Proton ordering, hydrogen bonds

The complex kinetic dependence on hydroxide-ion concentration was explained by the mechanism in (27). Proton removal from the phenylazo-resorcinol monoanion by the hydroxide ion to give the dianion occurs by two different routes. One route is first order with respect to the hydroxide ion with rate coefficients and and is assumed to consist of a direct attack by the hydroxide ion on the hydrogen-bonded proton. The other route leads to a complex dependence of the rate of approach to equilibrium on the hydroxide-ion concentration, and involves prior opening of the hydrogen bond (rate coefficients kj and followed by proton removal (rate coefficients and k j). Equation (28) is derived from the mechanism... 

According to [59], at 25 °C the diffusion coefficient of protons in water is Dh+ = 9.28 10 cm /s, while the self-diffusion of water molecules is three times lower, = 2.8 10" cm /s. The hydrogen-bonded network, that restrains the self-diffusion of water molecules, paves the way for the rather fast proton transfer along the hydrogen bonds. Curiously, ordering water molecules within the network in itself does not accelerate proton transfer. Actually, according to recent data [60], the rate of proton diffusion in the... 

The hydration reaction has been extensively studied because it is the mechanistic prototype for many reactions at carbonyl centers that involve more complex molecules. For acetaldehyde, the half-life of the exchange reaction is on the order of one minute under neutral conditions but is considerably faster in acidic or basic media. The second-order rate constant for acid-catalyzed hydration of acetaldehyde is on the order of 500 M s . Acid catalysis involves either protonation or hydrogen bonding at the carbonyl oxygen. 

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