Dehydration reaction, steps

PO—SM Coproduction. The copioduction of propylene oxide and styrene (40—49) includes three reaction steps (/) oxidation of ethylbenzene to ethylbenzene hydroperoxide, (2) epoxidation of ethylbenzene hydroperoxide with propylene to form a-phenylethanol and propylene oxide, and (3) dehydration of a-phenylethanol to styrene. 

The dehydration reactions have somewhat higher activation energies than the addition step and are not usually observed under strictly controlled kinetic conditions. Detailed kinetic studies have provided rate and equilibrium constants for the individual steps in some cases. The results for the acetone-benzaldehyde system in the presence of hydroxide ion are given below. Note that is sufficiently large to drive the first equilibrium forward. 

Adiponitrile may also be produced by the electrodimerization of acrylonitrile (Chapter 8) or by the reaction of ammonia with adipic acid followed by two-step dehydration reactions ... 

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. 

StepS 9-1° of F Sure 29-7 Dehydration and Dephosphorylation Like mos /3-hydroxy carbonyl compounds produced in aldol reactions, 2-phospho glvcerate undergoes a ready dehydration in step 9 by an ElcB mechanism (Section 23.3). The process is catalyzed by enolase, and the product i... 

Problem 29.12 ls the pro-R or pro-5 hydrogen removed from citrate during the dehydration in step 2 of the citric acid cycle Does the elimination reaction occur with syn or anti geometry ... 

Steps 1, 3 Phosphate transfers, steps 2, 5, 8 isomerizatlons step 4 retro-aldoi reaction step 5 oxidation and nucleophilic acyl substitution steps 7, 10 phosphate transfers step 9 F.2 dehydration... 

The final two stages are very straightforward. Oxidative scission of the C3-C5 double bond in 6 with ozone provides triketone 5 which, without purification, is subjected to a base-induced intramolecular aldol/dehydration reaction. The crystalline product obtained from this two-step sequence (45 % overall yield) was actually an 85 15 mixture of ( )-progesterone and a diastereomeric substance, epimeric at C-17. Two recrystallizations afforded racemic progesterone [( )-(1)] in diastereomerically pure form. 

Acid-Catalyzed Dehydration of Secondary or Tertiary Alcohols An El Reaction Step 1... 

Figure 19.1 A schematic view of the common formaldehyde-induced modifications in proteins. Reactive methylol adducts are formed in the initial reaction between formaldehyde and cysteine or the amino groups of basic amino acid residues. The methylol adduct can subsequently undergo a dehydration reaction to form a Schiff s base. Adducted residues can undergo a second reaction to form methylene bridges or can convert to the ethoxymethyl adduct after the ethanol dehydration step.

Davies et al. describe the preparation of both oxazole- and thiazole-containing derivatives of combretastatin. By formation of the ketoamide intermediate 60, in a 54% yield (Scheme 14), both classes of compounds may be obtained by altering the last step of the reaction [58]. To produce the oxazole 61 a cyclo-dehydration reaction was performed using triphenylphosphine-iodine-triethylamine, and the thiazole compound 62 was formed by thiona-tion using Lawesson s reagent, with an excellent yield (94%). 

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). 

For isoenzymes I and II, the CO2 hydration rates are independent of buffer at high buffer concentrations, indicating thereby that a reaction step other than the buffer-dependent step becomes rate limiting. Studies of both hydration and dehydration reactions at high concentrations of buffers in H20 and DoO indicated that the kinetic parameter, kCSLt, for isoenzyme II has large isotope effect (k jkV) 3-4) (45b). This is consistent with involvement of H+ transfer in the rate-limiting step. The H+ transfer half-reaction is composed of at least two steps,... 

