A pka values

These above equations suggest that amines with pKa In the range of 4-6 will be more rapidly nltrosated than those with pKa values In the range of 9-11. This has been borne out In practice many times. Amines such as N-methylanlllne with a pKa value of 4.84, piperazine (pKa value 5.9, 9.8), and amlno-pyrlne (pKa value 5.04) are much more rapidly nltrosated than plperdlne (pKa value 11.2), dlmethylamlne (pKa value 10.72), and pyrrolidine (pKa value 11.27). Under many of the conditions studied. It has been shown that the reactivity of ascorbic acid Is sufficiently rapid that It can successfully compete with most all amines when present In approximately 2 mole ratio excess of the nltrosatlng agent. 

In the case of the rhenium aqua-ion [Re(OH2)3(CO)3]+ (33b) the question has been posed whether complex-anion can be considered to be a Bronsted acid. Titrations with hydroxide in water yielded a pKa value of 7.55 which is exceptionally low for a +1 cation. After the deprotonation of one coordinated water molecule, polymer formation over (/r-OH) bridges was initiated and the two compounds [Re3(/T3-OH)(/T-OH)3(CO)9r (35) and [Re2(/i-OH)3(CO)6] were (36) isolated and structurally characterized (Scheme 6). 

Hardly surprisingly the tendency of alkanes to lose proton and form carbanions is not marked, as they possess no structural features that either promote acidity in their H atoms, or are calculated to stabilise the carbanion with respect to the undissociated alkane (cf. carboxylic acids, p. 55). Thus CH4 has been estimated to have a pKa value of 43, compared with 4.76 for MeC02H. The usual methods for determining pKa do not, of course, work so far down the acidity scale as this, and these estimates have been obtained from measurements on the iodide/organo-metallic equilibria ... 

The operation of (d) is seen in cyclopentadiene (14) which is found to have a pKa value of 16 compared with 37 for a simple alkene. This is due to the resultant carbanion, the cyclopentadienyl anion (15), being a 6n electron delocalised system, i.e. a 4n + 2 Hiickel system where n = 1 (cf. p. 18). The 6 electrons can be accommodated in three stabilised n molecular orbitals, like benzene, and the anion thus shows quasi-aromatic stabilisation it is stabilised by aromatisation ... 

The acidity of 30 has been measured with reference to Streitwieser s acidity scale (87) a pKA value of 15.8 for 30 as compared to 22.8 for fluorene and 18.5 for 9-phenylfluorene demonstrates a significant increase in acidity (87). 

A number of carboxylic acids other than acetic were investigated as solvents or promoters. All of these acids which were stable to reaction conditions were found to be effective in promoting glycol ester production (e.g., propionic, pivalic, benzoic, etc.). However, other Br nsted acids of non-carboxylic nature were not found to be effective promoters. Thus penta-chlorophenol, although it has a pKa value (4.82) very close to that of acetic acid (4.76), is not a comparable promoter (Table I, reaction 13). Likewise, phosphoric acid (pK 2.15) is not an effective solvent or co-solvent with acetic acid (Table I, reaction 8). Experiments with lower concentrations of these acids in sulfolane solvent also showed that carboxylic acids are unique in promoting glycol formation. The promoter function of carboxylic acids thus appears not to be dependent (only) upon their acidity, but on some other chemical or structural property. 

Kuznesoff and Jolly56 have reported a pKa value of 3.5 for the acid GeHsCC H, 12, in water. We have correlated the pKa values of XCO2H in water with the LDR equation to obtain the relationship ... 

Sodium valproate exhibits basic properties. Titration of an aqueous solution of sodium valproate with aqueous hydrochloric acid gave a pKa value of... 

Titration of valproic acid with aqueous sodium hydroxide using acetone-water as the sample solvent and extrapolated to pure water gave a pKa value of 4.6 (proton lost). 

Titration experiments on the nitrile-terminated dendrimers in water show for DAB-dendr-(CN)A pKa values of 3.2 and 4.8. The corresponding calculated pKa values are 3.1 and 4.1 respectively (using the pKalc program, version 2.0, Com-pudrug chemistry). For DAB-dendr-(CN)s only the two inner nitrogen atoms can be protonated in acetonitrile, due to the low basicity of the four other ones. This is confirmed with calculated pKa values of the four outer tertiary nitrogen atoms in DAB-dendr-(CN)s, ranging from 2.0 to 3.2. The presence of the electropositive nitrile-functions and the protonated inner tertiary amines can account for this phenomenon. 

From this equation it appears that the effective parent acid is at a pKa value of 3.92. The regression coefficients of both the inductive effect and resonance effect terms indicate... 

Seijeant235 used values of 3.08 and 4.77 respectively for the pK 1 and pK2 values of m-aminobenzoic acid in water at 25 °C and derived a pKa value for the neutral acid of 4.20. He concluded that the value of am for NH2 is effectively 0.00. Seijeant also considered p-aminobenzoic acid and derived a value for ap of NH2 equal to —0.57. 

The values of perhydropyrido[l,2-f)]pyridazine and its 2-oxo derivative were found to be 2.80 0.04 and 7.32 0.03, respectively (72KGS220), whereas that of anhydro 4-hydroxy-2-methyl-5,6,7,8-tetrahydropyrido(l,2-6]pyridazinium hydroxide (16) was determined by spectrophotometry to be 2.77 (71CPB159). UV spectroscopic measurements in sulfuric acid gave a pKa value of -0.25 for pyrido[l,2-6]cinnoline derivative (17, R = H) (74JHC125). 

Streptonegrin (negrin, 5-amino-6-[7-amino-5,8-dihydro-6-methoxy-5,8-dioxo-2-quinolinyl]-4-[2-hydroxy-3,4-dimethoxyphenyl]-3-methyl-2-pyridinecarboxylic acid) [3930-19-6] M 506.5, m 262-263°, 275°(dec). Purified by TLC on pH 7-buffered silica gel (made from a slurry of Silica Gel 60 and 4(X)ml of 0.05M phosphate buffer pH 7.0) and eluted with 5% MeOH/CHClj. The extracted band can then be recrystd from Me2CO or dioxane as almost black plates or needles. It is soluble in pyridine, Me2NCHO, aqueous NaHCOy (some dec), and slightly soluble in MeOH, EtOH, EtOAc and H2O. It has a pKa value in the range 6.2-6.4 (dioxane/H2O 1 1) and UV 248, 375-380nm (e 38400 and 17400). [Weinreb et al. JACS 104 536 1982 Rao et al. JACS 85 2532 1963]. It is an antineoplastic and causes severe bone marrow depression [Wilson et al. Antibiot Chemother 11 147 7967]. 

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