Basicity of ammonia

Suppose for example we wished to compare the basicities of ammonia and pyndme... 

Also the value reported by Haney and Franklin of 186 1 kcal/mol is in agreement. The latter workers determined the proton affinity of ammonia as 207 kcal/mol which is approximately 21 kcal/mol larger than that for phosphine. The greater basicity of ammonia as compared to phosphine is showi by the difference of about 20 pH units in their relative basicities in aqueous solutions. The difference in the basicities of the aqueous solutions of 23-32 kcal/mol, which is comparable to that in the gas phase, leads to the somewhat surprising conclusion that solvent effects appear to play an unessential part in the relative basicities of PH3 and NH3 in aqueous solutions. The proton affinities of HjO and H2S, 164 and 170 kcal/mol, respectively, are in the reverse order. 

In summary, the chemistry of ammonia solutions is remarkably parallel to that of aqueous solutions. The principal differences are in the increased basicity of ammonia and its reduced dielectric constant. The latter not only reduces the solubility of iotuc materials, it promotes the formation of ion pairs and ion clusters. Hence even strong acids, bases, and salts are highly associated. 

The amide 2 is paracetamol, the popular analgesic. Amines are much more nucleophilic than phenols (compare the basicities of ammonia and water) so reaction with acetic anhydride gives... 

Substitution by Alkyl Groups As an example, consider the relative basicities of ammonia and methylamine. Alkyl groups are electron-donating toward cations, and methylamine has a methyl group to help stabilize the positive charge on nitrogen. 

If we want to know the basicity of ammonia, we must look up the pKA of its conjugate acid, the ammonium cation, NH4, protonated ammonia. Its pK is 9.24 which means that ammonia is a weaker base than hydroxide—the pKA for water (the conjugate acid of hydroxide) is 15.74 (p. 190). Now we can summarize the states of ammonia at different pH values. 

Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. 

TABLE 28-2 Basicities of Ammonia and Some Amines in Water ... 

A major problem with Arrhenius s acid-base theory is that some substances, like ammonia, produce basic solutions and react with acids, but do not contain hydroxide ions. In 1923 Johannes Bronsted, a Danish chemist, and Thomas Lowry, an English chemist, independently proposed a new way to define acids and bases. An acid donates hydrogen ions (also called a proton donor) a base accepts hydrogen ions (also called a proton acceptor). These definitions not only explain all the acids and bases covered by Arrhenius s theory, they also explain the basicity of ammonia and ions such as carbonate, and phosphate, P04 ... 

The picture of acids and bases as hydrogen ion donors and acceptors explains not only the properties of the acids and bases we have described thus far, but also the basicity of ammonia (NH ) and ions such as the carbonate ion (GOg ). The neutral ammonia molecule can accept a hydrogen ion from a water molecule to produce a solution that is basic because it contains hydroxide ions. In this reaction, water donates the hydrogen ion and therefore is the acid. 

Table 6.9 relates the basicity of ammonia and some very simple allg lamines by tabulating the values of the related ammonium ions. 

The measure of acidity/basicity and discrimination between Lewis and Bronsted acidity is a delicate task This topic is the subject of other chapters of this series Molecular Sieves - Science and Technology (cf. Volume 4, Chapter 1, Volume 5, Chapter 2, Volume 6, Chapters 3-6), to which the reader is referred for detailed information. We limit to note that means for measuring the acidity and basicity of MTS are the same as with other systems titration, adsorption volumetry, gravimetry and microcalorimetry, temperature-programmed desorption (basically of ammonia), infrared and NMR spectroscopies and catalytic probe reactions. 

The relative order of basicities of ammonia and methylamines in aqueous solution is well-known to be... 

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