Amine acid-base properties

In the polyazamacrocyclic phenols [64], an acidic function, the phenol group, has an intra-annular orientation while the basic units, the nitrogen atoms of the macrocycles, have no defined orientation. In water at 25°C, the pATa values of the phenols were measured and compared with those of other macrocyclic and non-macrocyclic phenols (Kimura et al., 1987a,b). Because the nitrogen atoms and the phenol function both possess acid-base properties, more than one pXg value could be measured. By the use of UV measurements, the values of the phenol group could be distinguished from those of the amines. 

The structures of the amino acids reveal any functional groups that result In acidic or basic properties. To determine the acid-base properties of these compounds, look for carboxyl groups and amine groups. 

Since the discovery of the exceptional basicity of l,8-bis(dimethylamino)-naphthalene (Alder et al., 1968) and its unusual kinetic behaviour (Hibbert, 1973, 1974, 1975) there has been considerable interest in the acid-base properties of hindered diaminonaphthalenes. 1,8-Bis(dimethylamino)naph-thalene (pK — 12.1 at 25°C and ionic strength 0.1 mol dm-3) (Alder et al., 1968 Hibbert, 1974 Chiang et al., 1980) is more basic than most aliphatic amines and the pAT-values of the partially methylated diamines [52] illustrate the dramatic effect of introducing the fourth methyl group (Alder et al., 1968). Reaction of protonated l,8-bis(dimethylamino)naphthalene with... 

This is a good time to have another brief look at Sections 4.3.5 and 4.5.4, and compare how we used similar reasoning to consider the likely stability, or otherwise, of anions and cations in order to predict the acid-base properties of aromatic amines... 

All these electrolytes are neutral in Bronsted acid-base properties. Although rather exceptional, an acid, a base, or a pH buffer may be added to the supporting electrolyte of neutral salts. The acid-base system to be selected depends on the purpose of the measurement. We often use trifluoromethanesulfonic acid (CF3S03F1) as a strong acid acetic acid, benzoic acid, or phenol as a weak acid an amine or pyridine as a weak base and tetraalkylammonium hydroxide (ILtNOH) as a strong base. Examples of buffer systems are the mixtures of picric acid and its R4N-salt and amines and their PlCl04-salts. Here, we should note that the acid-base reactions in aprotic solvents considerably differ from those in water, as discussed in Chapter 3. 

Although very few detailed studies have been carried out, the acid-base properties of polyheteroatom six-membered rings are fairly predictable. The fully reduced systems behave as typical secondary amines, although the inclusion of extra heteroatoms often reduces their basicity. Thus the reduced dithiazines (68) (thialdine) and (69), and the related thiadiazine (70), are weak bases with pKz values of 2.7, 2.8 and 2.0 respectively (58JCS2893). The weak basicity is due to the inductive effect of the /3-heteroatoms. This inductive effect of /3-heteroatoms is also apparent in the dioxazine (71), which exhibits no tendency to... 

For many molecules, due to extensive redistribution of electron densities, acid-base property in the excited state differs considerably from that in the ground state [33 For instance, aromatic amines are weakly basic in the ground state. But many of them become acidic in the excited state and readily donate a proton to a proton acceptor to produce the anion in the excited state. Such a molecule, which behaves as an acid in the excited state, is called a photoacid similarly, photobases are those that display basic properties in the excited state. In many cases, excited state proton transfer (ESPT) results in dual emission bands. One of these emission bands arises om the neutral excited state and bears mirror image relation with the absorption spectrum. The other emission band is due to the excited deprotonated (anion) or protonated species and exhibits a large Stokes shift. 

No detailed study of the acid-base properties of these six-membered heterocycles has been carried out, but the behavior is fairly predictable. Fully reduced systems behave as typical secondary amines... 

Formation of the tetrahedral intermediate carbinolamine and subsequent elimination of water are amenable to acid-base catalysis and do not require a metal surface. The relative rates of adduct formation and subsequent dehydration to imine or enamine depend on the structure of alcohol and amine, and on the nature and strength of acidic and basic sites on the catalyst surface. It must be stressed that several side-reactions (e. g. dimerization and oligomerization, dehydration) are also acid or base-catalyzed, and good selectivity for the desired product requires proper tuning of the redox and acid-base properties of the catalyst. This is crucial in catalyst development when choosing a suitable support, additive, or modifier. Even traces of impurities remaining on the surface from the catalyst precursor can strongly influence product distribution [10]. 

More About the Acid-Base Properties of Amines... 

The interaction of R/,NH3-g (primary, secondary, and tertiary amine) with HA is determined mainly by the relative acid-base properties of the components. Amines, markedly more basic than A (the anion of the water-immiscible acid), are expected to form an ion pair (R/jNH/ -p A ). For A" that is much more basic than R NH -p, H bonding is the main interaction. 

Acid-Base Titrations in Nonaqueous Solvents. It is a fact that the apparent acidity or basicity of a compound is strongly dependent on the acid-base properties of the solvent. For example, very strong acids such as HCl and HNO3 cannot be individually titrated in water because water is sufficiently basic that these acids appear to be totally ionized. Very weak bases, such as amines, cannot be successfully titrated with strong acid in water. Many acids or bases that are too weak for titration in an aqueous medium, however, become amenable to titration in appropriate nonaqueous solvents. As a consequence, there are now many neutralization methods that call for solvents other than water [23-25]. 

The earliest advantages recognized arose from the use of amphiprotic solvents, those that have both acidic and basic properties. The prototype is water. Significant differences in acid-base properties are seen in the case of either protogenic solvents (more acidic than water), for example acetic acid, or protophilic solvents (more basic than water), for example ethylenediamine. In the protogenic cases it was found that bases too weak to be titrated in water could be successfully titrated with a strong acid dissolved in the same solvent. For example, primary, secondary, and tertiary amines can be titrated in acetic acid with perchloric acid in acetic add as titrant. Medicinal sulfonamides, which have a primary amino group, can be titrated... 

Physical, Spectroscopic, and Acid-Base Properties of Amines Qualitative and quantitative characteristics of the functional group. 

The acid-base properties of amines are an extension of what you know about ammonia They are weaker acids and stronger bases than are water or alcohols. It will repay you many times over, however, to go over the information involving pAS/s of these molecules. The qualitative ability to handle acid-base concepts is one of the more useful capabilities you can take out of a course in organic chemistry. 

Polyacrylates as binders consist of copolymers of acrylate and methacrylate esters. Other unsaturated monomers (e.g., styrene and vinyltoluene) may also be incorporated, but usually to a lesser extent. Copolymers formed exclusively from acrylates and/or methacrylates are termed straight acrylics. The comonomers differ as regards the alcohol residues of the ester group, which also allow incorporation of additional functional groups. Choice of suitable monomers allows wide variation of the physical and chemical properties of the resulting polymer. Hydrophilicity, hydrophobic-ity, acid base properties as well as can be adjusted resins containing hydroxyl, amine, epoxy, or isocyanate groups can also be produced. 

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