Bases, weak

Weak bases also react with water to establish an equilibrium system. Ammonia is a t3q ical weak base. It reacts with water to form the ammonium ion and the hydroxide ion  

Like a weak acid, a weak base is only partially ionized. There s a modified equilibrium constant expression for weak bases — the K. You use it exactly the same way you use the K (see Weak acids for the details) except you solve for the [Off]. 

Gil/e me that proton Bronsted-LoWer acid-base reactions 

With the Arrhenius theory, acid-base reactions are neutralization reactions. With the Bronsted-Lowery theory, acid-base reactions are a competition for a proton. For example, take a look at the reaction of ammonia with water  

Ammonia is a base (it accepts the proton), and water is an acid (it donates the proton) in the forward (left to right) reaction. But in the reverse reaction (right to left), the ammonium ion is an acid and the hydroxide ion is a base. 

Some strong bases, such as Sr(OH)2, contain two OH ions. These bases completely dissociate, producing two moles of OH per mole of base. For example, Sr(OH)2 dissociates as follows  

A weak base is analogous to a weak acid. Unlike strong bases that contain OH and dissociate in water, the most common weak bases produce OH by accepting a proton from water, ionizing water to form OH . 

In this equation, B is generic for a weak base. Ammonia, for example, ionizes water according to the reaction  

Many substances behave as weak bases in water. Weak bases react with water, abstracting protons from H2O, thereby forming the conjugate acid of the base and OH ions  

The equilibrium-constant expression for this reaction can be written as 

Water is the solvent, so it is omitted from the equilibrium-constant expression. The most commonly encountered weak base is ammonia  

As with K,y and K , the subscript b in Ky denotes that the equilibrium constant refers to a particular type of reaction, namely the ionization of a weak base in water. The constant Ky, the base-dissociation constant, always refers to the equilibrium in which a base reacts with H2O to form the corresponding conjugate acid and OH.  

T TABLE 16.4 lists the Lewis structures, conjugate acids, and Ky values for a number of weak bases in water. Appendix D includes a more extensive list. These bases contain one or more lone pairs of electrons because a lone pair is necessary to form the bond with H. Notice that in the neutral molecules in Table 16.4, the lone pairs are on nitrogen atoms. The other bases listed are anions derived from weak acids. 

Analyze We are given the concentration of a weak base and asked to determine the concentration of OH . 

The extent of ionization of a weak base is quantified with the base ionization constant, K ). For the general reaction in which a weak base ionizes water, we define as follows  

By analogy with K, the smaller the value of A t, the weaker the base. Table 15.8 lists some common weak bases, their ionization reactions, and values for The p scale can also be applied to so that pA ), = -log Ky,. 

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The sodium ethanoate which is largely dissociated, serves as a source of ethanoate ions, which combine with any hydrogen ions which may be added to the solution to yield more of the acid. The addition of hydrogen ions has therefore much less effect on such a solution than it would have on water. In a similar manner, the solution of the salt of a strong acid and a weak base, in the presence of a weak base, has a pH that is insensitive to additions of alkali. 

C7H9N402- M.p. 337 C, an alkaloid obtained from cacao seeds or prepared synthetically. Constitutionally it is similar to caffeine, and is also a weak base. It is usually administered as the sodium compound combined with either sodium ethanoate or sodium salicylate, and is employed almost entirely as a diuretic. Physiologically theobromine resembles caffeine, but its effect on the central nervous system is less, while its action on the kidneys, is more pronounced. 

C7HgN402. Occurs to a small extent in tea, but is chiefly prepared synthetically. Like caffeine, it is a very weak base which forms water-soluble compounds with alkalis. It has a similar pharmacological mechanism to that of caffeine and is used, in combination with ethy-lenediamine. as a diuretic and a bron-chodilator. 

The hydroxide of lithium, although soluble in water, is a weak base owing to the great attraction between the Li" and OH ions (p. 74) the hydroxide of beryllium is really a neutral, insoluble... 

Aqueous ammonia can also behave as a weak base giving hydroxide ions in solution. However, addition of aqueous ammonia to a solution of a cation which normally forms an insoluble hydroxide may not always precipitate the latter, because (a) the ammonia may form a complex ammine with the cation and (b) because the concentration of hydroxide ions available in aqueous ammonia may be insufficient to exceed the solubility product of the cation hydroxide. Effects (a) and (b) may operate simultaneously. The hydroxyl ion concentration of aqueous ammonia can be further reduced by the addition of ammonium chloride hence this mixture can be used to precipitate the hydroxides of, for example, aluminium and chrom-ium(III) but not nickel(II) or cobalt(II). 

Conversion of the salt of a weak base into the free base e.g.y if... 

Conversion of the salt of a weak base into the free base. Prepare a column of a strong base anion resin (such as Amberlite IRA-40o(OH) ) washed with distilled water as above. Drain off most of the water and then allow 100 ml. of A//2.Na.2C03 solution to pass through the column at 5 ml. per minute. Again wash the column with 200 ml. of distilled water. Dissolve 0-05 g. of aniline hydrochloride in 100 ml. of distilled water and pass the solution down the column. The effluent contains aniline in solution and free from all other ions. 

