Donor-acceptor compounds, definition

Chemistry of cyclopropane derivatives, which combine different sorts of activating substituents, has less definitely been investigated and their synthetic potentials have only recently been explored. Whereas geminally donor-acceptor-substituted systems 6 were treated in preceeding contributions to this series 5 6 7), this review will be confined to vicinally activated compounds 7. 

In 1923, Brpnsted and Lowry defined acids and bases on the basis of the transfer of protons. A Brpnsted-Lowry acid is any species that can donate a proton, and a Brpnsted-Lowry base is any species that can accept a proton. These definitions also include all the Arrhenius acids and bases because compounds that dissociate to give H30+ are proton donors, and compounds that dissociate to give OH are proton acceptors. (Hydroxide ion accepts a proton to form H20.)... 

Some liquid covalent halides can act as nonaqueous solvents " based on Lewis acid-base behavior, according to the donor-acceptor definition. The self-dissociated ions consist of a cation formed by subtraction of a hahde ion from the neutral compound, while the anion is formed by its addition (equation 24). Salts derived from such covalent halides can be considered as titration products of the parent acidic and basic compounds (equations 25 and 26). In such cases, both the cation and the anion usually possess a stable coordination number with a high geometrical symmetry. 

A large number of covalently linked systems are currently being synthesized and investigated, differing in the nature of A, B, and L, as well as in the number of functional units in the supramolecular system (nuclearity). It is common to call simple two-component donor-acceptor systems such as that of Eq. 2 dyads , and progressively more complex systems triads , tetrads , pentads , etc.. Systems where all the A and B units are organic molecules are dealt with in Chapter 1 of this section. The present chapter deals with systems where at least one of the A/B functional units is a transition metal coordination compound. From this definition, however, are excluded (a) systems where A and/or B are porphyrins or related species (dealt with in Chapter 2) and (b) systems of high nuclearity with dendritic structures (dealt with in Chapter 9). 

According to the Bronsted-Lowry definition, an acid is a proton donor and a base is a proton acceptor. Compounds that have both a hydrogen and an unshared pair of electrons can potentially react as either an acid or a base, depending on the reaction conditions. For example, water, has both unshared pairs of electrons and hydrogens. Therefore it can act like an acid or a base. 

Because few plant enzymes have been purified to a homogeneous state, final and definitive conclusions cannot yet be drawn concerning their substrate specificities. Many aminotransferase preparations hitherto studied have not shown absolute specificity for a single amino donor compound or amino acceptor compound. Where an incompletely purified enzyme catalyzes a multiplicity of reactions, it is obviously difficult to be certain whether the reactions are all being catalyzed by the same enzyme or by different enzymes present in the preparation. With this reservation, it is nevertheless possible to draw a number of conclusions from currently available data. The outer limits of specificity can fi-equently be ascertained, even though it may sometimes be found subsequently that purer preparations will exhibit more restricted substrate specificities. 

This electrophile/nucleophile dichotomy can be looked upon as a special case of the acid/base idea. The classical definition of acids and bases is that the former are proton donors, and the latter proton acceptors. This was made more general by Lewis, who defined acids as compounds prepared to accept electron pairs, and bases as substances that could provide such pairs. This would include a number of compounds not previously thought of as acids and bases, e.g. boron trifluoride (39),... 

Those photosensitive systems mentioned above consist of at least one vinyl compound which has an electron donating or accepting property. When both acceptor and donor are non-polymerizable, the system is not photosensitive. Photopolymerization of styrene is not sensitized by the ECZ-CH3CN pair. The definition of donor and acceptor is a matter of relativity. Styrene is by no means neutral, but there should be no objection to considering it as a weaker donor than VCZ or ECZ and a weaker acceptor than AN or CH8CN. Photoirradiation of AN, VCZ or styrene alone in a neutral solvent, such as benzene, or in bulk does not bring about any appreciable rate of polymerization. 

---