Proton donor function

Moncrief summarized the work of Cohn and the information in the early literature. As early as 1923, it was known that rupture of the heterocyclic ring, as well as substitution of the imino hydrogen atom, results in the loss of sweetness. Thus, o-carboxybenzenesulfonamide and N-alkyl derivatives of saccharin are tasteless. This loss of sweetness would be expected, as the NH group is the only proton-donor function available in the molecule. 

In the overwhelming majority of cases, this acid-base interaction limits itself only to the first stage - formation of heteromolecular adduct. Interactions occurring in all stages of the scheme [9.13] are typical of binary solvents such as triethylene amine-pyridine or N-diethylaniline. Proton-donor function towards the amine component reveals itself distinctly in the case of diphenylamine. 

If amines differ substantially in their energies of proton affinity, one of them has a proton-donor function in binary mixtures of these amines, so that it acts as an acid ... 

Section 27 21 Often the catalytically active functions of an enzyme are nothing more than proton donors and proton acceptors In many cases a protein acts m cooperation with a coenzyme, a small molecule having the proper func tionahty to carry out a chemical change not otherwise available to the protein itself... 

The kinetics of reactions cataly2ed by very strong acids are often compHcated. The exact nature of the proton donor species is often not known, and typically the rate of the catalytic reaction does not have a simple dependence on the total concentration of the acid. However, sometimes there is a simple dependence of the catalytic reaction rate on some empirical measure of the acid strength of the solution, such as the Hammett acidity function Hq, which is a measure of the tendency of the solution to donate a proton to a neutral base. Sometimes the rate is proportional to (—log/ig)- Such a dependence may be expected when the slow step in the catalytic cycle is the donation of a proton by the solution to a neutral reactant, ie, base but it is not easy to predict when such a dependence may be found. 

Some materials such as water, alcohols, carboxylic acids and primary and secondary amines may be able to act simultaneously as proton donors and acceptors. Cellulose and poly(vinyl alcohol) are two polymers which also function in this way. 

The reduction of a benzenoid ring, except in benzoic acid derivatives, occurs only in the presence of a proton donor having a pKa of 19 or less (pKa of ammonia is about 33). With the exception of the vinyl group, the other functional groups listed above do not require a proton donor of this acidity in order to be reduced, although the course of reduction may then be complex, e.g. as with esters. " Consequently, a variety of functional groups should be capable of selective reduction in the presence of a benzenoid ring if the reaction medium does not contain an acid of pKa <19. A few examples of such selective reductions have been reported in the steroid literature. 

Unless a proton donor is added, the lithium-ammonia reduction of an cnone leads to the lithium enolate and lithium amide. The latter is a sufficiently strong base to rapidly convert the mono-alkylated ketone into its enolate, which can be further alkylated. The function of the... 

An important implication of the Bronsted definitions of acids and bases is that the same substance may be able to function as both an acid and a base. For example, we have seen that a water molecule accepts a proton from an acid molecule (such as HC1 or HCN) to form an H30+ ion. So water is a base. I lowever, a water molecule can donate a proton to a base (such as O2- or NH3) and become an OH ion. So water is also an acid. We describe water as amphiprotic, meaning that an H20 molecule can act both as a proton donor and as a proton acceptor. 

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

A These molecules are equipped with acidic functionality only and, hence, they can participate in the hydrogen bonds as proton donors. The most representative examples from this group are chloroform (CHCI3) or dichlo-romethane (CHjClj). 

AB The molecules belonging to the last category are equipped with both the acidic and the basic functionalities and, hence, they can interact through the hydrogen bonds both as proton donors and proton acceptors. Acohols and carboxylic acids are among the most representative examples from this group. 

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