Benzene's Puzzle

Faraday first isolated benzene in 1825 from illuminating gas and obtained "CH" as its empirical formula. In 1834 Mitscherlich determined its molecular formula as C₆H₆, which was difficult to explain at the time because it seemed to violate the tetravalency of carbon.

YouTube video: the molecular orbitals of benzene

Michael Faraday FRS (22 September 1791 – 25 August 1867) was an English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic induction, diamagnetism and electrolysis.

Eilhard Mitscherlich (7 January 1794 – 28 August 1863) was a German chemist, who is perhaps best remembered today for his discovery of the phenomenon of isomorphism (crystallography) in 1819.

Benzene is a highly unsaturated hydrocarbon, very special and very stable, that does not undergo the usual reactions of alkenes or alkynes. That's puzzling! Why is that?

Benzene (C₆H₆) does not react with aqueous KMnO₄, Br₂ or Markovnikov reagents (RM), nor does it give alkene additions

Structures proposed for benzene in the 19th century: Dewar, Ladenburg, benzvalene and Claus

Kekulé structure (1865): two cyclohexatrienes in equilibrium, although benzene is actually a regular hexagon

The heat released when benzene is hydrogenated is much lower than that anticipated if the three double C=C bonds behaved as "normal" alkenes. This experimental fact indicates that benzene is much more stable than expected.

The hydrogenation of cyclohexene releases 28.6 kcal/mol

When cyclohexene is hydrogenated, 28.6 kcal/mol are released.

The hydrogenation of 1,4-cyclohexadiene releases 57.2 kcal/mol, twice that of cyclohexene: there is no special stabilization

When 1,4-cyclohexadiene (isolated C=C double bonds) is hydrogenated, exactly the expected amount of heat is released, 2 × 28.6 = 57.2 kcal/mol. The isolated double bonds behave as two ordinary double bonds and have no additional stabilization.

The hydrogenation of 1,3-cyclohexadiene releases 54.9 kcal/mol, 2.3 kcal/mol less than expected: conjugation stabilizes it

When conjugated 1,3-cyclohexadiene is hydrogenated, 54.9 kcal/mol are released, 2.3 kcal/mol less than expected. This means that the conjugated double bonds have a special stabilization and do not behave like isolated ones.

Hydrogenation of benzene: 85.8 kcal/mol would be expected for three isolated double bonds, or 78.9 with conjugation, but 49.3 are measured; benzene is about 30 kcal/mol more stable

If benzene were a simple set of three isolated C=C double bonds in a six-membered ring, 3 × 28.6 = 85.8 kcal/mol would be released on hydrogenation. Subtracting 2.3 kcal/mol for each of the three conjugations, we would arrive at 85.8 − 3 × 2.3 = 78.9 kcal/mol. However, only 49.3 kcal/mol are released: benzene is nearly 30 kcal/mol more stable than expected for the simple cyclohexatriene proposed by Kekulé. Where does that stabilization come from?