficient energy to react, (3) having the proper orientation to react.
+
-
Rate = (A)(B) X (e"*dRT) X (8)
An introdnctio~~ to entropy and its relation to probability can be nicely illustrated by the carbene insertion reaction. This reaction is so vigorous that there is no selectivity between different kinds of hydrogens. A molecule like isopentane therefore reacts to give four 202
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Journal of Chemical Education
The use of energetics in the elementary course as discussed here is easy for the student to grasp, and if referred back to with sufficient regularity it gives the student a good grasp of the kinds of reactions which are reasonable, and those which are unreasonable, and it permits him to develop a judgment about reactions at an early stage which would otherwise come only after long experience.
