Bronsted base nitrogen bases

Nucleophilicity. A distinction is usually made between nucleophilicity and Lowry-Bronsted basicity [213]. The latter involves specifically reaction at a proton which is complexed to a Lewis base (usually H2O), while the former refers to reactivity at centers other than H. Linear correlations have been shown for gas-phase basicity (proton affinity) and nucleophilicity of nitrogen bases toward CH3I in solution [214] where the solvent is not strongly involved in charge dispersal. In each case, reaction of the base/nucleophile... 

Aprotic protophillic solvents all nitrogen bases without protons, e.g., pyridine, and other conjugated amines, and fully alkylated amines or amides. The solvents function as Bronsted bases as well as Lewis bases and may have high permittivity. 

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

Most of the imprinted sorbents used in MISPE were prepared using MAA and ethylene glycol dimethacrylate (EDMA) as monomers. In these cases elution of more weakly bound analytes such as triazines [19,30,32], 7-hydroxy-coumarin (2) [26] or theophylline (12) [28] can be achieved using methanol or water as elution solvent. For more strongly bound analytes such as stronger nitrogen Bronsted bases, efficient elution has been achieved using eluents of the same base solvent but with the addition of small amounts of acids (e.g. acetic acid, trifluoroacetic acid (TEA)) or base (e.g. triethylamine (TEA)) [20-22,25,33]. 

Caffeine and theophylline have pharmaceutically important chemical properties. Both arc weak BrOnsted bases. The reported pK values are 0.8 and 0.6 fur caffeine and 0.7 for theophylline. These values represent the basicity of the imino nitrogen at position 9. As acids, caffeine has a pK., above 14. and theophylline, a pK., of 8.8. In theophylline, a proton can be donated from position 7 (i.e.. it can act as a Bronsted acid). Caffeine cannoi donate a proton from position 7 and does not act as a Bretnsted acid at pH values under 14. Caffeine docs have electrophilic sites at positions I. 3. and 7. In addition to its Bronsted acid site at 7. theophylline has electrophilic sites at I and 3. In condensed terms, both compounds arc electron-pair donors, but only theophylline is a proton donor in most pharmaceutical system.s. 

Whereas the Lewis acid-base theory does not contradict Bronsted theory, as "bases" in Bronsted theory must have a pair of nonbonding electrons in order to accept a proton, it expands the family of compounds that can be called "acids" any compound that has one or more empty valence-shell orbital and provides an explanation for the instantaneous reaction of boron triflouride (BF3) with ammonia (NH3). The nonbonding electrons on the nitrogen in NH3 are donated into an empty orbital on the boron to form a new covalent bond, as shown in Eq. 13. 

The acid-base chemistry of amino acids is more complicated than shown in Equations 16.48 and 16.49, however. Because the COOH group can act as an acid and the NH2 group can act as a base, amino acids undergo a self-contained Bronsted-Lowry acid-base reaction in which the proton of the carboxyl group is transferred to the basic nitrogen atom ... 

X-ray crystallographic data), a bicarbonate ion at the active site is shown in red, the zinc cation at the active site is green, a water molecule is shown in blue, and the basic sites that coordinate with the zinc cation (as Lewis bases) or remove the proton from water to form hydroxide (as Bronsted-Lowry bases) are magenta (these bases are nitrogen atoms from histidine imidazole rings). No hydrogen atoms are shown in any of these species. As you can see, a remarkable orchestration of Lewis and Bronsted-Lowry acid-base reactions is involved in catalysis by carbonic anhydrase. 

Table 16.5 lists a number of common weak bases and their ionization constants. Note that the basicity of all these compounds is attributable to the lone pair of electrons on the nitrogen atom. The ability of the lone pair to accept a ion makes these substances Bronsted bases. 

Whenever any metal salt and any Bronsted base (an anion or other molecule with electronegative atoms, such as nitrogen or oxygen, that has the capacity to donate electrons) come into contact, coordination is likely to occur to give a complex compound. For example, when solid nickel chloride (yellow) reacts with a stream of ammonia gas, it is converted to purple hexamminenickel(II) chloride ... 

PROBLEM 13.51 Pyridine and imidazole are modest Bronsted bases at nitrogen, whereas pyrrole is not. 

Acyl compounds and nitriles are Bronsted bases and Lewis bases as well, with the carbonyl oxygen or the nitrile nitrogen being the center of basicity (Fig. 18.16). 

Bronsted oti values for the addition of five nitrogen bases to benz-aldehydes are presented in the third column of Table... 

A Bronsted-Lowry acid-base reaction involves the transfer of a proton from the acid to the base. An ammonia molecule, with an unshared electron pair on the nitrogen, is the proton receiver-the base-in this reaction. See Section 17.2. 

Amines are organic bases whose acid—base chemistry is like that of ammonia. For example, methyl-amine behaves as a Bronsted base because the nonbonded electron pair of the nitrogen atom can accept a proton from an acid such as hydronium ion. When methylamine accepts a proton, the conjugate acid, methyl-ammonium ion, is produced. 

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