Dipole repulsions

Several structural factors have been considered as possible causes of the anomeric effect. In localized valence bond terminology, it can be recognized that there will be a dipole-dipole repulsion between the polar bonds at the anomeric carbon in the equatorial conformation. This dipole-dipole interaction is reduced in the axial conformation, and this factor probably contributes to the solvent dependence of the anomeric effect. 

The addition of methyl 3-amino-2-butanoate to ( )-4-[(5 )-arylsulfinyl]-5-(2-chlorophenyl)-4-penten-2-one in refluxing methanol gave the dihydropyridine as a single diastereomer. The stereochemical outcome is rationalized as occurring from attack of the nucleophile on the conformation in which dipole-dipole repulsion is minimized17. 

A variation of this approach has recently been provided by Lyakhov et al. [598] who, from measurements of water adsorption on CuS04 5 H20, on MgS04 7 H20, and on their respective dehydration products, discern a correlation between strengths of surface bonding and S—T behaviour. At low surface coverages, the mutual dipole—dipole repulsions in the adsorbed layer inhibit water loss, in part by a blocking action on loss of water of crystallization and in part by polarization effects which provide a... 

Chloroketone (33) prefers to adopt the axial conformation (33a) because of dipole repulsion between C-Cl and C=0, The less hindered side of the carbonyl group is therefore trana to the chlorine giving (35) in the right conformation (35a) - trans diaxlal - for rearrangement. 

Mtrans isomer. Both groups dipole repulsion... 

Kevan (1974) and Tachiya (1972) point out that CKJ use an SCF approximation to calculate the medium polarization energy, but in everything else they use the adiabatic approximation. This somewhat inconsistent procedure, which may be called the modified adiabatic approximation, gives results similar to those obtained by FFK. Varying the dipole moment and the polarizability in the semicontinuum models varies the result qualitatively in the same direction. It increases the electron-solvent attraction in the first shell and also increases the dipole-dipole repulsion. Both hv and I increase with the dipole moment, but not proportionately. 

Aside from the interaction of the radical with the solvent there are several factors that can make the ethane unstable or stabilize the resulting free radical. They are the stabilization of the radical by resonance, steric strain in the ethane, and dipole-dipole repulsion in the ethane. Steric inhibition of both resonance and solvation in the radical favor association to the dimer. 

The inhibited unimolecular decomposition of symmetrically di-substituted benzoyl peroxides into radicals also obeys the Hammett rho-sigma relationship. Unfortunately, no extensive activation parameter data are available. The effect of the substituent changes on the rates at the single temperature has been explained in terms of dipole-dipole repulsion in the peroxide.122... 

When the reaction is well-established as a radical one it is still possible to find explanations of polar substituent effects, usually in terms of ion-radical intermediates, dipole-dipole repulsion, solvation, or charge-transfer structures like those postulated for -complexes.488 Reac-... 

It was shown (801) that the diastereoselectivity of a-fluoroalkyl nitrones is reversed to that of the corresponding a-alkyl nitrones. This fact supports the conclusion that the conformation, due to the relief of the dipole repulsion between the fluorine atom and the oxygen atom of the nitrone is preferred in a-fluoroalkyl nitrones. 

The case is interesting in 1,2 dibromocyclohexane. The moment for the diaxial isomer is assumed to be zero, while the calculated and observed values for the diequatorial form are respectively 3.09 D and 3.12 D. The trans diaxial form is more stable than the trans diequatorial, although the latter is favoured sterically. The reason for the lesser stability of the trans diequatorial form is that it is destabilized by dipole repulsion. 

When there are chances for the presence of both anti and gauche forms in an equilibrium, the anti isomer is preferred, because the steric and dipole repulsion come into play in the latter, as is illustrated from the example of ethylene glycol. The two forms of this glycol are ... 

Experimental data for 1,2-dicyanoethane are available. In the gas phase, the anti conformer is more stable while in the liquid phase, where dipole-dipole repulsive effects are deemphasized, the gauche is more stable67. ... 

Because of the higher CH acidity (by 7-8 pKa units) of alkylidene malonates (441), as compared with that of dialkyl malonates (87JA809), one-pot syntheses of alkylidene aminomethylenemalonates (442) could be carried out readily with a wide variety of primary and secondary aliphatic and cycloaliphatic amines, anilines, naphthylamines and heterocyclic amines, trialkyl orthoformate, and alkylidene malonates (83MI1 86MI9). It was proposed that the higher CH acidity of alkylidene malonates was a consequence of the electrostatic (dipole-dipole) repulsion effects... 

By exploiting electrostatic field effects (unfavourable through-space charge-dipole repulsion) to increase the nucleophilic susceptibility of cyclohexanones, more efficient catalysts (16) and (17) for epoxidafion through in situ dioxirane formation have been designed. ... 

Fig. 2.1 Dipole and hydrogen bond interactions. A schematic representation of (a) head-to-tail dipole-dipole attractive interactions (e.g., in tri-n-octylamine) (b) head-to-head dipole-dipole repulsive interactions caused by steric hindrance (e.g., in dibutyl sulfoxide) (c) chainlike dipole-dipole interactions (e.g., in 1-octanol) (d) a cyclic, hydrogen-bonded dimer (e.g., in hexanoic acid).

Two distinct dynamic NMR effects occur in 1,3-dioxocane and its derivatives having a gem-dimethyl group at the 2- and/or 6-positions (chemical, not conformational numbering) (76JA2059). These molecules have unsymmetrical boat-chair conformations (e.g. 432) which minimize non-bonded and dipole-dipole repulsions... 

For (a) and (b) the gauche is the more stable conformer and has a greater population because of H-bonding with F and N. The anti conformer is more stable in (c) because there is no H-bonding with Br and dipole-dipole repulsion causes OH and Br to lie as far from each other as possible. 

Adatom pair Distance of strongest bond Bond energy meV Expected dipole repulsive energy meV... 

From an experimental point of view there was little evidence to form a basis for reliable speculation. Up to 1936 only the triphenyl methyl (and related) radicals had been studied quantitatively in solution and these had appreciable activation energies for recombination of about 11 kcal.3 A number of authors felt that there might be a moderate dipole-dipole repulsion in the head-to-head approach (i.e., C to C) of two alkyl radicals. Others postulated a repulsion arising from the necessity to rehybridize the carbon orbitals from that which would obtain in the radicals to that which would obtain in the final hydrocarbon. 

Dipole-dipole repulsions may invalidate the assumption that, e.g., the dipole moment of 4-(p-chlorophenyl)-l-methylpiperidine is the vector sum of that of p-chlorophenylcyclohexane and W-methylpiperidine.93,94 The assumption119 that the iV-methylpiperidine moment is equally inclined to the N-Me and to the two N—C bonds may be invalid.125... 

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