Proton reactions with hydroxide

There are two possible types of mechanism for the uncatalyzed hydrolysis of epoxides, a simple SN2 reaction of the substrate with water and a reaction of the protonated substrate with hydroxide ion. Another question to be answered concerns the position of attack of the nucleophile in substituted ethylene oxides. Experiments by Long and Pritchard [150] with H2180 indicate that in the uncatalyzed hydrolysis of propylene oxide two-thirds of the overall reaction occur via attack at the primary carbon. The corresponding percentage for the reaction of isobutylene oxide has not been determined precisely, but it is 20 % at least, probably much more. Attack at the primary carbon predominates also in the uncatalyzed reaction of propylene oxide with chloride ion [152]. 

The small isotope effects observed in proton transfer from cyanocarbon acids to various bases shown in Table 3 (for example feH/feD = 1.46 for proton transfer from malononitrile to water) are compatible with an extremely product-like transition state in which the proton is almost fully transferred [113] (Sect. 4.3). Similar conclusions may be reached from the small isotope effects observed for chloroform (feH/feD = 1.41 0.01 [114] and 1.11 0.05 [171]) and phenylacetylene (kH/kD = 0.95 0.09 [143]) for reaction with hydroxide ion, and for reaction of disulphones with water (feH/feD = 2.2 0.1 [65]). In all these cases the magnitude of the Bronsted exponent is close to the limiting value of unity as expected for a product-like transition state. 

When the p T in Mechanism (76) is above 7, the interconversion of EA and EAH will involve reaction with hydroxide, rather than with protons, k(, will be unimolecular, and fc5[OH ] will replace k. The p f now is displaced by the stickiness of the substrate only when EA and EAH are rapidly equilibrated, and the degree of displacement decreases as kf, becomes equal to or less than 2 [see Ref. (i) for a fuller discussion]. 

Moles of protons available for reaction with hydroxide ion... 

Find the most acidic hydrogen in each of the following and write a chemical equation for the proton-transfer process that occurs on reaction with hydroxide ion. Use curved arrows to show electron flow and label the acid, base, conjugate acid, and conjugate base. 

One can write and balance the two corresponding redox half-reactions either with protons or with hydroxide ions ... 

The mechanism of Scheme 8 is compatible with this observation. At low concentrations of hydroxide ion the rate of collapse of the tetrahedral intermediate to reactants must be faster than its reaction with hydroxide ion ( -1 At2[OH ]) the observed rate constant is dependent upon the concentration of hydroxide ion with the diffusion-controlled step, being rate-limiting, The calculated pAT,-values for the protonated amine of the tetrahedral intermediates are well below that for water. Proton transfer from the tetrahedral intermediate to hydroxide ion is therefore in the thermodynamically favourable direction and it is to be expected that the rate-limiting step for this process is the diffusion-controlled encounter of the proton donor and acceptor. 

The intrinsic reaction coordinate for neutral, base and acid catalyzed hydrolyses of formamide with a single H2O, HjO and OH reactant has been determined at MP2(full)/6-3 lG(d,p)//4-31G level [83]. Of the four pairs of reactants or protonated intermediates, N-protonated formamide is the most easily hydrolyzed with the calculated barrier for the breaking of the C-N bond being 6 Kcal-mol. Nucleophilic addition to the carbonyl carbon is barrierless for the reaction with hydroxide ion, but the resulting stable intermediate must overcome a barrier of 19 Kcal mol for scission of the C-N bond. While O-protonation is energetically favored over N-protonation by 14 Kcal-mof, nucleophilic addition to carbon in 0-protonated formamide faces a barrier of 24 Kcal-mof to yield a tetrahedral intermediate, which faces a further barrier of 16 Kcal-mof for the breaking of the C-N bond. The neutral reactants have to overcome a barrier of 44 Kcal-mof to yield products. 

Some work has to be done on the enolization of ketones with tertiary amino groups [105, 106]. The reactions of these compounds with iodine are complex processes which consume several moles of iodine per mole of ketone. Nevertheless, the pH-rate profile for enolization of 4-dimethylaminobutan-2-one appears to be sigmoid with the rare proportional to the concentration of the deprotonated form. The plateau rate constant and that for the enolization of 4-diethylamino-butanone and 5-ethylaminopentan-2-one are seven to eight powers of ten greater than the rate constants for the spontaneous enolization of simple aliphatic ketones. The results suggest that these compounds react with intramolecular catalysis. A mechanism as symbolized by 70 seems unlikely since the plateau rate constant for the enolization of 71 is similar to that for the enolization of 72 [105]. Instead, the kinetically equivalent process, involving reaction of the protonated form with hydroxide ion seems more likely. If this is correct, reaction via the six-membered cyclic transition state 73 is faster than via the... 

The formation of the above anions ("enolate type) depend on equilibria between the carbon compounds, the base, and the solvent. To ensure a substantial concentration of the anionic synthons in solution the pA" of both the conjugated acid of the base and of the solvent must be higher than the pAT -value of the carbon compound. Alkali hydroxides in water (p/T, 16), alkoxides in the corresponding alcohols (pAT, 20), sodium amide in liquid ammonia (pATj 35), dimsyl sodium in dimethyl sulfoxide (pAT, = 35), sodium hydride, lithium amides, or lithium alkyls in ether or hydrocarbon solvents (pAT, > 40) are common combinations used in synthesis. Sometimes the bases (e.g. methoxides, amides, lithium alkyls) react as nucleophiles, in other words they do not abstract a proton, but their anion undergoes addition and substitution reactions with the carbon compound. If such is the case, sterically hindered bases are employed. A few examples are given below (H.O. House, 1972 I. Kuwajima, 1976). 

In Figure the hydronium ion acts as an acid because it donates a proton to a base. The hydroxide anion acts as a base because it accepts a proton from an acid. When a hydronium ion with charge +1 transfers a proton to a hydroxide ion with charge -1, the two resulting water molecules have zero charges. The pair of charges becomes a neutral pair. A proton transfer reaction such as this one, in which water is one product and a pair of charges has been neutralized, is called a neutralization reaction. 

Soil pH is the most important factor controlling solution speciation of trace elements in soil solution. The hydrolysis process of trace elements is an essential reaction in aqueous solution (Table 3.6). As a function of pH, trace metals undergo a series of protonation reactions to form metal hydroxide complexes. For a divalent metal cation, Me(OH)+, Me(OH)2° and Me(OH)3 are the most common species in arid soil solution with high pH. Increasing pH increases the proportion of metal hydroxide ions. Table 3.6 lists the first hydrolysis reaction constant (Kl). Metals with lower pKl may form the metal hydroxide species (Me(OH)+) at lower pH. pK serves as an indicator for examining the tendency to form metal hydroxide ions. 

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