Dehydration reaction mechanism

Reaction Mechanism and Kinetics. The equiHbria involved ia the hydration—dehydration of ethylene first proposed (117) can be expressed as follows ... 

The reaction mechanism involves deprotonation of the carboxylic anhydride 2 to give anion 4, which then adds to aldehyde 1. If the anhydride used bears two a-hydrogens, a dehydration takes place already during workup a /3-hydroxy carboxylic acid will then not be isolated as product ... 

Problem 29.4 Write a mechanism for the dehydration reaction of /3-hydroxybutyryl ACP to yield crotonyl ACP in step 7 of fatty-acid synthesis. 

With these results, we named the new enzyme as phenylacetaldoxime dehydratase (EC 4.99.1.7). It was also suggested that the enzyme utilizes FMN as an electron acceptor, because the value was increased about five times under anaerobic condition and the sulfite ion could replace FMN, although the enzyme requires oxidized form of FMN. It was revealed that the enzyme is a quite unique enzyme whose apparent function is to catalyze a dehydration reaction. The reaction mechanism is of much interest. 

The reaction mechanism is shown in Figure 4 and is adapted from work by Fiego et al. [9] on the acid catalysed condensation of acetone by basic molecular sieves. The scheme has been modified to include the hydrogenation of mesityl oxide to MIBK. The scheme begins with the self-condensation of acetone to form diacetone alcohol as the primary product. The dehydration of DAA forms mesityl oxide, which undergoes addition of an addition acetone to form phorone that then can cyclise, via a 1,6-Michael addition to produce isophorone. Alternatively, the mesityl oxide can hydrogenate to form MIBK. 

Cellulose pyrolysis kinetics, as measured by isothermal TGA mass loss, were statistically best fit using 1st- or 2nd-order for the untreated (control) samples and 2nd-order for the cellulose samples treated with three additives. Activation parameters obtained from the TGA data of the untreated samples suggest that the reaction mechanism proceeded through an ordered transition state. Sample crystallinity affected the rate constants, activation parameters, and char yields of the untreated cellulose samples. Various additives had different effects on the mass loss. For example, phosphoric acid and aluminum chloride probably increased the rate of dehydration, while boric acid may have inhibited levoglucosan... 

After the first dehydration step, the reaction propagates by successive dehydration-methanolation steps, competing with poly-merization-cyclization-aromatization processes. The existence of dehydration-methanolation mechanism is inferred from the constant presence of a small amount of methanol (from in situ C-NMR observation) on the catalyst. Further evidence has been acquired in favor of the carbenium ion chain-growth mechanism from the l C-NMR study of CO incorporation into the products during the conversion of methanol (46). 

The ester mechanism also explains well Gandini s and Plesch s observations on the dehydration of aromatic carbinols by acids [60], and it is actually an old item in the stock-cupboard of organic reaction mechanisms which has proved useful in explaining numerous organic reactions. This respectable antiquity and usefulness seem to have done nothing to make it more acceptable to many polymer chemists. 

J. L. Boucher, M. Delaforge, D. Mansuy, Dehydration of Alkyl- and Arylaldoximes as a New Cytochrome P450-Catalyzed Reaction Mechanism and Stereochemical Characteristics , Biochemistry 1994, 33, 7811. 7818. 

Figure 13.19 summarizes the reaction mechanism starting from Sn(OH) + or Sn(OH)e in acidic media in alkaline media. As in the case of Pd, Sn02 oxide is spontaneously formed by dehydration due to an internal oxolation reaction promoted by a strong polarization of the O-H bond of the hydroxide. Thermodynamically stable species with respect to pH are presented in Fig. 13.20. Various molecular cationic species with different hydroxylated levels are possible in an acidic medium, whereas only Sn(OH)g is expected for a basic pH.

More recent studies in this field have revealed the importance of the intrinsic acidic sites on the aluminas in directing the course of the dehydration. Recently the consideration of stereochemical factors involved in the dehydration and the use of gas chromatography as an analytical tool has led to a better understanding of this reaction, with a resultant better appreciation of the reaction mechanism. 

One of the most fruitful approaches to the elucidation of reaction mechanisms in organic chemistry is the study of the effect of structure on the reactivity and the course of the reaction. This approach is used extensively in homogeneous reactions and found to be equally rewarding in the study of the mechanism of dehydration of alcohols over alumina catalysts. Much information was obtained by changing the configuration of the alcohols. 

