Aprotic solvents dipole moments

Parker37 defined class 4 as solvents "which cannot donate suitable labile hydrogen atoms to form strong hydrogen bonds with an appropriate species and proposed the designation dipolar aprotic solvents he extended their range down to s > 15 and quoted as examples acetone, acetonitrile, benzonitrile, dimethylformamide, dimethyl sulphoxide, nitrobenzene, nitromethane (41.8) and sulfolane (tetramethylene sulphone) (44), where e varies from 21 to 46.5, and the dipole moment p from 2.7 to 4.7 debye. 

Dipolar aprotic solvents possess large dielectric constants Sr > 15), sizeable dipole moments (p > 8.3 x 10" ° Cm= 2.5 D), and average E/-values of 0.3 to 0.5. These solvents do not act as hydrogen-bond donors since their C-H bonds are not strongly polarized enough. 

It should also be feasible to extend further the types of reaction that can be accelerated. For example, large solvent effects have been observed in kinetic studies of many reactions involving anions.46,47,5° 51 In many cases the solvents are aprotic but not truly apolar, in the sense that their molecules have large dipole moments, for example, (CH3)2S=0, CH3CON(CH3)2. Derivatives of polyethylenimine can be made that have substituents mimicking these in chemical structure. For example, acylation of the polymer will produce CH3CO—N=C loci on the macromolecule. Such modified polymers should manifest substantial catalytic effects. 

If we consider just essentially using dipole moment as a surrogate for dielectric constant, with minor conceptual adjustments like some changes in the parameterization constants, then from the above, for nonpolar and polar aprotic solvents the correlation is good enough that it may be possible to parameterize with dipole moment, but there is no clear indication that this would have any advantage. Furthermore, water, the most important solvent, belongs to the polar protic class, for which there is no correlation. 

The dipole moments and dielectric constants of aprotic solvents range from near-zero values for the hydrocarbons to moderate values for solvents like... 

Polar aprotic solvents, on the other hand, have dipole moments and are still able to solvate cations by electron donation from an oxygen atom, but they lack the ability to form hydrogen bonds because any hydrogen atoms they may have are on carbon. Examples include DMF and DM So (dimethyl sulfoxide). 

An apolar aprotic solvent is characterized by a low relative permittivity (sr < 15), a low dipole moment [ju < 8.3 10 Cm = 2.5 D), a low value ca. 0.0... 0.3) cf. Table A-1, Appendix), and the inability to act as a hydrogen-bond donor. Such solvents interact only slightly with the solute since only the non-specific directional, induction, and dispersion forces can operate. To this group belong aliphatic and aromatic hydrocarbons, their halogen derivatives, tertiary amines, and carbon disulfide. 

In contrast, dipolar aprotic solvents possess large relative permittivities (sr > 15), sizeable dipole moments p > 8.3 10 ° Cm = 2.5 D), and average C.f values of 0.3 to 0.5. These solvents do not act as hydrogen-bond donors since their C—H bonds are not sufficiently polarized. However, they are usually good EPD solvents and hence cation sol-vators due to the presence of lone electron pairs. Among the most important dipolar aprotic solvents are acetone, acetonitrile [75], benzonitrile, A,A-dimethylacetamide [76, 77], A,A-dimethylformamide [76-78], dimethylsulfone [79], dimethyl sulfoxide [80-84], hex-amethylphosphoric triamide [85], 1-methylpyrrolidin-2-one [86], nitrobenzene, nitro-methane [87], cyclic carbonates such as propylene carbonate (4-methyl-l,3-dioxol-2-one) [88], sulfolane (tetrahydrothiophene-1,1-dioxide) [89, 90, 90a], 1,1,3,3-tetramethylurea [91, 91a] and tetrasubstituted cyclic ureas such as 3,4,5,6-tetrahydro-l,3-dimethyl-pyr-imidin-2-(l//)-one (dimethyl propylene urea, DMPU) [133]. The latter is a suitable substitute for the carcinogenic hexamethylphosphoric triamide cf. Table A-14) [134]. 

Other examples of this type of reaction are Sn2 reactions between azide ion and 1-bromobutane [67], bromide ion and methyl tosylate [68], and bromide ion and iodoethane [497]. In changing the medium from non-HBD solvents (HMPT, 1-methylpyrrolidin-2-one) to methanol, the second-order rate constants decrease by a factor of 2 10 [67], 9 10 [68], and 1 10 [497], respectively. The large decrease in these rate constants in going from the less to the more polar solvent is not only governed by the difference in solvent polarity, as measured by dipole moment or relative permittivity, but also by the fact that the less polar solvents are dipolar aprotic and the more polar solvents are protic cf. Section 5.5.2). 

A second limitation of the Hughes-Ingold theory concerns the fact that the solvent is treated as dielectric continuum, characterized by one of the following its relative permittivity, e, the dipole moment, fi, or by its electrostatic factor, EF, defined as the product of and [27]. The term solvent polarity refers then to the ability of a solvent to interact electrostatically with solute molecules. It should be remembered, however, that solvents can also interact with solute molecules through specific inter-molecular forces like hydrogen bonding or EPD/EPA complexation cf. Section 2.2). For example, specific solvation of anionic solutes by pro tic solvents may reduce their nucleophilic reactivity, whereas in dipolar aprotic solvents solvation of anions is less,... 

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