Rabbit-ear effect

The term is not just whimsy on the author s part certain stereoelectronic phenomena arising from the presence of lone pairs on heteroatoms in a 1,3-relationship were once called the rabbit-ear effect , and a photograph of the eponymous creature even appeared on the cover of the Swedish journal Kemisk Tidskrift. History of the term, photograph Eliel EL (1990) From Cologne to Chapel Hill. American Chemical Society, Washington, DC, pp 62-64... 

If the geometry of VI and VII is very similar to bicyclo [2.2.1] heptane, and if the gauche states are more important in the C-O C-O bond than in the C-C C-O bond, it may be concluded that Vll is less stable than VI. In fact, the add-hydrolysis data112 show that VII is more reactive than VI. Hie difference of the hydrolysis rates is affected by the difference of the angles between C3-C7-C4 and C1-O7-C4 as well as the difference of Eliel s rabbit-ear" effect between O2-CV, and O2-O7. 

Fig. 23.—The Rabbit-ear Effect Interactions in the Staggered Conformations of an Acetal Segment.

If the two lone pair sp orbitals on oxygen are drawn in the customary skittle shapes, the major lobes in a Newman projection look like two rabbit s ears. The direct electrostatic effect of lone pair electrons was therefore dubbed the rabbits ears effect, but failed to find acceptance, as much from the overtones of Beatrix Potter as from the fact that the shape of electron density resembled not two, but one ear (Figure 2.11). 

Polydioxolanes exist predominantly in gauche conformation, because in the anti- form the polymer would be of higher energy due to the repulsion of the non-bonded electrons of the oxygen atoms ( rabbit ears effect) ... 

Figure 15. Rabbit ear effect in (a) 2-halogenotetrahydropyrans 97 and (b) 1,3-diazane derivatives 99.

In the initial publications (28, 198) concerning the rabbit ear effect, its electrostatic nature was emphasized, since electric dipoles were attributed to the atom-lone pair systems. Such point of view encouraged and enabled other workers to criticize this concept rather successfully (see Section III.B.l). 

In 1968, Hutchins et al. [41] reported that there is a widespread phenomenon in structural chemistry that the conformations are strongly disfavored if the unshared electron pairs on nonadjacent atoms are parallel or syn-axial, as is the case, for example, in 40 in Fig. 2.15. This effect is thought to be due to the repulsion of electric dipoles engendered by the unshared electron pairs. For obvious reasons, Eliel named this phenomenon the rabbit-ear effect. ... 

There are two reasons for this First, there are two destabilizing syn-parallel interactions between the four pairs of unshared electrons on two oxygen atoms (rabbit-ear effect) and second, in the gauche-gauche conformation there are two stabilizing ew fo-anomeric effects. 

By using NMR spectroscopy, Hutchins et al. [41] studied the confoimatiOTis of variously substituted 1,3-diazanes and found striking support for the rabbit-ear effect (Fig. 2.17). 

Hutchins RO, Kopp LD, Eliel EL (1968) Repulsion of syn-axial electron pairs. The rabbit-ear effect . J Am Chem Soc 90 7174-7175... 

Eliel EL (1969) Rabbit ear effect Polar interactions of hetero atoms. Kem Tidskr 81... 

Polydioxolans are also assumed to exist in the gauche form, because the rabbit-ear effect in the antiform is larger (1 kcal mole"1) than the gauche interaction of the methyl groups (usually considered to be 0.9 kcal mole ). 

---