Unimolecular dehydration

Formic acid decomposition has been studied on the (110), (001), and (100) surfaces of Ti02 [23-25,40-51]. The degree to which surface reducibility influences the reaction paths e.g., dehydrogenation vs. dehydration unimolecular reactions vs. bimolecular ones) will be explored in more detail... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

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. 

Unimolecular pyrolysis of the tautomers of monothioformic acid (two conformers of thiol- and two conformers of thiono-) have been studied by ab initio methods with STO-3G and 6-31 G basis sets. The barrier heights for dehydrogenation (via a four-centre transition state) and dehydrogensulfldation (via a three-centre transition state) of thiol formic acid are 67.47 and 67.09 kcalmol" respectively. Dehydration of 5-cw-HCSOH occurs via a three-centre transition state with an activation energy of 81.18 kcalmoG this is much greater than for dehydration of the s-trans form, which occurs via a four-centre transition state with a barrier of only 68.83 kcalmol" ... 

Saito and Niiyama (241) investigated the transient behavior of ethanol dehydration catalyzed by Baj sPW O. When the ethanol feed was stopped after a steady state had been attained, ethylene continued to form for a prolonged period, whereas ether, formation decreased rapidly. This transient behavior, as well as the kinetics under stationary conditions, was well simulated with a model based on the assumption that the ethylene and ether are formed by unimolecular and bimolecular reactions in the bulk, respectively. 

For all four alcohols in the zeolitic catalysts with small enough crystallite sizes—when diffusion limitations also disappear—dehydration kinetics are well approximated by the exponental function, a fact that is explicable in terms of the unimolecular decay of molecules of butyl alcohol adsorbed on identical active sites. With isobutyl alcohol, for example, the rate coefficient k may be written... 

This reaction is an dehydration acid-catalyzed.12 The hexaaquocop-per cation behaves as a weak cationic acid in copper-salt solution.13 Protonation of the hydroxy group produces an oxonium ion that decomposes unimolecularly into carbocation 21 and water. Water is removed from the reaction equilibrium by means of a water-separating device. Carbocation 21 eliminates an -proton with formation of the energetically favorable conjugated diene 9. 

The MNDO method has been employed405 to study the reaction pathway and to optimize the structures of reactant, product, and transition state of the acid-catalysed rearrangement of 1,2-propylene glycol, and the unimolecular dehydration of protonated a,co-diols in the gas phase has been examined406 by tandem mass spectrometric experiments. It has been shown that the reaction of l,2-diarylcyclopropane-l,2-diols (342) with acids yields primarily the a,//-unsaturated ketones (343) in which the aryl... 

Unimolecular cleavage in this case corresponds to the dehydration of the acid, but in the case of protonated esters the cleavage pathway depends on the nature of the alkoxy group [Eqs. (3.68)-(3.70)]. 

Vieira AJSC, Steenken S (1987a) Pattern of OH radical reaction with 6- and 9-substituted purines. Effect of substituents on the rates and activation parameters of unimolecular transformation reactions of two isomericOH adducts. J PhysChem 91 4138-4144 Vieira AJSC, Steenken S (1987b) Pattern of OH radical reaction with N6,N6-dimethyladenosine. Production of three isomeric OH adducts and their dehydration and ring opening reactions. J Am Chem Soc 109 7441-7448... 

Hydroxyls can act as nucleophiles, although they are less nucleophilic than amines or thiols. Under acidic conditions, hydroxyls can be eliminated in a dehydration reaction (Fig. 79). Elimination reactions can occur as an El reaction (elimination unimolecular) or E2 reaction (elimination bimolecu-lar). The El elimination mechanism proceeds through formation of a carbo-cation intermediate as the rate-determining step with loss of water whereas the E2 mechanism is second order with the base abstraction of a proton and loss of the leaving group occurring simultaneously (120). 

The least expensive method for synthesizing simple symmetrical ethers is the acid-catalyzed bimolecular condensation (joining of two molecules, often with loss of a small molecule like water), discussed in Section 11-10B. Unimolecular dehydration (to give an alkene) competes with bimolecular condensation. To form an ether, the alcohol must have an unhindered primary alkyl group, and the temperature must not be allowed to rise too high. If the alcohol is hindered or the temperature is too high, the delicate balance between substitution and elimination shifts in favor of elimination, and very little ether is formed. Bimolecular condensation is used in industry to make symmetrical ethers from primary alcohols. Because the condensation is so limited in its scope, it finds little use in the laboratory synthesis of ethers. 

If the first step in the oxidation of hydrocarbons is the production of peroxy radicals (HO2, RO2) and peroxides (ROOH, HOOH, ROOR ), we see that the next stage must involve the decomposition of these intermediates, cither by free radical attack on the peroxides or by unimolecular decomposition. An additional mode of destruction is by surface dehydration to form IT2O + aldehyde. 

Thermal reaction of larger carboxylic acids on titanium dioxide surfaces traced reaction pathways similar to those of formic acid. The (110 -faceted surface of TiOiCOOl) selectively decomposed acetate and propionate via unimolecular dehydration to form ketene and methyl ketene. The dehydration yields from both reactants were quite similar, as were the desorption temperatures of the products [46]. 

Interactions with other molecules can affect the ultimate reaction selectivity of the process. For example, the catalytic dehydration of formic acid on TiO2(001) occurred as a unimolecular process at high temperatures and low formate coverage, while a bimolecular dehydrogenation process dominated at near-saturation coverage of the titania crystal. 

The catalytic dehydration reaction of formic acid on TiO2(110) is suggested to involve the unimolecular decomposition of formate ions (HCOO (a)) as rate-determining step. The formate-surface interaction activates the unimolecular decomposition of formate to preferentially yield CO(g) and OH (a). An acidic proton of a HCOOH molecule, which encounters the surface in a steady state, reacts with the resultant OH (a) to form H2O as shown in Scheme 1. 

Scheme 1. Dehydration reaction pathway involving the unimolecular decompo ion of formate on Ti02(l 10)...

Both unimolecular dehydration of alcohols to olefins and dehydrogenation to CO and aldehydes have been reported in the literature (Tolstopyatova et al., 1961 Tosun and Rase, 1972 Ganichenko, 1967 Minachev, 1973), but permanent dehydration activity was observed only at reaction temperatures higher than 300°C due to the contamination of the oxide surfaces with product water molecules. 

Lim et al. used a palladium complex for the cationic polymerization of TH F and the ROMP of NB [10]. The same group also showed that even condensation and chain polymerization could be performed simultaneously in one step (Scheme 11.45). This was achieved by the use of unimolecular compounds which can simultaneously act both as an initiator for chain polymerization, and as an end-capper for condensation polymerization. The method provides a simple means of combining NMRP with a condensation polymerization to yield interesting and useful block copolymers [207]. Another interesting new system for the combination of chain (AROP of CL) and step (dehydration polycondensation) polymerizations for polyester-based new material, in which scandium trifluoromethane sulfonate catalyzed both polymerization modes, was reported by Takasu et al. (Scheme 11.46) [208]. 

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