426 5. QUINONES 



Molecular Structure 



p-Benzoquinone does not exhibit aromatic characteristics, inasmuch as 

 the benzenoid resonance in the ring is impossible, and this is evident in 

 the definite distortion of the ring from a regular hexagon, as shown in the 

 crystallographic X-ray diffraction study of Robertson (1935) (structure I). 

 The C=0 bond length was found to be appreciably less than expected 



(« (n) 



for a pure double bond (1.22-1.24 A) and this was attributed to resonance 

 with the ring double bonds, but the electron diffraction study of vaporized 

 39-benzoquinone (structure II) by Swingle (1954) gave a more normal 

 length, which is also consistent with the results obtained on similar mol- 

 ecules and the relatively low resonance energy (3-4 kcal/mole). 



The surprising observation by Hassel and Naeshagen (1930) that p- 

 benzoquinone, usually assumed to be symmetrical and planar, has a dipole 

 moment of around 0.67 debye has been confirmed several times in dif- 

 ferent solvents (Hammick et al., 1935) and in the vapor phase (Coop and 

 Sutton, 1938). The earlier theories of distortion due to thermal impacts 

 or vibration of the C=0 group in the plane of the ring have been shown 

 to be incorrect (Kofod, 1953), and it is now generally held that the molecule 

 does not possess a permanent moment, the results arising because of po- 

 larization of the electron clouds around the oxygen atoms in the applied 

 electric field (Paoloni, 1958; Charney, 1961). Hydroquinone possesses a 

 permanent dipole moment (1.4 debyes) but this is less than expected be- 

 cause the partial double bond character of the C — bonds limits free ro- 

 tation of the hydroxy groups (Lander and Svirbely, 1945). The dipole 

 moments for the other common quinones are: o-benzoquinone 5.1 debyes, 

 1,2-naphthoquinone 5.6 debyes, and 1,4-napthoquinone 1.3 debyes. As- 

 suming these values and applying a molecular orbital treatment, Kubo- 

 yama (1958, 1959) has calculated the charge distributions over the mole- 

 cules. The fractional electronic charge on the oxygen atoms was determined 

 as lying between — 0.46 and — 0.55, and on the carbon atoms of the 

 C=0 bond between + 0.27 and + 0.35, the residual positive charge being 

 distributed throughout the remainder of the ring carbon atoms. 



Substitution of a hydroxy or amino group in a position adjacent to the 

 carbonyl group usually provides conditions suitable for hydrogen bonding. 

 Such intramolecular hydrogen bonds have been demonstrated spectro- 

 scopicaUy in 2-hydroxy-l,4-naphthoquinone and naphthazarin (Brock- 



