Ethylene protonation rate constant

Rate constant for protonation/s Rate constant for deprotonation/l mol s One central ethylene group is substituted by a 1,2-phenylene group. 

Besides the effect of solvent polarity, the C=C rotation in many push-pull ethylenes is sensitive to acid catalysis (143). This is probably explained by protonation of the acceptor groups, for example, the oxygen atoms in C=0 groups (16), which increases their acceptor capacity. Small amounts of acids in halogenated solvents, or acidic impurities, may have drastic effects on the barriers, and it is advisable to add a small quantity of a base such as 2,4-lutidine to obtain reliable rate constants (81). Basic catalysis is also possible, but it has only been observed in compounds containing secondary amino groups (38). 

In so far as values of pATn2o for the hydration of alkenes are known or can be estimated,47 values of pATR can be derived by combining rate constants for protonation of alkenes with the reverse deprotonation reactions of the carbocations. The protonation reactions seem much less likely to be concerted with attack of water on the alkene than the corresponding substitutions. Indeed arguments have been presented that even protonation of ethylene in strongly acidic media involves the intermediacy of the ethyl carbocation.97,98... 

In conclusion, overwhelming evidence points today to the general acid catalysis of these reactions and to the formation of a short-lived intermediate with the characteristics of a symmetrical open ion. This behaviour includes an enormous range of nucleophili-city of the substrate, from ethylene to 1,1-diethoxyethylene. Thus, the postulation of a rr-complex precursor in the mechanism of acid-catalysed hydration of olefins is now unjustified, and the second order rate constants experimentally obtained are in fact a reflection of the protonation reaction of the hydronium ion onto the double bond. 

Sawamoto and Higadiimura have continued their investigation of the stop-flow spectroscopic behaviour of the polymerisation of p-methoxystyrene and added triflic acid to the series of catalysts previously tried. Again, they observed the typical absorption around 380 nm, attributed to the protonated monomer and calculated an initiaticm rate constant at 30 °C in ethylene chloride (assuming a bimolecular interaction orders not determined) kj = 5 x 10 s . This value is expectedly much... 

Table I shows that hydroxyl radicals react rapidly with both monomers examined. High rate constants would also be expected for other monomers. Hydrated electrons do not react rapidly with ethylene, but a slow reaction cannot be excluded. Hydrated electrons react with other monomers with rate constants around 1-2 X 1010M-1 sec."1. A similar high reactivity would be expected for unexamined vinyl monomers. There is no reason to suppose that the hydration of the electron plays an important part in the reaction hence, electrons generated by radiation in other media should also react with vinyl monomers. For vinylpyridine, comparison of the neutral form with the protonated form shows a significantly higher rate constant for reaction with the protonated form, but other such comparisons (where possible) have not yet been made.

Extension of these studies to formic acid media (containing 4 vol. % ethylene glycol and 1.3 vol. % water) showed that for protodeboronation of 4-methoxy-benzeneboronic acid at 25 °C) rates were invariant of a tenfold variation in acidity produced by adding sodium formate (0.05-0.20 M) to the medium (Table 194), and in this range the concentration of molecular formic acid is essentially constant. This was, therefore, assumed to be the reactive species. At higher acidities the rate increased, which was attributed to the increase in concentration of hydronium ions and protonated formic acid ions which bring about reaction more readily625. 

A rough correlation between the relative rates of deprotonation and the Jq h for the bridgehead protons in the series 7, 8 and 9, was observed by Gloss and Larrabee. Taking the C-H coupling constant as a measure of the acidity of hydrocarbons, the acidity of the proton in bicyclobutane is expected to be between that of acetylene and ethylene (Jq-h values for acetylene, ethylene and bicyclobutane are 248,156 and 205 Hz respectively). 

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