Carbon-hydrogen acids, dissociation

Direct hydrogen atom abstraction occurs less frequently from the nucle-obases, despite the expected modest carbon—hydrogen bond dissociation energy of the carbon—hydrogen bonds in the methyl groups of thymidine and 5-methyl-2 -deoxycytidine due to resonance stabilization of the incipient radicals. The respective radicals are also formed by deprotonation of the nucleobase radical cations, intermediates involved in electron transfer that are produced via one-electron oxidation. Amine radicals are also postulated as intermediates produced from the spontaneous decomposition of chloramines that arise from reactions of nucleosides with hypochlorous acid." " However, the majority of nucleobase radical intermediates arise from the... 

The hydrogen ion flux that is provided by carbonic acid dissociation also can attack calcite (CaCO ) ... 

If a methyl group replaces a hydrogen atom on the carbon of the C==N bond across which addition of water occurs, a considerable reduction in the extent of water addition is observed. Conversely, the existence of such a blocking effect can be used as a provisional indication of the site at which addition of water occurs, while the spectrum and acid dissociation constant of the methyl derivative provide a useful indication of the corresponding properties of the anhydrous parent substance. Examples of the effect of such a methyl group on equilibria are given in Table IV. 

Oxidation of alkanes involves the removal of an electron from either a carbon-hydrogen or a carbon-carbon o-bond. These are dissociative processes where the radical-cation cannot be detected as an intermediate in either fluorosulphuric acid or acetonitrile. 

Carbonic acid dissociates to produce bicarbonate and hydrogen ions... 

The fluorotelomer carboxylic acids (FTCAs) and fluorotelomer unsaturated carboxylic acids (FTUCAs) are degradation products of FTOHs with the general structure F(CF2) CH2CH00H and F(CF2) CHCOOH respectively, where usually n = 6, 8 or 10 (Table 3.1). Similar to FTOHs, FTCAs and FTUCAs are also named based on the ratio of fluorinated carbons to hydrogenated carbons in the molecule. Although the acid dissociation constants are not known for the FTCAs and the FTUCAs, it is assumed they will also dissociate in the natural environment ... 

Acid-Catalyzed Elimination Reactions. The simplest kind of elimination reaction is catalyzed by acids and proceeds through a transitory carbonium ion (p. 44). Consider tert-butyl alcohol. In the presence of acid, an oxonium ion is formed (I) which can dissociate into water and a carbonium ion (II). As with all carbonium ions, there are then four courses of reaction open. (1) It can react with another water molecule or anion. (2) It can rearrange. (3) It can abstract a hydrogen atom with a pair of electrons from another molecule. (4) It can attract an electron pair from the carbon-hydrogen bond of an adjacent carbon atom so as to liberate a proton and to form an olefin (III to IV). The fourth possibility is the process by which many acid-catalyzcd elimination reactions occur. 

Because bicarbonate is a small ion, it is freely filtered at the glomerulus. The bicarbonate load delivered to the nephron is approximately 4,500 mEq/day. To maintain acid-base balance, this entire filtered load must be reabsorbed. Bicarbonate reabsorption occurs primarily in the proximal tubule (Fig. 51-1). In the mbular lumen, filtered bicarbonate combines with hydrogen ion secreted by the apical Na+-H+-exchanger to form carbonic acid. The carbonic acid is rapidly broken down to CO2 and water by carbonic anhydrase located on the luminal surface of the brush border membrane. The CO2 then diffuses into the proximal tubular cell, where it reforms carbonic acid in the presence of intracellular carbonic anhydrase. The carbonic acid dissociates to form hydrogen ion, that can again be secreted into the tubular lumen, and bicarbonate that exits the cell across the basolateral membrane and enters the peritubular capillary. 

The carbonic acid dissociates, releasing hydrogen ions, which are buffered by nonbicarbonate buffers (i.e., proteins, phosphate, and hemoglobin) and bicarbonate. Thus on the basis of physicochemical factors, increases in PaC02 raise the serum bicarbonate concentration. In general, in acute respiratory acidosis, the bicarbonate concentration increases by 1 mEq/L above 24 for each 10-mm Hg increase in PaC02 above 40 (see Table 51 ). 

Polymethacrylic acid, PMA, is composed of vinyl polymer backbone as with PAA molecules, but a carbons attached to carboxyl groups are bonded to methyl groups instead of hydrogen atoms. This rather hydrophobic poly-carboxylic acid shows a peculiar acid dissociation behavior. As is illustrated... 

The 5,8-dihydroxy-4(3//)-quinazolinones possess three protons capable of dissociation in aqueous buffer, viz. N3—H and the protons of the 5- and 8-hydroxyl groups (Scheme 107). Initial acid dissociation of the N3 proton does not take place at low pH because electron-rich substituted quinazolin-4(3//)-ones possess pK values greater than 10. The first dissociation is from the 5-hydroxyl which affords an anion (697) which is stabilized by internal hydrogen bonding expressed by pK 7.3 (7.8). Cleavage of the halogen-carbon bond in the 2-substituent in (697) involves a rate-determining reaction of the hydroquinone monoanion and dianion species, since there is an approximate 100-fold difference in rate constants between the precursor 2-chloro (696 X = Cl) and... 

In clean natural water the pH can be calculated from the content of free CO2 and hydrogen carbonates using the expression for the first dissociation constant of carbonic acid. Dissociation to the 2nd degree can be neglected as its effect becomes significant only as pH > 8.3. Due to the inaccurate determination of free CO2 the calculation provides only rough results. On the contrary, from a known value of pH and the content of HCO3 the content of free CO2 can be calculated. 

Carbonic acid dissociates to give hydrogen ions and bicarbonate ions. 

Reagents such as n-butyllithium, methyllithium, and phenyllithium manufactured by this process are commercially available. Carbanions can also be formed by an acid-base reaction involving heterolytic dissociation of a carbon-hydrogen bond by a strong base. An example is the deprotonation of limonene (86) by -butyllithium complexed with tetramethylethylenedia-mine (TMEDA), as shown in equation 5.61. Note that there is more than one kind of allylic proton in 86, but the deprotonation preferentially produces the least-substituted carbanion. ... 

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