Baeyer's Strain Theory (1885)

Adolf Baeyer was Kekulé's first graduate student. In Heidelberg in the mid-1850s, just as Kekulé was developing his ideas on tetravalent carbon, they worked together in their shared laboratory, Kekulé's kitchen. Baeyer was studying compounds of cacodyl, the dimethyl arsenic radical! (Kekulé lived to the age of 66; Baeyer, to 82.)

By 1885, the year he was knighted, Baeyer had spent 8 years as Liebig's successor in the prestigious professorship of chemistry at the Munich Academy of Science. Though his laboratory included a large number of students and researchers, Baeyer did much of his research with his own hands, but in the following study of explosive polyacetylene compounds such as HOOC-CC-CC-COOH he was "most enthusiastically" assisted by a Dr. Homolka.

Chemical Laboratory, Academy of Science Munich (1893)

(1) (2) (3)

(1) Aldolf von Baeyer 1835-1917 (Nobel Prize 1905)
(2) Johannes Thiele (who drank to excess) 1865-1918 Invented the theory of partial valences
(3) Richard Willstätter (Nobel Prize 1915) 1872-1942. Baeyer's successor in Munich, fled to Switzerland in 1939.

(4) (5) (6)

(4) Walter Dieckmann 1869-1925 Inventor of the Dieckmann condensation of diesters (next semester)
(5) Henry Lord Wheeler 1867-1914. Yale alumnus and first organic chemistry professor (1896-1911)
(6) Viktor Villiger - Discovered Baeyer-Villiger oxidation of ketones by peroxyacids

On Polyacetylene Compounds
Berichte der Deutschen Chemischen Gesellschaft, 2269-2281 (1885)

Baeyer was struck by the tendency of the polyacetylenes to explode, which led him to the following discussion on pp. 2278-80:

1. Theory of Ring Closure and the Double Bond

Ring closure is apparently the only phenomenon that can supply information about the arrangement of atoms in space. Since a chain of 5 or 6 members can be closed easily, while with one of more or fewer members it is difficult or impossible, spatial factors are apparently involved.

The previously proposed general rules on the nature of carbon atoms are the following:

I. Carbon is as a rule tetravalent.

II. The four valences are equivalent, as shown by the fact that there is only one monosubstitution product of methane.

III. The four valences are equivalently arranged in space and correspond to the corners of a regular tetrahedron inscribed in a sphere.

IV. The atoms or groups attached to the four valences cannot exchange places. Evidence: there are two tetrasubstitution products abcd of methane, LeBel-van't Hoff Rule.

V. Carbon atoms can bind to one another with 1, 2, or 3 valences.

VI. These compounds can form either open or closed-ring chains.

I should like to add the following to these generally accepted rules:

VII. The four valences of the carbon atom point in the directions connecting the center of the sphere to the corners of the tetrahedron, forming an angle of 109°28' with one another. The direction of attachment can undergo alteration, but a strain is generated increasing with the size of the deflection.

A notion of the significance of this rule comes easily starting from the Kekulé's ball model and assuming that the wires, like elastic springs, can be distorted in all directions. Combining this with the assumption that the directions of the bonds always corresponds to the direction of the wires, one has a true picture of the hypothesis proposed in the seventh rule.

If one now tries, most significantly only with models, to connect a large number of carbon atoms without constraint, that is in the directions of the tetrahedron axes or of the wires, one obtains either a zig-zag line or a 5-membered ring, which is obvious since the angle in a regular pentagon is 108°, only slightly different from the 109°28' angle between the valences. To make a larger or smaller ring one must bend the wires, that is, a strain would result in the sense of the seventh rule.

How well this view agrees with the facts can be seen by analyzing the rings built from several methylene groups.

The simplest methylene ring is ethylene, which can be considered a dimethylene. According to the seventh rule and under the condition that both axes bend by the same amount, preparing its double bond requires bending the axes until they are parallel, that is each axis must distort by 1/2 * 109°44' from its resting position. In trimethylene, which one can think of as an equilateral triangle, the angle the axes must make with one another is 60°, and the bending of each must be 1/2(109°28'-60°) = 24°44'; in tetramethylene 1/2(109°28'-90°) = 9°44'; in pentamethylene, corresponding to the 108° angle of a regular pentagon 1/2(109°28'-108°) = 0°44'; in hexamethylene corresponding to the 120° angle of a regular hexagon 1/2(109°28'-120°) = -5°16', that is the axes must be spread by 5° away from one another. The following collection makes this relationship clear:

Dimethylene is indeed the weakest ring, which can be opened by hydrogen bromide, bromine and even iodine; trimethylene is broken only by hydrogen bromide but not by bromine; finally tetramethylene and hexamethylene are difficult or impossible to break. The only consideration that one might throw in from the viewpoint of facts is that six-membered carbon rings are very common, while on the contrary five-membered rings have thus far been found only rarely and in complicated compounds. This objection should not weigh too heavily, since the six-membered ring is almost always found only in the form of hydrogen-poor compounds like benzene, and it is thus very possible that pentamethylene under the same conditions would be a little easier to form and a little more stable than hexamethyleneº

Click here for Sachse's criticism of Baeyer's Strain Theory