Basicity constants amines

As we discussed in Section 1 14 it is more useful to describe the basicity of amines m terms of the pA a s of their conjugate acids than as basicity constants Always bear m mind that... 

Basicity constant Ki, (Section 1 14) A measure of base strength especially of amines... 

Various amines find application for pH control. The most commonly used are ammonia, morpholine, cyclohexylamine, and, more recently AMP (2-amino-2-methyl-l-propanol). The amount of each needed to produce a given pH depends upon the basicity constant, and values of this are given in Table 17.4. The volatility also influences their utility and their selection for any particular application. Like other substances, amines tend towards equilibrium concentrations in each phase of the steam/water mixture, the equilibrium being temperature dependent. Values of the distribution coefficient, Kp, are also given in Table 17.4. These factors need to be taken into account when estimating the pH attainable at any given point in a circuit so as to provide appropriate protection for each location. 

The base properties of the amine are represented by its basicity constant (basicity dissociation constant, Kh), which identifies the amount of the amine (in moles) that is ionized (i.e., available to raise pH) in liquid water-condensate at any given temperature and pressure. The dissociation reaction for a primary amine is shown in equation 1, and the value of the dissociation constant is shown in equation 2. 

Basicity also may be regarded as a measure of amine hydrolysis, and it governs the increase in pH level, per additional unit measure of amine. The law of diminishing returns applies of course, and beyond a certain point there is little further increase in system pH. Also, the basicity constant for individual amines varies considerably with temperature, so it is important to consider Kb at the relevant operating temperature, rather than simply at ambient temperature. 

The final pH, after neutralization, is determined by the remaining amine hydrolysis (basicity constant). 

In their studies of the effect of solvent upon the N—H stretching frequency in pyrrole, Fuson and Josien [1] have shown the distinction between the solvent-solute interaction which is a function of dielectric constant alone [2, 3] and that which is more specific, involving N—H hydrogen bonding. The most pronounced frequency shifts are those caused by pyridine [4] (K—M N bonding) and by acetone (N—H 0 bonding). The choice of pyrrole for these studies was presumably partly governed by convenience since the N—H band in pyrrole is considerably more intense than in the more basic secondary amines. We have attempted an extension of this work in two directions ... 

As discussed in Section 3.10.3, in the gas phase the basicity of simple amines follows the order NMe3 > NHMe2 > NH2Me > NH3 because of the electron donating effect of the methyl (Me) groups. In solution, however, we can define a basicity constant as the equilibrium constant for the reaction shown in Equation 3.4. Note it is important to specify temperature, solvent (usually water) and solution ionic strength, 1 Basicity constants are related to the acid dissociation constants (/Q of the base s conjugate acid via the dissociation constant of water, K = 10 14 at 25 °C. Thus Kbx K = Kw. 

A considerable amount of data on the protonation, and complex formation with metal ions of polymeric amines have been reported. A critical insight leads to the conclusion that much has to be done in order to reach a clear vision of the chemical properties of many polymers of this kind. Most protonation studies deal with the determination of basicity constants with potentiometric techniques, which alone give little information on the protonation mechanism only few studies have been substantiated by spectroscopic (nmr) and calorimetric measurements. 

Because amines are fairly strong bases, their aqueous solutions are basic. An amine can abstract a proton from water, giving an ammonium ion and a hydroxide ion. The equilibrium constant for this reaction is called the base-dissociation constant for the amine, symbolized by Kb. 

The above Kb for the equilibrium amine + H+ ammonium ion, bears a simple relation to the more customary but less general basicity constant Kb of the equilibrium amine -f- H20 ammonium ion -f- OH-, according to the expression Kh = Kb. [H20]/Kw = 55.5 io14 Kh while Kb" = 55.5 Kb is always quoted. This has no influence on the above ratio. 

The basicity constant is the measure of the equilibrium of an amine with water. 

Reactions of cationic carbonyl complexes with alcohols are less common than with amines ( 5.8.2.12.4). They occur only with the most reactive carbonyl complexes with high C—O force constants this is because alcohols are poorer nucleophiles and also less basic than amines. Metal carbonyls react with alkoxide ions. See Table 1 for complexes of the Mn, Fe and Co subgroups ". ... 

We found it convenient to compare acidities of carboxylic acids by measuring the extent to which they give up hydrogen ion to water the equilibrium constant for this reaction was called the acidity constant, Ka. In the same way, it is convenient to compare basicities of amines by measuring the extent to which they accept hydrogen ion from water the equilibrium constant for this reaction is called a basicity constant, K. ... 

Amines are considerably more basic than alcohols, ethers, or water. When an amine is dissolved in water, an equilibrium is established in which water acts as an acid and transfers a proton to the amine. Just as the acid strength of a carboxylic acid can be measured by defining an acidity constant Ka (Section 2.8), the base strength of an amine can be measured by defining an analogous basicity constant K. The larger the value of ifb (and the smaller the value of piTj,), the more favorable the proton-transfer equilibrium and the stronger the base. 

B. Expressing Relative Basicities of Amines The Basicity Constant... 

Relative basicities are expressed using the basicity constant, Kb, which is defined as the concentrations of the ionized amine products in water (ammonium salt and hydroxide) divided by the concentration of the un-ionized amine. Larger Kb s mean greater basicity. pKb is the negative logarithm of Kb the smaller the pKb the stronger the base. 

Two conventions are used to measure the basicity of amines. One of them defines a basicity constant Ky, for the amine acting as a proton acceptor from water ... 

Base Strength of Amines As Measured by Their Basicity Constants and the Dissociation Constants of Their Conjugate Acids ... 

Section 22.4 Basicity of amines is expressed either as a basicity constant K, (p/fb) of the amine or as a dissociation constant (pA a) of its conjugate acid. 

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