Acids protons and

Monoprotic weak acids, such as acetic acid, have only a single acidic proton and a single acid dissociation constant. Some acids, such as phosphoric acid, can donate more than one proton and are called polyprotic weak acids. Polyprotic acids are described by a series of acid dissociation steps, each characterized by it own acid dissociation constant. Phosphoric acid, for example, has three acid dissociation reactions and acid dissociation constants. 

Replacement of an aromatic/heteroaromatic proton with a trialkylsilyl group can confer a variety of synthetic advantages. The silyl moiety can mask a potentially acidic proton, and it can be readily removed by electrophiles, normally resulting in a process of ipso desilylation ... 

Hydrolysis and decarboxylation of (12) occur in hot acetic acid. The C02H group in (9) is not protected the first mole of Ph2P=CHg removes the acid proton and the second does the Wittig reaction. 

Moore et al. [189] showed that nalidixic acid has a strong intramolecular hydrogen bond between the acid proton and the ring carbonyl oxygen. This stabilized the molecule and weakened the acid (pKa 6.11). It is interesting in view of the previous photodegradation work that radical formation was most... 

It follows, then, that the proton level in pure water is located midway between the unitary level of acidic proton and the unitary level of basic proton, leading to the hydrated proton concentration at pH 7. 

Fig. 9-22. Unitary proton levels of hydrated and adsorbed hydronium ions (acidic proton) and of hydrated and adsorbed water molecules (basic proton) the left side is the occupied proton level (the real potential of acidic protons), and the right side is the vacant proton level. Hi/HjO) = unitary occupied proton level of adsorbed hydronium ions (acidic proton level) H20.d = unitary vacant proton level of adsorbed hydronium ions (acidic proton level) and unitary occupied proton level of adsorbed water molecules (basic proton level) OH = unitary vacant proton level of adsorbed water molecules (basic proton level) (pHi, ) = hydrated proton level at iso-electric point pR...

Aliphatic nitroalkanes can be categorized into six basic groups primary, secondary and tertiary nitroalkanes, terminal and internal gem-dinitroalkanes, and trinitromethyl compounds. Primary and secondary nitroalkanes, and terminal gem-dinitroalkanes, have acidic protons and find particular use in condensation reactions for the synthesis of more complex and... 

Dinitromethane has two acidic protons and reacts with Michael acceptors to form bis-adducts. " Secondary nitroalkanes can only react with one equivalent of Michael acceptor. In the absence of steric effects primary nitroalkanes usually react with two equivalents of Michael acceptor to form bis-adducts. Depending on the reaction stoichiometry, 1,4-dinitrobutane can be reacted with methyl acrylate to form either the bis-adduct (129) or the tetra-adduct (130) in good yield. " ... 

Primary and secondary nitroalkanes, dinitromethane, and terminal em-dinitroaliphatic compounds like 1,1-dinitroethane, all contain acidic protons and have been used to generate Mannich products. Formaldehyde is commonly used in these reactions although the use of other aliphatic aldehydes has been reported. The nitroalkane component is frequently generated in situ from its methylol derivative, a reaction which also generates formaldehyde. Ammonia, " aliphatic amines, " hydrazine, and even urea have been used as the amine component of Mannich reactions. 

Primary nitramines have acidic protons and are able to undergo condensation reactions to form functionalized nitramines. These reactions are discussed in Section 5.13 because the products have potential application as energetic polymer precursors or find use for the synthesis of other explosives. 

A final explanation of the slow deswelling is that above pH 6.5, the gel must deprotonate by transferring its protons to hydroxyl ions and buffer species. Except at very high pH, hydroxyl ion concentrations are quite low. Neither citrate nor phosphate buffer can be very helpful. The highest pK for citrate is approximately 6.4, well below the pKa of the gel amines (7.8). Thus proton transfer from amine to citrate is not favored. Phosphate buffer has pK values 2.15,7.10, and 12.12. The first two pKs are lower than 7.8, so the corresponding phosphate groups are weaker bases than the gel amines. At and below pH 11.0, the phosphate group with pK = 12.12 will retain its acidic proton, and therefore will also not be able to extract a proton from the gel. 

Excited-state proton transfer relates to a class of molecules with one or more ionizable proton, whose proton-transfer efficiency is different in the ground and excited states. The works of Forster [2-4] and Weller [5-7] laid the foundation for this area on which much of the subsequent work was based. Forster s work led to the understanding of the thermodynamics of ESPT. He constructed a thermodynamic cycle (Forster cycle) which, under certain acceptable approximations, provides the excited-state proton-transfer equilibrium constant (pK f,) from the corresponding ground-state value (pKa) and electronic transition energies of the acid (protonated) and base (deprotonated) forms of the ESPT molecule ... 

Aspartic proteases hydrolyze the amide bond as a result of concerted effort by an aspartic acid and an aspartate (Box 13). The aspartic acid protonates and activates the peptide A (Scheme 3.5.2) towards nucleophilic attack, and the aspartate is re-... 

This elimination is catalyzed by the enzyme enolase and follows an Elcb mechanism. The enzyme supplies a base to remove the acidic proton and generate a carbanion in the first step. In addition, a Mg2+ cation in the enzyme acts as a Lewis acid and bonds to the hydroxy group, making it a better leaving group. 

Alkyl groups in dithioles (3a, b) possess acidic protons and condense with aldehydes to form 5-vinyldithioles (109). This type of reactivity has been widely used in the preparation of some l,6,6aA4-trithiapentalenes (5) or precursors (7iAHC(i3)i6i) by reaction with carbon disulfide (Scheme 17). 

This product forms via an E2 elimination mechanism. Consequently, the elimination reaction is only favored if a frarcs-periplanar relationship exists between the acidic proton and the bromide. In the case of the starting material used in the fast reaction, this is the case. However, looking at the starting material used in the slow reaction, no trarcs-periplanar relationship exists between the acidic proton and the bromide. 

We talked about regio- and stereoselectivity in connection with El and E2 reactions. With ElcB, the regioselcctivity is straightforward the location of the double bond is defined by the position of (a) the acidic proton and (b) the leaving group. 

Much has been learned from synthetic complexes (Section 17-E-7). The requirements to mimic oxyhemoglobin are the formation of a 5-coordinate heme precursor having a proximal base (imidazol, pyridine, or other) and hindering pathways that would lead to irreversible formation of /i-peroxo dimers. The lifetime of the working models is increased by exclusion of acidic protons and nucleophiles from the 02 binding site and working at low temperatures. 

Substances containing more than one acidic proton are called po/yprofic adds. Diprotic acids contain two acidic protons, and triprotic acids contain three acidic protons. Acid protons dissociate one at a time and have different Ka andpJC constants. Carbonic acid (H2CO3) is a diprotic acid. 

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