Aminoazobenzene is a very weak base, and consequently it will not form salts with weak organic acids, such as acetic acid, although it will do so with the strong mineral acids, such as hydrochloric acid. Aminoazobenzene is a yellowish-brown compound, whilst the hydrochloride is steel blue. The colour of the latter is presumably due to the addition of the proton to the phenyl-N-atom, the cation thus having benzenoid and quinonoid forms ... 

On the other hand, the two nitro groups make 2,4-dinitrophenylhydrazine a very weak base, and it has therefore to be used in reasonably concentrated acid solution. 

Aniline hydrochloride, being a salt formed from a very weak base and a strong acid, undergoes considerable hydrolysis in aqueous solution to aniline... 

The amines are comparatively weak bases, so that a certain amount of free amine will be produced by salt hydrolysis unless an excess of acid is present. The reaction mixture must be kept very cold during the process (which is exothermic in character), otherwise the diazonium salt may be partially converted into the corresponding hydroxy compound ... 

Determination of the dissociation constants of acids and bases from the change of absorption spectra with pH. The spectrochemical method is particularly valuable for very weak bases, such as aromatic hydrocarbons and carbonyl compounds which require high concentrations of strong mineral acid in order to be converted into the conjugate acid to a measurable extent. 

For the nitration of the very weak base, acetophenone, there is reasonable agreement between observed and calculated activation parameters, and there is no doubt that nitration of the free base occurs at acidities below that of maximum rate. In this case the equilibrium concentration of free base is much greater than in the examples just discussed and there is no question of reaction upon encounter. ... 

A widely used protecting agent is 2-cyanoethanol (- 3-hydroxypropanonitrile, hy-dracrylonitrile), which is condensed with phosphates with the aid of DCC. The 2-cyanoethyl (Ce) group is quantitatively removed as acrylonitrile by treatment with weak bases (H.G. Kho-rana, 1965). 

Biosynthesis usually occurs under reductive and nucleophilic reaction conditions. Weak bases and acids are employed in nature as reagents or as catalysts. ... 

As early as 1889 Walker (320), using samples of thiazole, 2,4-dimethylthiazoie, pyridine, and 2,6-dimethylpyridine obtained from Hantzsch s laboratory, measured the electrical conductivity of their chlorhydrates and compared them with those of salts of other weak bases, especially quinoline and 2-methylquinoline. He observed the following order of decreasing proton affinity (basicity) quinaldine>2,6-dimethyl-pyridine>quinoline>pyridine>2,4-dimethylthiazole> thiazole, and concluded that the replacement of a nuclear H-atom by a methyl group enhanced the basicity of the aza-aromatic substrates. 

Acids differ m the degree to which they ionize Those that ionize completely are called strong acids, those that do not are weak acids Likewise strong bases ionize completely weak bases do not... 

Step 3 IS new to us It is an acid-base reachon m which the carbocation acts as a Br0n sted acid transferrmg a proton to a Brpnsted base (water) This is the property of carbo cations that is of the most significance to elimination reactions Carbocations are strong acids they are the conjugate acids of alkenes and readily lose a proton to form alkenes Even weak bases such as water are sufficiently basic to abstract a proton from a carbocation... 

Pyridine and pyrrole are both weak bases but pyridine is much more basic than pyrrole When pyridine is protonated its unshared pair is used to bond to a proton and... 

This reaction is normally carried out m the presence of a weak base such as pyridine which reacts with the hydrogen chloride that is formed... 

A synthetically useful reaction known as the Michael reaction, or Michael addition, involves nucleophilic addition of carbanions to a p unsaturated ketones The most common types of carbanions used are enolate 10ns derived from p diketones These enolates are weak bases (Section 18 6) and react with a p unsaturated ketones by conjugate addition... 

In practice this reaction is difficult to carry out with simple aldehydes and ketones because aldol condensation competes with alkylation Furthermore it is not always possi ble to limit the reaction to the introduction of a single alkyl group The most successful alkylation procedures use p diketones as starting materials Because they are relatively acidic p diketones can be converted quantitatively to their enolate ions by weak bases and do not self condense Ideally the alkyl halide should be a methyl or primary alkyl halide... 

From acyl chlorides (Sections 15 8 and 20 4) Alcohols react with acyl chlorides by nucleo philic acyl substitution to yield esters These reactions are typi cally performed in the presence of a weak base such as pyri dine... 

Amines like ammonia are weak bases They are however the strongest uncharged bases found m significant quantities under physiological conditions Amines are usually the bases involved m biological acid-base reactions they are often the nucleophiles m biological nucleophilic substitutions... 

Amines are weak bases but as a class amines are the strongest bases of all neu tral molecules Table 22 1 lists basicity data for a number of amines The most impor tant relationships to be drawn from the data are... 

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