Acid dependence

As constituents of proteins the amino-acids are important constituents of the food of animals. Certain amino-acids can be made in the body from ammonia and non-nitrogenous sources others can be made from other amino-acids, e.g. tyrosine from phenylalanine and cystine from methionine, but many are essential ingredients of the diet. The list of essential amino-acids depends partly on the species. See also peptides and proteins. 

I) The melting point of pure maleic acid depends to a marked degree upon the rate of heating, and values between 133° and 143-144° may be observed. Slow heating (about 20 minutes) gives a value of 133-134° with more rapid heating... 

Acetylation with acetic anhydride is comparatively expensive because of the cost of the reagent. The use of the inexpensive glacial acetic acid depends upon the displacement of the reversible equilibrium ... 

The operation of the nitronium ion in these media was later proved conclusively. "- The rates of nitration of 2-phenylethanesulphonate anion ([Aromatic] < c. 0-5 mol l i), toluene-(U-sulphonate anion, p-nitrophenol, A(-methyl-2,4-dinitroaniline and A(-methyl-iV,2,4-trinitro-aniline in aqueous solutions of nitric acid depend on the first power of the concentration of the aromatic. The dependence on acidity of the rate of 0-exchange between nitric acid and water was measured, " and formal first-order rate constants for oxygen exchange were defined by dividing the rates of exchange by the concentration of water. Comparison of these constants with the corresponding results for the reactions of the aromatic compounds yielded the scale of relative reactivities sho-wn in table 2.1. 

The continued effectiveness of the nitronium ion in relatively dilute solutions has been indicated by comparing the dependence of the rates on the concentration of sulphuric acid, with the acidity-dependence of the ionisation of model compounds. The (formerly or Cq) acidity... 

If it be assumed that the ionising characteristics of nitric acid are similar to those of the organic indicators used to define the scales of acidity, then a correspondence between the acidity-dependence of nitration and would suggest the involvement of the nitronium ion, whereas a correspondence with Hq would support the h)rpothesis that the nitric acidium ion were active. The analogies with and Hg are expressed in the first and last pairs of the followii equations respectively. The symbol AQ represents anthraquinone, the indicator originally used in this way for comparison with the acidity dependence of the rate of nitration of nitrobenzene ... 

There is increasing evidence that the ionisation of the organic indicators of the same type, and previously thought to behave similarly, depends to some degree on their specific structures, thereby diminishing the generality of the derived scales of acidity. In the present case, the assumption that nitric acid behaves like organic indicators must be open to doubt. However, the and /fp scales are so different, and the correspondence of the acidity-dependence of nitration with so much better than with Hg, that the effectiveness of the nitronium ion is firmly established. The relationship between rates of nitration and was subsequently shown to hold up to about 82 % sulphuric acid for nitrobenzene, />-chloronitrobenzene, phenyltrimethylammonium ion, and p-tolyltrimethylammonium ion, and for various other compounds. ... 

Although the difference between the Hg and scales is sufficient to permit such a gross mechanistic distinction to be made, the acidity-dependence of nitration deviates from a close correspondence with the... 

The correlations of rates with acidity functions provide a convenient means of treatii results, and their uses will frequently be illustrated. However, their status is empirical, for whilst the acidity dependence of nitration becomes less steep with increasing temperature, the slope of... 

That the rate profiles are close to parallel shows that the variations in rates reflect the changing concentration of nitronium ions, rather than idiosyncrasies in the behaviour of the activity coefficients of the aromatic compounds. The acidity-dependences of the activity coefficients of / -nitrotoluene, o- and -chloronitrobenzene (fig. 2.2, 2.3.2), are fairly shallow in concentrations up to about 75 %, and seem to be parallel. In more concentrated solutions the coefficients change more rapidly and it... 

Another reason for treating with caution the results for benzene in solutions more acidic than 68% is discussed below ( 2.5). The acidity-dependences of rates of nitration at 25 °C have been established for the compounds listed in table 2.5. 

If the concentration of effective aromatic species does vary with acidity, as sometimes happens if the compound is substantially proto-nated, then the acidity-dependence of the rate will be less steep than usual, because the concentration of the active free base diminishes significantly with increasing acidity. This situation has been observed in certain cases ( 8.2). The fall in the concentration of the active species can be allowed for from a knowledge of its pK and the acidity function which, for the particular compound, gives the best measure of the acidity of the medium. Then the corrected acidity-dependence of the rate resembles that observed with compounds the concentration of which does not change significantly with acidity. The nitration of minor species is discussed later ( 8.2). 

