Chemistry conjugate acids

Alkylation of 2-methylaminothiazole (204) with ROH in 85% sulfuric acid gives 2-methylimino-3-alkyl-4-thiazoIine (54). 2-Amino-4-rnethyl-thiazoie alkylated with an excess of isopropanol, however, gives 95% of 2-isopropylamino-4-methyl-5-isopropylthiazole (56). The same result is obtained with cyclohexanol (242). These results and those reported in Sections III.l.C and IV.l.E offer interesting new synthetic possibilities in thiazole chemistry. The reactive species in these alkylations is the conjugate acid of 2-aminothiazole. and the diversity of the products obtained suggests that three nucleophilic centers may be operative in this species. 

In aqueous solution, closo borane anions are very stable as their conjugate acids, which possess acidity similar to sulfuric acid, yet their chemistry is remarkably different. Large unipositive cations, such as Tl", Cs", Rb", [(CH2)4N]", and [(CH2)3S]", yield water-iasoluble salts of ... 

Since pH rather than pOH is most widely used in environmental chemistry equations, it is most common to use an acidity constant for the conjugate acid of the base. In this case the equilibrium is expressed as... 

The acid-base chemistry of nicotine is now well known and investigations have shown that nicotine in tobacco smoke or in smokeless tobacco prodncts can exist in pH-dependent protonated or nnprotonated free-base forms. In tobacco smoke, only the free-base form can volatilize readily from the smoke particnlate matter to the gas phase, with rapid deposition in the respiratory tract. Using volatility-based analytical measurements, the fraction of nicotine present as the free-base form can be quantitatively determined. For smokeless tobacco products, the situation differs because the tobacco is placed directly in the oral cavity. Hence, the pH of smokeless tobacco prodncts can be measured directly to yield information on the fraction of nicotine available in the nnprotonated free-base form. It is important to characterize the fraction of total nicotine in its conjugate acid-base states as this dramatically affects nicotine bioavailability, because the protonated form is hydrophilic while the nnprotonated free-base form is lipophilic and thus readily diffuses across membranes (Armitage and Turner 1970 Schievelbein et al. 1973). As drug delivery rate and addiction potential are linked (Henningfield and Keenan 1993), increases in delivery rate due to increased free-base levels affect the addiction potential. 

The dicarbonyl [12539-66-1] available from 1,10-(N2)2B10H8 is another important species because of the scope ofits chemistry. Carbonyls of P 12 -12] can be formed from CO and the conjugate acid of [B12H12]2 . The B10- and B12-carbonyls exhibit very similar reactivity (99). The carbonyls can be considered anhydrides of carboxylic acids and accordingly react with alcohols and amines-to give esters and amides ... 

Most likely, the precursor for the formation of H202 in natural waters is the superoxide ion (02 -), which may have an even greater potential than H202 to affect geochemical and biological processes in the ecosystem. Therefore estimates of its lifetime and steady-state concentration are important. The aqueous chemistry of 02 was extensively reviewed by Bielski and coworkers (53). In aqueous solution 02 is in equilibrium with the conjugate acid ... 

The properties of O-, in contrast to these of the conjugated acid HO, are still poorly studied. The review by J. Lee and Grabowsky (1992) describes mass spectrometry of this radical. In conditions that are usual for synthetic organic chemistry, O- displays less reactivity than its acid OH. Oxidation of acetone by radicals O and OH leads to quite different products (Morkovnik Orhlobystin 1979) see combined Scheme 1-83 ... 

Just like sodium ions, chloride ions are spectator ions in acid-base chemistry. Their job is to provide a charge balance to the cations in solution. So, in calculating the pH of lidocaine hydrochloride we ignore the chloride ion. Now we could draw out the structure or write the molecular formula of lidocaine and its conjugate acid, but it is tedious to do so. Let s do what most chemists do, and postulate a temporary abbreviation for these species. How about using L for lidocaine, and HL+ for its conjugate acid Now, we can write an equation for the acid ionization equilibrium reaction. 

Common base classes and examples of bases used in organic chemistry are shown in Figure 3.1 along with their conjugate acids and associated pvalues. 

Conjugate add-base pairs Recall from Chapter 3 that the definition of adds and bases most useful to analytical chemistry is that of Bronsted, in which a conjugate acid-base pair is related by the reaction... 

Bromine addition to cyclopropanes is slow in the absence of light unless the ring is heavily substituted . In the presence of light, reaction is rapid since halogen radical attack on substituted cyclopropanes is fast even at — 78°C. In the presence of light, HBr is produced and cyclopropane reacts more competitively with proton acids than with bromine. Addition reactions to cyclopropane are generally slow in the absence of acid catalysts and therefore the conjugate acid is probably involved in reaction. The more facile reaction of cyclopropane with HBr compared with bromine is in contrast with alkene chemistry where bromine addition is the more rapid ... 

In pharmaceutical chemistry. pKh values arc not used to compare compounds that arc Lewis ba.ses. In.sleud, If a pK of an amine is given, it refers to its salt acting as the conjugate acid. Fur example, aniline with a pKj of 4.6 refers to... 

Control of pH is vital in synthetic and analytical chemistry, just as it is in living organisms. Procedures that work well at a pH of 5 may fail when the concentration of hydronium ion in the solution is raised tenfold to make the pH 4. Fortunately, it is possible to prepare buffer solutions that maintain the pH close to any desired value by the proper choice of a weak acid and the ratio of its concentration to that of its conjugate base. Let s see how to choose the best conjugate acid-base system and how to calculate the required acid-base ratio. 

The consequence of this replacement gives pyridine a reduced susceptibility to electrophilic substitution compared to benzene, while being more susceptible to nucleophilic attack. An avenue of chemistry not possible with benzene is the formation of pyridinium salts by donation of the nitrogen lone pair electrons. The resultant salts are still aromatic, however they are much more polarized. This is reflected by the apparent acidity of the corresponding conjugate acid (pK, 5.2) compared to the acidity of the corresponding conjugate acid of piperidine (pK, 11.1). 

In aprotic media, O2-- exhibits exceptional reactivity as a nucleophile towards aliphatic halogenated hydrocarbons and carbonyl carbons with adequate leading groups. Chapter 7 discusses the reaction chemistry of O2-- with electrophiles, reductants, transition metals, and radical species, and the chemistry of its conjugate acid (HOO) is outlined in Chapter 5. 

An estimate for the differences in basicity between the pyridyl and the benzimidazol-l-yl moieties were obtained by calculating pKa values for the conjugated acids of the two hydrogen-bond acceptors in each SR. 3-(benzimi-dazol-l-yl)methylpyridine pKa = 4.71 + 0.10 and 5.72 +0.30 for pyridyl and benzimidazol-l-yl moieties, respectively. 3-(2-methylbenzimidazol-l-yl)mefhyl-pyridine pKa = 4.72 +0.10 and 5.85 + 0.18 for pyridyl and benzimidazol-l-yl moieties, respectively. Calculations were performed using Advanced Chemistry Development (ACD/Labs) Software Solaris V4.76 ( 1994-2005 ACD/Labs). 

The Br0nsted-Lowry definitions of acids and bases are widely used in organic chemistry. As noted in the preceding equation, the conjugate acid of a substance is formed when it accepts a proton from a suitable donor. Conversely, the proton donor is converted to its conjugate base. A conjugate acid-base pair always differ by a single proton. 

---