Hydrogen bond acceptance/electron pair solvents

Even for a simple reaction, involving just one reactant species and one product species, such as a keto-enol tautomerism or a cis-trans isomerization, the above equation for a given solvent is complicated. StUl, in specific cases it is possible to unravel the solvent effects of cavity formation, for the solute species have different volumes, polarity/polarizability if the solute species differ in their dipole moments or polarizabilities, and solvent Lewis acidity and basicity if the solutes differ in their electron-pair and hydrogen-bond acceptance abilities. 

If covalent, ionic and metallic bonds are explained in electrical terms, students are better prepared to accept that hydrogen bonds, van der Waals forces, solvent-solute interactions etc. are also types of chemical bonding. Where learners see covalent bonds as electron pairs attracted to two different positive cores, they have a good basis for subsequently learning about electronegativity and bond polarity. 

Electron Pair Donicity and AbiUty to Accept a Hydrogen Bond The ability of solvent molecules to donate a free electron pair from their donor atoms (O, N, or S) to coordinate with acceptor atoms of solutes is a measure of the solvent donicity. It can also be construed as its basicity in the Lewis and the Brbnsted senses, because it also describes the ability of the solvent molecules to accept a proton from a Bronsted acid to be protonated or to form a hydrogen bond. 

The dissolution of a solute in a solvent always affects the solvent-solvent interactions in the vicinity of the solnte particles in addition to the solnte-solvent interactions that take place (Marcus, 1998b). This may be viewed in several stages. First, a cavity in the solvent is formed, to accommodate the solute, which breaks down the cohesive forces of the solvent. Next dispersion forces take effect. They apply to nonpolar and hardly polarizable solutes and solvents, as well as to polar and polarizable ones. Other forces that become active provide contributions from interactions of polar molecnles with polar or polarizable ones and from donor acceptor interactions, such as electron-pair or hydrogen-bond donation and acceptance, whether from or to the solute, the solvent, or both. 

The solvents of this class are often called dipolar aprotic solvents. They are polar and of very weak acidity (proton donor capadty, hydrogen bond donor capacity and electron pair acceptability). However, with regards to basidty (proton accept-... 

The chemical properties of solvents have obviously a strong bearing on their applicability for various purposes. The solvents should selectively dissolve the desired solutes and not some others, they should be inactive in the chemical reactions undergone by the solutes, but solvate, again selectively, reactants, transition states, intermediates, and products. These aspects of the behaviour can be achieved by the proper blend of the chemical properties of structuredness, polarity, electron-pair and hydrogen bond donation and acceptance ability, softness, acidity and basicity, hydrophilicity or hydrophobicity, and redox properties, among others. Such chemical characteristics can often be derived from physical properties, but in other cases must be obtained from chemical interactions, for instance by the use of chemical probes ( indicators ). 

The solvating ability of solvents depends not only on their general polarity, which is a non-specific property, but in a large part to their ability to interact in a specific manner with the solute. This may take place by the donation of a nonbonding pair of electrons from a donor atom of the solvent towards the formation of a coordinate bond with the solute, therefore exhibiting Lewis basicity, or the acceptance of such a pair from a solute, an exhibition of Lewis acidity of a protic or protogenic solvent towards the formation of a hydrogen bond between it and... 

This polarity index measures the intermolecular attraction between a solute and a solvent, whereas the Hildebrand solubility parameter is defined for pure solvent. For example, ether is not very polar and has a Hildebrand value of 7.4—about the same as hexane, which has a value of 7.3. However, ether can accept protons in the form of hydrogen bonds to its nonbonding electron pairs, and consequently its polarity index is 2.8 compared to 0.1 for hexane. 

The term hydrogen-bond acceptor (HBA) refers to the acceptance of the proton of a hydrogen-bond. Therefore, HBA solvents are also electron-pair donor (EPD) solvents. Hydrogen-bond donor (HBD) refers to the donation of the proton. Therefore, HBD solvents behave as protic solvents. 

Meanwhile, fluorinated alcohols cannot be good proton acceptors due to their electron-deficient lone pairs on the oxygen atoms. The (3 scale of both TFE and HFIP is 0.00, which is apparently smaller than those of ethanol (0.77), ether (0.47), water (0.18), or even toluene (0.11) [33]. Here, the (3 scale, i.e. the hydrogen-bond acceptor basicity of a solvent, describes the ability to accept a proton (donate an electron pair) in a solute-solvent hydrogen-bonding system [4]. These acidities (pFCa [31, 34]), hydrogen-bonding parameters (a and (3 scale... 

Cheletropic Cycloadditions. If an atom can both donate an electron pair and accept an electron pair to move to a higher covalency, it can act as a dienophile to a diene. Such is the case with the sulfur atom in sulfur dioxide and with phosphoms in tricovalent halides. This type of cycloaddition is called cheletropic, and results in the formation of 5-membered rings. The reaction of butadiene with SO2 is particularly important it is conducted on a commercial scale to produce the cyclic sulfone 5.18 (called sulfolene). Its hydrogenation product 5.19 (sulfolane) is widely used as a nonaqueous highly polar solvent. Substituted dienes also participate in the cycloaddition. Here, the sulfur can donate its electron pair while accepting an electron pair to form a new carbon-sulfur bond. 

Taft and coworkers described the formulation of three scales of solvent properties which were used to unravel and rationalize solvent effects on many types of physico-chemical properties. A tt scale of polarity/polarizabilities describes the solvent s ability to stabilize a charge or a dipole by virtue of its dielectric effect. The n values have been shown to be generally proportional to molecular dipole moments. The a scale of hydrogen bond donor acidities provides a measure of the solvent s ability to donate a proton. The jS scale of hydrogen bond acceptor basicities quantifies the solvent s ability to donate an electron pair (accept a proton). 

A complementary scale, the acceptor number [6], is a measure of the ability of the solvent to accept an electron pair, and is closely related to hydrogen bonding ability. It is the normalised NMR chemical shift of the complex of triethylphosphine oxide with the test compound relative to that with the strong acceptor antimony pentafluoride (equation 12.3). 

As was noted in Chapter 3, the presence of one or more unshared pairs of electrons confers both Brpnsted and Lewis basicity, and nucleophilicity, on the molecule. This may be manifested as the ability to solvate cations, to accept hydrogen bonds, to stabilize normal species or activated complexes by association with positive charges or electron-poor regions. Values of the empirical parameters that purport to measure basicity, p, B., DN, pA"gjj+ (aq), and so on, for different basic solvents accordingly differ even in their order of strengths. The hard/soft classification does not explain all the differences. 

The Kamlet-Taft a scale [48], on the other hand, was designed to show the net hydrogen bond donation or electron pair acceptance ability of a solvent, being... 

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