Unimolecular dissociation water

The major difference between the two mechanisms is the second step. The second step in the reaction of tcrt-butyl alcohol with hydrogen chloride is the unimolecular dissociation of tcrt-butyloxonium ion to tcrt-butyl cation and water. Heptyloxonium ion, however, instead of dissociating to an unstable primar y car bocation, reacts differently. It is attacked by bromide ion, which acts as a nucleophile. We can represent the transition state for this step as ... 

Randall and Alberly (136) have studied the binding of various ligands to aquocobalamin using stopped flow techniques. This work suffers from the fact that it is not clear if the added ligand is displacing coordinated water or coordinated benzimidazole. One might be led to believe that the reaction studied in this work is in fact displacement of benzimidazole because the kinetics are at least inconsistent with a mechanism in which unimolecular dissociation of coordinated water is the rate limiting step. 

The intermediate reaction complexes (after formation with rate constant, fc,), can undergo unimolecular dissociation ( , ) back to the original reactants, collisional stabilization (ks) via a third body, and intermolecular reaction (kT) to form stable products HC0j(H20)m with the concomitant displacement of water molecules. The experimentally measured rate constant, kexp, can be related to the rate constants of the elementary steps by the following equation, through the use of a steady-state approximation on 0H (H20)nC02 ... 

Where (H20) is the first order rate constant for the uncatalyzed reaction at a given water concentration, k3 = A2/[H20], A2 = Ac/[C1 ], kc = ki (catalyzed) + kx (uncatalyzed), and [Cl-] is the concentration of chloride ion from neutral salts. They again oppose the Swain mechanism on the basis a) that the unimolecular dissociation of the pentacovalent complex should be subject to electrophylic catalysis, b) that the steric effects are too great, c) that symmetrical chloride exchange is... 

The high stability of isolated 2 was confirmed by collisional activation (+NCR+) that caused only minor dissociation by elimination of water forming furan. The latter reaction is calculated to be the lowest-energy unimolecular dissociation of 2 that is 4 kj mol-1 exothermic, but is kinetically hampered by an energy barrier to intramolecular hydrogen transfer [69]. The kinetic stability of isolated 2 in the gas phase contrasts its properties in aqueous solution, where the enol is predicted to react rapidly with H30+ or OH- and isomerizes to the more stable lactone 3. The equilibrium constant for the isomerization 3 2 is calculated to be extremely small in water, Keq=5.7xlO 20, so enol 2 would be very difficult to generate and study in solution. 

The rate constants for unimolecular dissociation reactions were adjusted to tne high pressure of the supercritical water system by using high pressure rate constants, k, where applicable. For dissociations still in the low pressure... 

The ionic intermediate dissociates water and with OH" yields r-butyl alcohol as the final reaction product. In this mechanism the slowest (rate determining or rate limiting) process is the dissociation of bromide therefore the overall reaction rate does not depend on the methoxide ion concentration. This is the reason why this mechanism is called Sj.jl (substitution, nucleophilic, unimolecular). It is obvious that the 8 1 mechanism will be favored in... 

Like the reaction of terr-butyl alcohol with hydrogen chloride step 2, in which tert-butyloxonium ion dissociates to (CH3)3C and water, is rate-deter-mining. Because the rate-determining step is unimolecular-, the overall dehydration process is refened to as a unimolecular elimination and given the symbol El. 

The most exciting application of bond order indices concerns the description of chemical reactions involving the simultaneous change of several bonds. An example is the unimolecular decomposition of ethanol, which can happen at high temperature or IR multiphoton excitation of the molecule. Out of the possible dissociation channels, the lowest barrier characterizes the concerted water loss of the molecule, yielding ethene and H20 [30]. 

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