Proton-donor function, hydroxyl group

The possibility exists that a surface hydroxyl group on silica can itself function as a proton donor (12),... 

In all the aprotic solvents (with no proton donor admixtures), the superoxide ion generally cannot act as an oxidant, taking into account a wide range of functionally substituted compounds. For example, in dry pyridine, 07 does not oxidize 1,2-dimethoxybenzene (Sawyer Gibian 1979). This ion, however, reacts with 1,2-dihydroxybenzene. For the OH form, the first step consists of proton transfer from the hydroxyl group to the superoxide ion. Next, reactions proceed with the participation of HOO. The latter is formed according to the disproportionation of Scheme 1-70. 

The carboxylic acid group is of particular interest as it contains both the hydroxyl —OH and carbonyl C=0 functionalities on the same molecule. Its acidic nature is exhibited, for example, in crystal structures where its H-bonds tend to be shorter and straighter than those of other proton donors... 

What is now required is the number of ways Q of distributing the TVy bonds among the functional groups of the system. Let us briefly summarize the rationale for this enumeration process and apply it, first, to the simple case of a system of molecules with one proton donor and one acceptor group which self-associate. An example of such systems is that of molecules with hydroxyl -OH groups. 

To discuss acid-base catalysis, it is helpful to recall the definitions of acids and bases. In the Brpnsted-Lowry definition, an acid is a proton donor and a base is a proton acceptor. The concept of general acid-base catalysis depends on donation and acceptance of protons by groups such as the imidazole, hydroxyl, carboxyl, sulfhydryl, amino, and phenolic side chains of amino acids all these functional groups can act as acids or bases. The donation and acceptance of protons gives rise to the bond breaking and re-formation that constitute the enzymatic reaction. 

The second method is to prepare a functionalized polymer by reacting some reagent with the living aluminium alkoxide, with the terminal hydroxyl group generated after quenching the polymerization with a proton donor, or with the flmctionality incorporated by the first method (Eq. 12-14). 

It should be noted that minerals, and oxide minerals in particular, have different types of OH groups, depending on the coordination of the O atoms, as revealed by spectroscopic studies. Goethite (a-FeOOH) has four types of surface hydroxyls whose reactivities are a function of the coordination environment of the O in the FeOH group (Sposito 1984 Sparks 2002). The FeOH groups are A-, B-, or C-type sites, depending on whether the O is coordinated with 1,3, or 2 adjacent Fe(III) ions. The fourth type of site is a Lewis acid-type site, which results from chemisorption of a water molecule on a bare Fe(III) ion. Only A-type sites are basic (can bind H+), and, on the contrary, A-type and Lewis acid sites can release a proton. The B- and C-type sites are considered unreactive. Thus, A-type sites can be either a proton acceptor or a proton donor (i.e., they are amphoteric). Other spectroscopic studies have shown that boehmite (y-AlOOH) and lepidocrocite (y-FeOOH) have two types of OH, presumably associated with different crystal faces (Lewis and Farmer 1986). 

---