Moderately weak bases

Strong acid with a weak base. The titration of a strong acid with a moderately weak base (K sslO-5) may be illustrated by the neutralisation of dilute sulphuric acid by dilute ammonia solution [curves 1 and 3, Fig. 13.2(a)]. The first branch of the graph reflects the disappearance of the hydrogen ions during the neutralisation, but after the end point has been reached the graph becomes almost horizontal, since the excess aqueous ammonia is not appreciably ionised in the presence of ammonium sulphate. 

The solution has a very weak acid (phenol) and a moderately weak base (phenolate ion). The weak base is stronger hian hie weak acid, so hie solution is basic. 

Very little accurate e.m.f. work has been done on the dissociation constants of bases, chiefly because moderately weak bases are very volatile, while the non-volatile bases, e.g., anilines, are usually very weak. An exception to this generalization is to be found in the aliphatic amino-acids which will be considered in connection with the subject of ampho-... 

We can see why the two classes of compounds differ as they do. The ease with which W is lost depends upon its basicity the weaker the base, the better the leaving group. For acid chlorides, acid anhydrides, esters, and amides, W is, respectively the very weak base Cl the moderately weak base RCOO and the strong bases R 0 and NH2 . But for an aldehyde or ketone to undergo substitution, the leaving group would have to be hydride ion ( H ) or alkide ion... 

This is the salt of a moderately weak base, 2-terbutylaniline and the strong acid HCl. The p Tb for 2-terbutylaniline is 14.00 — 3.80 = 10.20. The salt is composed of 2-terbutyl anilinium ions, NH3CgH4(C4H9) ions and CP ions. The cation is an acid which will donate a proton to water ... 

The parent compound is a moderately weak base (pK 3.59), comparable with substituted aminopyrazines. The 2,3-dihydro compound 5, however, is very much stronger (pK 9.32). The base-weakening effect of the nitrogen in the 7-position is clearly demonstrated by comparison with the dihydroimidazopyridine (7). The latter compound has a pKa of 12.51 units. Similar effects are found in the imidazopyridazine and imidazo-pyrimidine series. No evidence has been found for covalent hydration in any of the triazaindenes studied. ... 

LCAO calculations have been performed on this heterocycle, and the resulting electron density map suggests that the 4-position is the most susceptible to nucleophilic substitution. The ultraviolet spectrum in methanol of imidazo[l,2-a]quinoxaline has a maximum at 315 nm and a minimum at 266 nm. The compound is a moderately weak base with a pKa (50% ethanol) of 4.6. No evidence for the site of protonation has been published, although the 3-position seems favored both from the theoretical calculations and by comparison with imidazo[l,2-a]pyrazines (see Chapter XXI). 

A primary B and C secondary and D tertiary bromoalkanes, reacting with (a) a good nucleophile that is a moderately weak base (b) a strong, bulky base (c) a good nucleophile that is a moderately strong, nonbulky base (d) like (a), a good but not particularly strongly basic nucleophile and (e) a weak and essentially nonbasic nucleophile. 

If homolytic reaction conditions (heat and nonpolar solvents) can be avoided and if the reaction is conducted in the presence of a weak base, lead tetraacetate is an efficient oxidant for the conversion of primary and secondary alcohols to aldehydes and ketones. The yield of product is in many cases better than that obtained by oxidation with chromium trioxide. The reaction in pyridine is moderately slow the intial red pyridine complex turns to a yellow solution as the reaction progresses, the color change thus serving as an indicator. The method is surprisingly mild and free of side reactions. Thus 17a-ethinyl-17jS-hydroxy steroids are not attacked and 5a-hydroxy-3-ket-ones are not dehydrated. 

The pH of a buffer does change slightly on addition of moderate amounts of a strong acid or strong base. Addition of H+ ions converts an equal amount of weak base B- to its conjugate acid HB ... 

Ti(OPr1)4-mediated nucleophilic ring opening of 2,3-epoxy-alcohol with primary amine requires more rigorous conditions, and the product is a complex mixture. Lin and Zeng22 found that this problem could be overcome and moderate to good yields could be obtained under weak base conditions by in situ /V-acylation of the aminolysis product with benzoyl chloride. 

