Mills-Nixon effect indan

Taylor has collected the above and similar data and compared the ratio of reactivities of the ortho and para positions of compounds of type 19 (expressed as log/odog/p) with the ratio of reactivities of the equivalent positions, a and c, in compounds of type 20 and found that the latter ratio was lower, i.e., a relative increase in the reactivity of the para position (c) has occurred upon ring formation. This fall in the ratio log fa log fc increases along the series X = S < 0 (< NH < CHg) in 20. As this trend parallels the increase in strain in the fused bridging ring it was argued that ring strain was the primary cause of the reduction in ratio. Position a is a-aromatic and position c is j8-aromatic therefore the above concept represents an extension by Taylor of an earlier explanation of the Mills-Nixon effect in indane. Further substitution... 

The manifestation of a Mills-Nixon effect in benzocyclopropene (1) was examined by a variety of methods. Thus examination of the PE spectra of cyclo-proparenes and appropriate model compounds provided no information on the question.Similarly, NMR investigations using vicinal coupling constants or orthobenzylic coupling constants, involving methyl-substituted cydoproparenes revealed no evidence for sterically induced bond fixation. Theoretical calculations led to contradictory results. Early extended Hiickel calculations supported the concept in indanes but in the opposite sense than that suggested by Mills and Nixon. Semiempirical calculations for benzocyclopropene favored structure 258 over A substantial Mills-Nixon effect in the same direction resulted from... 

This paper records the results of a similar study of the bond structure of tetrahydronaphthalene and a comparison of these results with our earlier work on indan and o-xylene [lc], thus permitting a rather comprehensive evaluation of the Mills-Nixon effect. What is believed to be a novel steric effect on intramolecular hydrogen bonding also will be described. 

Work on the Mills-Nixon effect has been reviewed by Badger [2]. Most of the experimental work done in this field has involved a study of relative chemical reactivities and hence is subject to the severe limitations mentioned earlier [ lc]. From a study of the ease with which the benzoates of isomeric hydroxy-5,6,7,8-tetrahydroacetonaphthones underwent the Baker-Venkataraman transformation O Farrell et ah [3] recently concluded that the 1,2-bond of 5,6,7,8-tetrahydro-naphthalene has a higher bond order than the 2,3-bond. Similar work also led these authors to conclude that the 4,5-bond in indan is of higher order than the 5,6-bond. These workers did not attempt to assess the extent of the difference in bond order presumably exhibited by the bonds in question. 

The Mills-Nixon hypothesis had, as its foundation, certain differences in the chemical behaviour of indan (3) and tetralin (4) from which a localization of the aromatic 7r-bonds was predicted to occur in the direction depicted by la rather lb. The original experimental evidence upon which the effect was based was shown to be erroneous, but calculations at various levels of theory indicated that aromatic bond localization should exist and become more pronounced as the size of the annelated ring decreases In essence one can recognize that the structure of benzene has a symmetry such that both Kekule structures must contribute equally. With Q tetralin (4) (and the lower homologues) no such symmetry requirement exists and ring annelation could induce bond length alternation within the arene nucleus. As the strain imposed by the fused ring increases, the Mills-Nixon effect should increase. The hypothesis has been the subject of considerable discussion and the controversy is far from settled. 

An interpretation of this effect was put forward by Mills and Nixon first. It was based on the tetrahedral distribution of the carbon atom valencies. Consider bond angles around the carbon junction atom in indan as depicted in Fig. 2 ... 

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