Substituents bond moments

These interactions (dd, di, ii) are a function of dipole moment and polarizability. It has been shown that the dipole moment cannot be replaced entirely by the use of electrical effect substituent constants as parameters52. This is because the dipole moment has no sign. Either an overall electron donor group or an overall electron acceptor group may have the same value of /x. It has also been shown that the bond moment rather than the molecular dipole moment is the parameter of choice. The dipole moments of MeX and PhX were taken as measures of the bond moments of substituents bonded to sp3- and sp2-hybridized carbon atoms, respectively, of a skeletal group. Application to substituents bonded to sp-hybridized carbon atoms should require a set of dipole moments for substituted ethynes. 

Complexes of platinum(II) with /J-monothiodiketonates, derived by deprotonation of the parent acid (207), can be prepared from PtCl -. The dark red complex Pt(C3H(Ph)2SO 2 (R = R = Ph) shows IR bands at 1535 cm-1 [v(C=C)], 1410 cm-1 [v(O O)] and 1270 cm-1 [v(O S)].1824 Electronic spectra and dipole moment data for these complexes have been compared with the O.O -diketonate complexes.182s 1826 The structure of the phenyl derivative has been confirmed by X-ray crystallography.1827 Detailed dipole moment measurements using static polarization have been made with fluorinated jS-monothiodiketone complexes. Variations with substituent depend on the magnitude and vector directions of the Ph—X bond moments (aryl substituents), the inductive effect of the meta and para substituent on the phenyl ring, and the mesomeric effect of the substituent X.1828 A useful separation method for bis(monothiotrifluoroacetylacetonates) of platinum(II) is gas chromatography.1829... 

The differences in the value of the dipole moments of the complexes [M(RCS=CHCOCF3) ] (R = Ph or X-substituted phenyl) depend upon (a) the magnitude and vector direction of the Ph—X bond moments (b) the inductive effect arising from the difference in electron density of the C-l and C-5 atoms of the monothio-jS-diketone (this is affected by the nature and position of the substituent X on the phenyl ring) (c) the change in moment brought about by the mesomeric effect (56) of X.230-231... 

D, 004), and 3-acetoxy- and 3-(a-methylacrylyloxy) thietanes have been measured and compared with values calculated for various conformations. Calculated moments for a series of 2,2-diphenyl-3- and 3,4-chloro or cyano-substituted thietanes have been compared with observed moments to determine conformations.The substituents prefer a pseudoequatorial conformation when possible. The two C-Cl bond moments in cis- and fra s-2,2-diphenyl-3,4-dichlorothietane differ [/ (Ca-Cl) = 1.40D, t(C4-Cl) = 2.10D], which is said to be caused by differing interactions between the lone-pair electrons on sulfur and chlorine. 2-Methyl-thietane has a moment of 1.79 3,3-dimethylthietane, 1.76 or 1.87 D , and the Spiro-derivative, 2.6-dithiaspiro[3.3]heptane, 1.12D. ... 

On the basis of the vector sum in Equation 18 the molecular dipole moments of substitued allenes should be predictable with the bond moments from Table 3. 

In the nitriles, the X—C bond length is insensitive to the change from nitrogen to phosphorus, as might be expected since the triple bond is substituent invariant. The only noticeable exception is FCN/FCP where the F—C distance changes by 0.02 A. The trend is reflected in the dipole moments which, although substantially smaller in the phosphaalkynes, show similar variations with substituents in CsN and CsP molecules. 

Most alkenes are weakly polar. For example, propene has a dipole moment of 0.3 D. The dipole moments of alkenes containing substituents with bond moments of known direction can be used to establish the bond moment for an sp —sp carbon—carbon bond. The dipole moment of chlo-roethene is 1.4 D. Because chlorine is more electronegative than carbon, the chlorine atom has a partial negative charge. The net dipole moment of rn r-l-chloropropene is 1.7 D. It results from the cumulative effect of the carbon—carbon single bond and the carbon—chlorine bond. Because dipole moment of tntwr-l-chloropropene is larger than that of 1-chloroethene, the two contributing bond moments in tntwr-l-chloropropene reinforce each other. 

Experimental measurements of dipole moments give size but not direction We normally deduce the overall direction by examining the directions of individual bond dipoles With alkenes the basic question concerns the alkyl groups attached to C=C Does an alkyl group donate electrons to or withdraw electrons from a double bond d This question can be approached by comparing the effect of an alkyl group methyl for exam pie with other substituents... 

Since the six carbons shown above have 10 additional bonds, the variety of substituents they carry or the structures they can be a part of is quite varied, making the Diels-Alder reaction a powerful synthetic tool in organic chemistry. A moment s reflection will convince us that a molecule like structure [XVI] is monofunctional from the point of view of the Diels-Alder condensation. If the Diels-Alder reaction is to be used for the preparation of polymers, the reactants must be bis-dienes and bis-dienophiles. If the diene, the dienophile, or both are part of a ring system to begin with, a polycyclic product results. One of the first high molecular weight polymers prepared by this synthetic route was the product resulting from the reaction of 2-vinyl butadiene [XIX] and benzoquinone [XX] ... 

Figure 4.3. Energy versus bond rotation in methylsuccinic acid (schematic). The diagram shows the greater stability of staggered as compared with eclipsed forms, and the effect of size and dipole moment of substituents on the barriers. The slope of the curve at any point represents the force opposing rotation there. ( = energy of activation of rotation.) (After Gordon )...

The unequal distribution of electron density in covalent bonds produces a bond dipole, the magnitude of which is expressed by the dipole moment, having the units of charge times distance. Bonds with significant bond dipoles are described as being polar. The bond and group dipole moments of some typical substituents are shown in Table 1.7. 

---