Acid-catalyzed dehydration reactions

Thus we see thai what at first appears to be a complicated reaction i actually a sequence of simple steps involving familiar, fundamental types of reactions acid-catalyzed dehydration, nucleophilic addition to an ,jS-unsaturaled carbonyl compound, electrophilic aromatic substitution, and oxidation. 

A related reaction is dehydration, in which an alcohol is converted into an alkene and water by the elimination of H— and —OH from adjacent carbon atoms. The dehydration of an alcohol to form an alkene can be considered the reverse of the hydration of an alkene to form an alcohol (Section 27-17). Dehydration reactions are catalyzed by acids. 

In the realm of homogeneous catalysis we often encounter examples of acid- and base-catalyzed hydration-dehydration and hydrolysis, metal-catalyzed hydrolysis and autoxidation, photocatalytic oxidation and reduction, metal-catalyzed electron transfer, acid-catalyzed decarboxylation, photocatalytic decarboxylation, metal-catalyzed free-radical chain reactions, acid-catalyzed nucleophilic substitutions, and enzymatic catalysis. 

Problem 13.13. We have described acid-catalyzed dehydration (loss of water) of an alcohol to yield an alkene. However, Sec. 12.6.1 described the opposite reaction—acid-catalyzed hydration (addition of water) of an alkene to yield an alcohol. Which is correct ... 

A MECHANISM FOR THE REACTION ] Acid-Catalyzed Dehydration of Secondary or Tertiary Alcohols An El Reaction 307... 

As mentioned in Section 32.2, arene c/s-dihydrodiols of type 2 undergo facile dehydration reactions (usually catalyzed by Brpnsted acid) that rearomatize the ring and give phenols 166 (Scheme 32.21). Such dehydrations are far more rapid than for the corresponding dehydration of arene f rans-dihydrodiols (which may be accessed by hydrolysis of the epoxide ring in arene oxides... 

Scheme 10.18 shows the standard electron pushing for an acid-catalyzed dehydration reaction. Base-catalyzed reactions typically occur only when conjugated dienes are formed, and we do not cover these reactions here. Alcohols show specific-acid-catalyzed dehydrations, and thus the mechanism given starts with a reversible protonation of the alcohol. 

In his cephalosporin synthesis methyl levulinate was condensed with cysteine in acidic medium to give a bicyclic thiazolidine. One may rationalize the regioselective formation of this bicycle with the assumption that in the acidic reaction mixture the tMoI group is the only nucleophile present, which can add to the ketone. Intramolecular amide formation from the methyl ester and acid-catalyzed dehydration would then lead to the thiazolidine and y-lactam rings. The stereochemistry at the carboxylic acid a-... 

Zaitsev s rule as applied to the acid catalyzed dehydration of alcohols is now more often expressed in a different way elimination reactions of alcohols yield the most highly substituted alkene as the major product Because as was discussed in Section 5 6 the most highly substituted alkene is also normally the most stable one Zaitsev s rule is sometimes expressed as a preference for predominant formation of the most stable alkene that could arise by elimination... 

These common features suggest that carbocations are key intermediates m alcohol dehydra tions just as they are m the reaction of alcohols with hydrogen halides Figure 5 6 portrays a three step mechanism for the acid catalyzed dehydration of tert butyl alcohol Steps 1 and 2 describe the generation of tert butyl cation by a process similar to that which led to its for matron as an intermediate m the reaction of tert butyl alcohol with hydrogen chloride... 

In Problem 5 17 (Section 5 13) we saw that acid catalyzed dehydration of 2 2 dimethyl cyclohexanol afforded 1 2 dimethylcyclohexene To explain this product we must wnte a mecha nism for the reaction in which a methyl shift transforms a secondary carbocation to a tertiary one Another product of the dehydration of 2 2 dimethylcyclohexanol is isopropyhdenecyclopentane Wnte a mechanism to rationalize its formation... 

IS reversible with respect to reactants and products so each tiny increment of progress along the reaction coordinate is reversible Once we know the mechanism for the for ward phase of a particular reaction we also know what the intermediates and transition states must be for the reverse In particular the three step mechanism for the acid catalyzed hydration of 2 methylpropene m Figure 6 9 is the reverse of that for the acid catalyzed dehydration of tert butyl alcohol m Figure 5 6... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

Many of the reactions listed at the beginning of this section are acid catalyzed, although a number of basic catalysts are also employed. Esterifications are equilibrium reactions, and the reactions are often carried out at elevated temperatures for favorable rate and equilibrium constants and to shift the equilibrium in favor of the polymer by volatilization of the by-product molecules. An undesired feature of higher polymerization temperatures is the increased probability of side reactions such as the dehydration of the diol or the pyrolysis of the ester. Basic catalysts produce less of the undesirable side reactions. 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

Propylene-Based Routes. The strong acid-catalyzed carbonylation of propylene [115-07-1] to isobutyric acid (Koch reaction) followed by oxidative dehydration to methacrylic acid has been extensively studied since the 1960s. The principal side reaction in the Koch reaction is the formation of oligomers of propylene. Increasing yields of methacrylic acid in the oxydehydration step is the current focus of research. Isobutyric acid may also be obtained via the oxidation of isobutyraldehyde, which is available from the hydroformylation of propylene. The -butyraldehyde isomer that is formed in the hydroformylation must be separated. 

The dehydration of -hydroxy ketones is a closely related reaction. In the case of 5,6-disubstituted 3-ketones, the 6-substituent usually remains in the less stable configuration. With acid catalyzed elimination, prolonged treatment or high concentration may cause epimerization ... 

The initial reaction is probably the acid-catalyzed hydration of the triple bond, followed by dehydration of the 17-hydroxyl group... 

Because of the usefulness of the reaction, a number of ways have been devised for carrying out dehydrations. One method that works particularly well for tertiary alcohols is the acid-catalyzed reaction discussed in Section 7.1. For example, treatment of 1-methylcyclohexanol with warm aqueous sulfuric acid in a solvent such as tetrahydrofuran results in loss of water and formation of 1-methylcydohexene. 

The reaction is an F.1 process and occurs through the three-step mechanism shown in Figure 17.6). As usual for El reactions (Section 11.10), only tertiary alcohols are readily dehydrated with acid. Secondary alcohols can be made to react, but the conditions are severe (75% H2S04,100 °C) and sensitive molecules don t survive. Primary alcohols are even less reactive than secondary ones, and very harsh conditions are necessary to cause dehydration (95% H2S04,150 °C). Thus, the reactivity order for acid-catalyzed dehydrations is... 

Mechanism of the acid-catalyzed dehydration of an alcohol to yield an alkene. The process is an El reaction and involves a carbocation intermediate. 

Diethyl ether and other simple symmetrical ethers are prepared industrially by the sulfuric acid-catalyzed dehydration of alcohols. The reaction occurs by SN2 displacement of water from a protonated ethanol molecule by the oxygen atom of a second ethanol. Unfortunately, the method is limited to use with primary alcohols because secondary and tertiary alcohols dehydrate by an El mechanism to yield alkenes (Section 17.6). 

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