Baeyer Strain Theory (Web Page)

From WikidChem

Slide 15:

The reason that this slide is near empty is because the link explains the material much better than a few powerpoint slides can. The link indeed is very well done and should be read to gain a full understanding of Baeyer's Strain Theory. I will attempt to summarize the web page as well as clear up some parts that might be hard to follow.

The intro provides a bit of history of Adolf von Baeyer (a German). He had worked under Kekule for some time in the 1850's, and later succeeded Liebig as the head of the chemistry department at the Munich Academy of Science in the 1870's. His lab included many people who would go on to discover great new things about chemistry (6 of them are highlighted below the photograph of his lab).

The next thing in the web page is a translation of a paper Baeyer wrote on ring closure. He noticed that a chain of 5 or 6 carbons was closed easily, while one of fewer carbons was more difficult to close, so he hypothesized that spacial factors were involved. Then he went on to list 6 generally accepted rules on the nature of carbon atoms, plus a seventh that he added. They are fairly self-explanatory, but I will summarize them: 1) Carbon is tetravalent. 2) The 4 valences are equivalent. 3) The 4 valences point toward the corners of a regular tetrahedron. 4) The groups attached to the valences cannot change places(i.e enantiomers don't interconvert). 5) Carbon atoms can self-link with 1, 2, or 3 valences (single, double, or triple bonds). 6) Carbon chains can be open or closed into a ring. 7) The 4 valences of carbon form an angle of 109°28' with each other, and that angle can be altered but only with increased strain.

This 7th rule was the one that Baeyer proposed, and it adds quantitative specificity to van't Hoff's general idea of how carbon's valences are disposed. He offered a way to understand it by looking at Kekule's ball model. One could see that the metal wires, which represent bonds in space, can be moved around to point in different directions.

Note in slide 17 of the powerpoint of 11/13/06 that when Kekulé made double bonds in the benzene ring, using his models with four wires emerging from a carbon ball at tetrahedral angles, he joined the wires from adjacent carbons at sharp angles. Thus the wires would not have to bend or depart from tetrahedral angles. Baeyer assumes that the wires that form bonds must meet without a kink, so that multiple bonds (or small rings) require changing the angle between wires from tetrahedral. He did not actually assume that bonds would bend (as we now think of it), just that the angle between straight wires would have to change from tetrahedral. Baeyer proposed that the farther the angles between the wires must bend from 109°28', the more strain is associated with that molecule. He said if one were to connect several carbon atoms in a row without strain on the bonds, either they would form a zigzag chain or a 5-membered ring. There are 5 carbons in an unstrained carbon ring, he said, because the angle between the sides of a pentagon is 180°-(360°/5) = 108°, which only differs slightly from the natural tetrahedral angle. He then proposed that in larger or smaller rings a strain would arise making the ring more REACTIVE. Because strain corresponds to the angle difference from 109°28', Baeyer calculated this value for rings with 2-6 carbons and summarized the findings in the picture.

He then tested his idea in terms of reactivity. He found that the most reactive ring indeed was the one with 2 carbons, then trimethylene was less reactive, and tetramethylene and hexamethylene were nearly impossible to break. He then made a short remark on how six-membered rings are the most common, while five-membered rings are not as much. He attributes this mostly to the fact that most six-membered rings are in the form of benzene, whose double bonds make its arrangement in space planar. Still, according to his theory five-membered rings should be most common because this is the ring size with the least strain. Right?

Right! - JMM

In fact, thats not true. Baeyer was correct in most of his argument, however he made one error. This was to assume that, for purposes of reckoning strain, all rings must be planar. In fact his picture of carbon rings is correct except for the angle difference written under hexamethylene. It should read 0, as explained later by Sachse. Indeed 6-membered carbon rings have no strain on them at all, but I will allow this to be explained by the next group.