Acids than anion

Although acetylene and terminal alkynes are far stronger acids than other hydro carbons we must remember that they are nevertheless very weak acids—much weaker than water and alcohols for example Hydroxide ion is too weak a base to convert acety lene to its anion m meaningful amounts The position of the equilibrium described by the following equation lies overwhelmingly to the left... 

It IS not necessary to prepare and isolate the sodium alkanethiolate m a separate opera tion Because thiols are more acidic than water they are quantitatively converted to their alkanethiolate anions by sodium hydroxide Thus all that is normally done is to add a thiol to sodium hydroxide m a suitable solvent (water or an alcohol) followed by the alkyl halide... 

An a hydrogen of an aide hyde or a ketone is more acidic than most other protons bound to carbon Aldehydes and ketones are weak acids with pK s in the 16 to 20 range Their enhanced acidity IS due to the electron withdrawing effect of the carbon yl group and the resonance stabi lization of the enolate anion... 

Carboxylic acids are weak acids and m the absence of electron attracting substituents have s of approximately 5 Carboxylic acids are much stronger acids than alcohols because of the electron withdrawing power of the carbonyl group (inductive effect) and its ability to delocalize negative charge m the carboxylate anion (resonance effect)... 

With pA a s of approximately 10 phenols are stronger acids than alcohols but weaker than carboxylic acids They are converted quantitatively to phenoxide anions on treatment with aqueous sodium hydroxide... 

Pyrrole is soluble in alcohol, benzene, and diethyl ether, but is only sparingly soluble in water and in aqueous alkaUes. It dissolves with decomposition in dilute acids. Pyrroles with substituents in the -position are usually less soluble in polar solvents than the corresponding a-substituted pyrroles. Pyrroles that have no substituent on nitrogen readily lose a proton to form the resonance-stabilized pyrrolyl anion, and alkaU metals react with it in hquid ammonia to form salts. However, pyrrole pK = ca 17.5) is a weaker acid than methanol (11). The acidity of the pyrrole hydrogen is gready increased by electron-withdrawing groups, eg, the pK of 2,5-dinitropyrrole [32602-96-3] is 3.6 (12,13). 

Dimerization involves addition of the cyanamide anion to the nitnle group of an undissociated molecule to give the anion of cyanoguanidine, or dicyandiamide. This reaction takes place most readily at pH 8—10 where the reactants are present in favorable proportion. The product is a weaker acid than cyanamide and is protonated at once with generation of a new cyanamide anion. 

The pyrazole molecule resembles both pyridine (the N(2)—C(3) part) and pyrrole (the N(l)—C(5)—C(4) part) and its reactivity reflects also this duality of behaviour. The pyridinic N-2 atom is susceptible to electrophilic attack (Section 4.04.2.1.3) and the pyrrolic N-1 atom is unreactive, but the N-1 proton can be removed by nucleophiles. However, N-2 is less nucleophilic than the pyridine nitrogen atom and N(1)H more acidic than the corresponding pyrrolic NH group. Electrophilic attack on C-4 is generally preferred, contrary to pyrrole which reacts often on C-2 (a attack). When position 3 is unsubstituted, powerful nucleophiles can abstract the proton with a concomitant ring opening of the anion. 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

Ethyl acetoacetate is a stronger acid than ethanol and is quantitatively converted to its anion on treatment with sodium ethoxide in ethanol. 

B5H9 also acts as a weak Brpnsted acid and, from proton competition reactions with other boranes and borane anions, it has been established that acidity increases with increasing size of the borane cluster and that arachno-boranes are more acidic than nido-horancs ... 

Phenol has different chemical properties from those of typical alcohols. Display the electrostatic potential map for phenol. Does this suggest that phenol is likely to be a stronger or weaker acid than any of the compounds discussed above Compare the electrostatic potential map for 4-nitrophenol to that for phenol. What effect does substitution by nitro have on acid strength Explain your result by considering charge delocalization in the conjugate base. Draw all reasonable Lewis structures for phenoxide anion and for 4-nitrophenoxide anion. Which is more delocalized Is this consistent with experimental pKa s ... 

Phenol, C6H5OH, is a stronger acid than methanol, CH3OH, even though both contain an O-H bond. Draw the structures of the anions resulting from loss of H+ from phenol and methanol, and use resonance structures to explain the difference in acidity. 

Carbonyl compounds are more acidic than alkanes for the same reason that carboxylic acids are more acidic than alcohols (Section 20.2). In both cases, the anions are stabilized by resonance. Enolate ions differ from carboxylate ions, however, in that their two resonance forms are not equivalent—the form with the negative charge on oxygen is lower in energy than the form with the charge on carbon. Nevertheless, the principle behind resonance stabilization is the same in both cases. 

Cationic polymerization of cyclosiloxanes is well known but used much less frequently than anionic reactions. The most widely used catalysts include sulfuric acid and its derivatives, alkyl and aryl sulfonic acids and trifluoroacetic acid1 2,1221. Due to their ease of removal, in industrial applications acid catalysts are generally employed on supports such as bentonite clay or Fuller s earth. 

Alkylation takes place at the most acidic position of a reagent molecule for example, acetoacetic ester (CH3COCH2COOEt) is alkylated at the methylene and not at the methyl group, because the former is more acidic than the latter and hence gives up its proton to the base. However, if 2 mol of base are used, then not only is the most acidic proton removed but also the second most acidic. Alkylation of this doubly charged anion then takes place at the less acidic position (see p. 458). This technique has been used to alkylate many compounds in the second most acidic position. ... 

Pol /protic acids also show clearly the effect of charge on acidity. As mentioned earlier, the successive Za values for a pol /protic oxoacid decrease by approximately five orders of magnitude. The neutral parent acid always is a stronger acid than is the anion produced by removing one proton. 

Interest has been shown by several groups on the effect of solvent and of added anions upon the oxidation of alcohols. The oxidation of isopropanol proceeds 2500 times faster in 86.5 % acetic acid than in water at the same hydrogen ion concentration . The kinetics and primary kinetic isotope effect are essentially the same as in water. Addition of chloride ion strongly inhibits the oxidation and the spectrum of chromic acid is modified. The effect of chloride ion was rationalised in terms of the equilibrium,... 

Finally, we can see that, neutral meolecules, either in nitro-type or in aci-nitro-type, are more stable than acid-dissociated anions the anion formation is a high endothermic reaction. The energy difference between neutral molecules and acid-dissociated anions calculated at the MP2/6311+-I-G level is 1539 kJ/mol for nitro-type species, and 1683 kJ/mol for aci-nitro-type species. It is clear that, in these conditions, the acid dissociation of the neutral molecules can hardly occur.in pure nitromethane solutions. It provides another theoretieal support for nitromethane as an ideal model of aprotic solvents. 

In either neutral molecules or acid-dissociated anions, the nitro-type species are more stable than the aci-nitro-type species. The 1,3-intramolecular hydrogen rearrangment is a high barrier process. In the tautomeric system formed via the 1,3-hydrogen shift, the equilibrium is therefore strongly displaced to the side of nitro-type species. 

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