Aspartame is relatively unstable in solution, undergoing cyclisation by intramolecular self-aminolysis at pH values in excess of 2.0 [91]. This follows nucleophilic attack of the free base N-terminal amino group on the phenylalanine carboxyl group resulting in the formation of 3-methylenecarboxyl-6-benzyl-2, 5-diketopiperazine (DKP). The DKP further hydrolyses to L-aspartyl-L-phenyl-alanine and to L-phenylalanine-L-aspartate [92]. Grant and co-workers [93] have extensively investigated the solid-state stability of aspartame. At elevated temperatures, dehydration followed by loss of methanol and the resultant cyclisation to DKP were observed. The solid-state reaction mechanism was described as Prout-Tompkins kinetics (via nucleation control mechanism). 

In the deactivation mechanism, a key role is also played by acetone formed on the Zr02 through dehydration reactions (e.g. aldol-type condensation reactions) (Equation 6.32) ... 

The mechanism for bacterial synthesis of PHA is not the simple dehydration reaction between alcohol and carboxyl groups. It is more complicated and involves the coenzyme A thioester derivative of the hydroxyalkanoic acid monomer (produced from the organic feedstock available to the bacteria) [Kamachi et al., 2001], Growth involves an acyl transfer reaction catalyzed by the enzyme PHA synthase (also called a polymerase) [Blei and Odian,... 

In order to obtain some information on the reaction mechanism, the reaction of propargylic alcohol with acetone in the presence of a catalytic amount of 5a was monitored. The result indicated that the catalytic formation of the hexadienone proceeded via the initial isomerization of propargylic alcohol to dnnamaldehyde followed by aldol condensation between the produced aldehyde and acetone, and then dehydration. In fad, heating of propargylic alcohol in the presence of a catalytic amount of 5a gave only dnnamaldehyde (Scheme 7.41), and the separate reaction ofcinna-... 

It is proposed that hydrated or dehydrated titaniumperoxo compounds are formed in TS-1 by H2O2 chemisorption on the titanyl (Ti 0) group, and that these complexes constitute the actual oxidants [96]. In the particular case of alkane oxidation, a homolytic reaction mechanism is proposed, as is tentatively represented in scheme 6 [114]. 

From extensive analysis of recombinant proteins, and the crystal structure of A. thaliana protein, detailed reaction mechanisms have been proposed. The ANS reaction likely proceeds via stereospecific hydroxylation of the leucoanthocyanidin (flavan-3,4-cA-diol) at the C-3 to give a flavan-3,3,4-triol, which spontaneously 2,3-dehydrates and isomerizes to 2-flaven-3,4-diol, which then spontaneously isomerizes to a thermodynamically more stable anthocyanidin pseudobase, 3-flaven-2,3-diol (Figure 3.2). The formation of 3-flaven-2,3-diol via the 2-flaven-3,4-diol was previously hypothesized by Heller and Forkmann. The reaction sequence, and the subsequent formation of the anthocyanidin 3-D-glycoside, does not require activity of a separate dehydratase, which was once postulated. Recombinant ANS and uridine diphosphate (UDP)-glucose flavonoid 3-D-glucosyltransferase (F3GT, sometimes... 

The dehydration reactions initiated by eliminating a hydroxyl group from an enediol are discussed in the present article. The products (usually dicarbonyl compounds) of these eliminations are normally transient intermediates, and undergo further reaction. The final products formed are determined by the carbohydrate reacting, the conditions of reaction, and the character of the medium. Except for a Section on analytical methods (see p. 218), the subject matter is restricted to aqueous acids and bases. The presence of compounds other than the carbohydrate under study has only been considered where it has helped to elucidate the mechanism involved. The approach here is critical and interpretative, with emphasis on mechanism. An attempt has been made to demonstrate how similar reactions can logically lead to the various products from different carbohydrates a number of speculative mechanisms are proposed. It is hoped that this treatment will emphasize the broad functions of these reactions, an importance that is not fully recognized. No claim is made for a complete coverage of the literature instead, discussion of results in the articles that best illustrate the principles involved has been included. 

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