TABLE 8.1 The acidity dependence and Arrhenius parameters for the nitration of some cations in... 

TABLE 8.2 The acidity dependence of rates of nitration of some free bases in sulphuric acid... 

The case of i-methyl-4-quinolone is puzzling. The large proportion of the 3-nitro isomer formed in the nitration (table 10.3 cf. 4-hydroxyquinoline) might be a result of nitration via the free base but this is not substantiated by the acidity dependence of the rate of nitration or by the Arrhenius parameters. From r-methyl-4-quinolone the total yield of nitro-compounds was not high (table ro.3). 

Among the W-oxides of this series of compounds isoquinoline 2-oxide shows the simplest behaviour on nitration. The acidity dependence of the rate of nitration (table 8.1), and comparison with the 2-methoxyiso-quinolinium cation (v, R = Me) (table 10.3) show the oxide to be nitrated as its conjugate acid (v, i = H) in 76-83 % sulphuric acid. The... 

The same orientation is found in the nitration of 2-methylbenzimi-dazole, whilst 5-nitro- and 2-methyl-5-nitrobenzimidazole are further nitrated at C(g). The acidity dependence of the rate of nitration of... 

The acids m Table 1 7 span a range of more than 70 pK units (10 ° m K ) In this sec tion we 11 introduce some generalizations that will permit us to connect molecular struc ture with acidity at least insofar as trends m related compounds are concerned The mam ways m which structure affects acidity depend on... 

The strength of an acid depends on the atom to which the proton is bonded The two mam factors are the strength of the H—X bond and the electronegativity of X Bond strength is more important for atoms m the same group of the periodic table electronegativity is more important for atoms m the same row Electronegative atoms elsewhere m the molecule can increase the acidity by inductive effects... 

Oxidation of an alcohol yields a carbonyl compound Whether the resulting carbonyl compound is an aldehyde a ketone or a carboxylic acid depends on the alcohol and on the oxidizing agent... 

The lower equivalent weight of magnesium hydroxide compared to caustic soda, hydrated lime, and soda ash reduces the stoichiometric amounts necessary to neutralize a given amount of acid. Depending on relative alkah costs, magnesium hydroxide can offer the advantage of lower chemical costs. 

Wet-process acid is manufactured by the digestion of phosphate rock (calcium phosphate) with sulfuric acid. Depending on availabiHty, other acids such as hydrochloric may be used, but the sulfuric-based processes are by far the most prevalent. Phosphoric acid is separated from the resultant calcium sulfate slurry by filtration. To generate a filterable slurry and to enhance the P2O5 content of the acid, much of the acid filtrate is recycled to the reactor. 

The purified acid is recovered from the loaded organic stream by contacting with water in another countercurrent extraction step. In place of water, an aqueous alkafl can be used to recover a purified phosphate salt solution. A small portion of the purified acid is typically used in a backwashing operation to contact the loaded organic phase and to improve the purity of the extract phase prior to recovery of the purified acid. Depending on the miscibility of the solvent with the acid, the purified acid and the raffinate may be stripped of residual solvent which is recycled to the extraction loop. The purified acid can be treated for removal of residual organic impurities, stripped of fluoride to low (10 ppm) levels, and concentrated to the desired P2 s Many variations of this basic scheme have been developed to improve the extraction of phosphate and rejection of impurities to the raffinate stream, and numerous patents have been granted on solvent extraction processes. 

The cost and price of sulfuric acid depend in large part on raw material cost and on freight costs. In many areas, the deUvered cost of sulfur is the most important factor affecting sulfuric acid pricing. By-product raw material, ie, SO2, costs at smelters are essentially zero, but the remote locations of many smelters make freight costs significant. Nevertheless, the nondiscretionary nature of smelter acid means that it must be sold if the smelter is to operate. 