It is well documented that the isoimide is the kinetically favoured product and that isomerization yields the thermodynamically stable imide when sodium acetate is used as the catalyst. High catalyst concentrations provide maleimides with low isoimide impurity. The mechanism by which the chemical imidization is thought to occur is shown in Fig. 3. The first step in the dehydration reaction may be formation of the acetic acid-maleamic acid mixed anhydride. This species could lose acetic acid in one of the two ways. Path A involves participation by the neighboring amide carbonyl oxygen to eject acetate ion with simultaneous or subsequent loss of proton on nitrogen to form the isoimide. Path B involves loss of acetate ion assisted by the attack of nitrogen with simultaneous or subsequent loss of the proton on nitrogen to form the imide. If the cyclodehydration is run in acetic anhydride in the absence of the base catalyst, isoimide is the main reaction product. 

Dehydration of 3-dehydroquinate (step c), the first step in Eq. 25-3, is the first of three elimination reactions needed to generate the benzene ring of the end products. This dehydration is facilitated by the presence of the carbonyl group. After reduction of the product to shikimate (step d)19 a phosphorylation reaction (step e)20,21 sets the stage for the future elimination of Pj. In step/, condensation with PEP adds three carbon atoms that will become the a, P, and... 

For example, peptide bond formation can occur between two amino acids by a dehydration resulting from simple heating as depicted in figure 1.5c. In the cell, peptide bond formation also takes place, but several intermediate steps are involved and the reaction takes place not by dehydrating but in the wet environment of the cytosol. Similarly the phosphodiester bond depicted in figure 1.6c is not formed by a simple dehydration reaction in the cell but... 

The Reduction Reactions. The object of the next three reactions (steps 4 to 6 in fig. 18.12a) is to reduce the 3-carbonyl group to a methylene group. The carbonyl is first reduced to a hydroxyl by 3-ketoacyl-ACP reductase. Next, the hydroxyl is removed by a dehydration reaction catalyzed by 3-hydroxyacyl-ACP dehydrase with the formation of a trans double bond. This double bond is reduced by NADPH catalyzed by 2,3-trans-enoyl-ACP reductase. Chemically, these reactions are nearly the same as the reverse of three steps in the j6-oxidation pathway except that the hydroxyl group is in the D-configuration for fatty acid synthesis and in the L-configuration for /3 oxidation (compare figs. 18.4a and 18.12a). Also remember that different cofactors, enzymes and cellular compartments are used in the reactions of fatty acid biosynthesis and degradation. 

The overall reaction scheme for dehydration is identical for all butyl alcohols and for both our catalysts, but the relative amounts of various reaction intermediates and the relative values of the rate coefficients for various reaction steps can be dramatically different. Thus, for a given temperature and given catalyst loading in the reactor, and for a given gas-flow rate through the reactor and concentration of butyl alcohol in the gas flow, the observed reaction rates and selectivities with respect to various reaction products can be crucially different for different butyl alcohols and different catalysts (i.e., crystalline HZSM-5 or AAS). 

An elementary example of diis process is the reaction of an organometallic reactant widi a ketone (or aldehyde) followed by dehydration of the resulting alcohol to die olefin. This is truly a sequential process in that the product alcohol is dehydrated in a second, independent reaction step. It suffers as a useful synthetic method because regioisomers are often formed hi die elimination step. 

In all these cases it is possible to determine Kk directly by combining kn2o with ku, the rate constant for carbocation formation. The latter constant is readily determined spectrophotometrically by monitoring acid-catalyzed dehydration of the aromatic hydrate to the corresponding aromatic product. In principle, as we have seen, when the dehydration product is aromatic, carbocation formation is the rate-determining step of the reaction. However, the finite values of kp/kn2o for the phenanthrenonium ion and other areno-nium ions leading to moderately stable aromatic products imply a small correction for reversibility of this reaction step. 

The 2,3-dimethyl-1-pentene formed in the dehydration reaction must be optically pure because it arises from optically pure alcohol by a reaction that does not involve any of the bonds to the stereo-genic center. When optically pure 2,3-dimethyl-l-pentene is hydrogenated, it must yield optically pure 2,3-dimethylpentane—again, no bonds to the stereogenic center are involved in this step. 

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). 

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