Methylamine base ionization

In contrast to strong bases, a weak base ionizes only partially in dilute aqueous solution to form the conjugate acid of the base and hydroxide ion. The weak base methylamine (CH3NH2) reacts with water to produce an equilibrium mixture of CH3NH2 molecules, CH3NH3+ ions, and OH ions. 

Base ionization constants You won t be surprised to leam that like weak acids, weak bases also form equihbrium mixtures of molecules and ions in aqueous soluhon. Therefore, the equilibrium constant provides a measure of the extent of the base s ionization. The equilibrium constant for the ionization of methylamine in water is defined by this equilibrium constant expression. 

Table 17.2 lists ionization constants for some weak bases. All of these bases are similar to ammonia in that they have an N atom with a lone pair of electrons that accepts an H ion from water. For example, the base ionization of methylamine, CH3NH2, is... 

Write the chemical equation for the base ionization of methylamine, CH3NH2. Write the Ki, expression for methylamine. 

As we did with the acid dissociation expression, the concentration of water is omitted from the equilibrium expression, which gives the base dissociation constant, Ky, (or base ionization constant). Thus, for a weak base such as methylamine, the Ky, is written... 

Some commonly used buffers, such as sodium and potassium phosphate, are incompatible with ELSD, but there are ready alternatives. For example, ammonium acetate has similar buffering properties to potassium phosphate, and ammonium carbonate, ammonium formate, pyridinium acetate, and pyridinium formate are options for different pH ranges. Typical mobile phase modifiers that do not meet the volatility criteria can be replaced by a wide variety of more volatile alternates. For example, phosphoric acid, commonly used as an acid modifier fo control pH and ionization, can be replaced by trifluoroacetic acid other acids that are sufficiently volatile for use with FLSD include, acetic, carbonic, and formic acids. Triethylamine, commonly used as a base modifier, is compatible with FLSD other base modifiers that can be used are ethylamine, methylamine, and ammonium hydroxide [78]. 

The carboxyl group occupies a prominent place in protein structure and function and is commonly taken to be ionized as a carboxylate, particularly when paired in a salt bridge with a Whereas this may be the case in a protein environment, it is not obvious that a —COOH group will donate its proton to a base to form an ion pair in the gas phase. For example, SCF calculations with a polarized 6-31G basis set indicate that neither methy-lamine nor an arginine model is able to extract a proton from acetic acid so as to form an ion pair. On the other hand, there is some indication that correlation might stabilize the ion pair for formic acid -f methylamine. ... 

Dimethylaminomethylenamino-l-methyl-5-carbonitrile (89), obtained by the action of dmethylformamide and phosphoryl chloride on 4-amino-l-methyltriazole-5-cartoxamide, furnished 6-amino-7-methyl-8-azapurine (87% yield) when refluxed with aqueous ammonium acetate (4 equiv) for 10 min. Ammonium chloride could not replace the acetate. The 2-methyl isomer of 89 reacted similarly but the 3-methyl isomer (a much weaker base) reacted only slightly lowering the pH to increase its ionization only accelerated hydrolysis of the amidino group. Methylamine acetate solution gave, with the nitrile 89,7-methyl-6-methylamino-8-azapurine (81% yield), and butylamine acetate acted similarly (75% yield). ... 

The commercial preparation of ammonia is accomplished in huge quantities by the Haber process [Equation (16.12)], discussed in Section 16.4 under Nitrogen Fixation. In the laboratory the most common preparation is the treatment of ammonium salts with strong bases, as represented in Equation (16.13).The self-ionization of liquid ammonia (analogous to that of liquid water) is shown in Equation (16.14). NH3(/) is often used as a nonaqueous solvent. As shown in Equation (16.15), NH3 acts as a weak base in water and serves as a prototype for a number of nitrogen-containing bases such as methylamine, pyridine, and aniline, which you may recall from studying acid-base equilibria in earlier courses. 

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