Sulfates having alkyl groups from methyl to pentyl have been examined. With methyl as an example, the hydrolysis rate of dimethyl sulfate iacreases with the concentration of the sulfate. Typical rates ia neutral water are first order and are 1.66 x lO " at 25°C and 6.14 x lO " at 35°C (46,47). Rates with alkaH or acid depend on conditions (42,48). Rates for the monomethyl sulfate [512-42-5] are much slower, and are nearly second order ia base. Values of the rate constant ia dilute solution are 6.5 X 10 L/(mol-s) at 100°C and 4.64 X 10 L/(mol-s) at 138°C (44). At 138°C, first-order solvolysis is ca 2% of the total. Hydrolysis of the monoester is markedly promoted by increasing acid strength and it is first order. The rate at 80°C is 3.65 x lO " ... 

The principal constituents of rosin (qv) are abietic and related acids. Tall oil (qv) is a mixture of unsaturated fatty and aHcycHc acids of the abietic family. Refined tall oil may be high in rosin acids or unsaturated acids, depending on the refining process. Ethoxylates of rosin acids, eg, dehydro abietic acid, are similar to fatty acid ethoxylates in surfactant properties and manufacture, except for thek stabiHty to hydrolysis. No noticeable decomposition is observed when a rosin ester of this type is boiled for 15 min in 10% sulfuric acid or 25% sodium hydroxide (90). Steric hindrance of the carboxylate group associated with the aHcycHc moiety has been suggested as the cause of this unexpectedly great hydrolytic stabiHty. 

The dissociation of hypochlorous acid depends upon pH and, to a much lesser extent, temperature (6). At 25°C, it is - 0% at pH 5, about 50% at pH 7.5, and - 100% at pH 10, see Figure 1. Because of the acidity formed by chlorine gas, addition of soda ash (Na2C02) or sodium sesquicarbonate (Na2C03-NaHC03) is necessary to maintain the proper pH and to replenish alkalinity. 

The ratio between the isomers obtained in coupling with 1,3- and 1,5-naphtholsulfonic acids depends on the reactivity of the diazo component. Energetic ones, such as the 2,4-dinitrobenzenediazonium compound, essentially couple only with l-naphthol-3-sulfonic acid [3771-14-0] in the para position, but 4-chloro-benzenediazonium salt (a weaker diazo) attacks the ortho position. Both isomers result when mononitrobenzenediazonium compounds are used. The tendency to couple para is greater in l-naphthol-5-sulfonic acid [117-59-9] C QHgO S (21). For the combination of... 

The viscosity of a mixture of fatty acids depends on the average chain length n and can be calculated from the equations (4) ... 

The dissociation of hypochlorous acid depends on the pH. The unionized acid is present in greater quantities in acid solution, although in strongly acid solution the reaction with water is reversed and chlorine is Hberated. In alkaline solutions the hypochlorite ion OCL is increasingly Hberated as the pH is increased. The pH is important because unionized hypochlorous acid is largely responsible for the antimicrobial action of chlorine in water. Chlorine compounds are therefore more active in the acid or neutral range. The hypochlorites most commonly employed are sodium hypochlorite [7681-52-9] or calcium hypochlorite [7778-54-3]. 

Alkyl- and aryl-thiopyridazines are oxidized to sulfoxides, sulfones or sulfonic acids, depending on the reaction conditions. N- Oxidation can take place simultaneously. 

Thiophene and selenophene can be chloromethylated by treatment with formaldehyde and hydrochloric acid. Depending on the conditions, 2-chloromethyl or 2,5-bis(chloromethyl) derivatives are obtained. The chloromethylation of benzo[6]thiophene gives the 3-chloromethyl derivative and that of benzo[6]furan the 2-chloromethyl compound (71AHC(13)235). 

Specific acid catalysis is observed when a reaction proceeds through a protonated intermediate that is in equilibrium with its conjugate base. Because the position of this equilibrium is a function of the concentration of solvated protons, only a single acid-dependent term appears in the kinetic expression. For example, in a two-step reaction involving rate-determining reaction of one reagent with the conjugate acid of a second, the kinetic expression will be as follows ... 

The use of silver (II) salts, particularly argentic picolinate, as reagents for hydroxyl oxidation has also been disclosed recently. The reaction may be run in acid, neutral or basic media in aqueous or polar organic solvents at room or slightly elevated temperatures. Primary alcohols may be oxidized to aldehydes or acids depending on the conditions used. Amines and trivalent phosphorous compounds are more sensitive to oxidation with this reagent than are hydroxyl groups. 

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