Initial attempts to spread the amide as a monolayer film on aqueous subphases failed. Since patches of microcrystals could be observed by careful inspection, it was concluded that the aggregation forces in the sohd phase were too great to permit significant expansion of the molecules onto the adjacent surface of the air-water interface. It is well known that amides are weak bases that are protonated on the carbonyl oxygen by moderately strong aqueous acid. Accordingly, a series of subphases prepared from carefully purified sulfuric acid-... 

N-acylamino acids does not appear to have been studied previously. In solution, N-acyl-N-nitroso-a-amino acids are moderately stable to weak bases, such as triethylamine or sodium carbonate, but are decomposed rapidly at 0° to expel nitrogen on addition of sodium hydroxide (43). For example, treatment of N-nitroso-N-benzoyl-D,L-pheny1-alanine LXIIb with an aqueous sodium hydroxide solution at 0 gives benzoic acid and 1-hydroxy-3-phenylpropanoic acid LXVa in a 93% yield. The facile base-catalysed formations of a-hydroxy acids LXV are a general reaction and probably occur by intramolecular attack as shown in LXII->LXIII->LXIV. Oxadiazolone LXIV can decompose by various possible pathways to give LXV among which the carbonium ion pathway is least likely. 

The oxygen as heteroatom in ethers or carbonyl compounds is weak to moderate Lewis base. Nevertheless, a highly reactive metal carbene complex can interact with the oxygen to generate oxygen ylide. The interaction between ether and metal carbene functional groups is believed to be rather weak as demonstrated by the facts that other metal carbene reactions, such as G-H insertion and cyclopropanation, can proceed in ethereal solvents." These experiments demonstrate that the formation of the metal ylide is much less favored in the equilibrium shown in Equation (1). ... 

The experiments with reversible poisoning of alumina by small amounts of bases like ammonia, pyridine or piperidine revealed [8,137,142,145, 146] relatively small decreases of dehydration activity, in contrast to isomerisation activity which was fully supressed. It was concluded that the dehydration requires only moderately strong acidic sites on which weak bases are not adsorbed, and that, therefore, Lewis-type sites do not play an important role with alumina. However, pyridine stops the dehydration of tert-butanol on silica—alumina [8]. Later, poisoning experiments with acetic acid [143] and tetracyanoethylene [8] have shown the importance of basic sites for ether formation, but, surprisingly, the formation of olefins was unaffected. 

The polymerization of an NCA may be initiated by any moderately strong base or by nucleophiles. Weak nucleophiles, such as water, alcohols, or primary amines, generally initiate polymerization by the normal (nucleophilic) mechanism or by the carbamate mechanism. Tertiary amines and strong bases, such as methoxide, initiate polymerization by the active monomer mechanism. Secondary or primary amines may initiate polymerization by any one or all of these mechanisms. More than one mechanism may be active at any one time and frequently a polymerization may begin by the active monomer mechanism and then, at a later stage, propagate by the normal mechanism. 16 ... 

Derivatives of alkaline, alkaline earth metals and phenols, naphtols, an-troles, etc. (pK> 10). These compounds are salts in their nature and, as the salts of strong bases and moderately weak acids, can exist in water solutions. They are soluble only in polar solvents (water, liquid ammonia), are prone to form adducts with phenols, water, etc., have high thermal stability and cannot be transferred into the gas phase. 

COSMO models [27-30] were compared with the SM approach [22] by Klamt [43] and by Cramer and Truhlar [44]. A very recent paper by Klamt and coworkers [45] shows that improved calculated pKa values are obtained for the limited domain of strong to moderately weak acids by a cluster-continuum method in which the acid and conjugate base are each associated with one or a few solvent molecules and this cluster is then continuum-calculated with COSMO-RS. The authors point out, however, that for the calculation of pKa a consistent and generally applicable method is still lacking . This paper clarifies the problem raised in [41]. The matter is under study.1... 

---