Alcohol acid-catalyzed dehydration

We now have a new problem Where does the necessary alkene come from Alkenes are prepared from alcohols by acid catalyzed dehydration (Section 5 9) or from alkyl halides by dehydrohalogenation (Section 5 14) Because our designated starting material is tert butyl alcohol we can combine its dehydration with bromohydrm formation to give the correct sequence of steps... 

The general features of this elegant and efficient synthesis are illustrated, in retrosynthetic format, in Scheme 4. Asteltoxin s structure presents several options for retrosynthetic simplification. Disassembly of asteltoxin in the manner illustrated in Scheme 4 furnishes intermediates 2-4. In the synthetic direction, attack on the aldehyde carbonyl in 2 by anion 3 (or its synthetic equivalent) would be expected to afford a secondary alcohol. After acid-catalyzed skeletal reorganization, the aldehydic function that terminates the doubly unsaturated side chain could then serve as the electrophile for an intermolecular aldol condensation with a-pyrone 4. Subsequent dehydration of the aldol adduct would then afford asteltoxin (1). 

Zamaraev and Thomas provide a concise summary of work done with a family of classic catalytic test reactions—dehydration of butyl alcohols—to probe the workings of acidic molecular sieve catalysts. This chapter echoes some of the themes stated by Pines and Manassen, who wrote about alcohol dehydration reactions catalyzed by solid acids in the 1966 volume of Advances in Catalysis. 

For the structural optimization of the tricyclic triazolium salt 119 the cw-tricyclic lactam 126 was chosen as the precursor for the synthesis of the tetracyclic triazolium salt 127. The diastereo- and enantiopure y-lactam 126 was synthesized following a procedure reported by Ennis et al. (Scheme 32) (Ennis et al. 1996 Nieman and Ennis 2000). a-Tetralone (124) was a-alkylated with ethyl bromoacetate and subsequently hydrolyzed to the corresponding carboxylic acid. Condensation with (R)-phenylglycinol yielded the lactam 125 as a single stereoisomer. Stereoselective reduction, dehydration of the alcohol, and acid-catalyzed enamine hydrolysis provided the cis-tricyclic lactam 126. The one-pot procedure that had previously been successful in the synthe-... 

Alcohols undergo acid-catalyzed dehydration reactions to form alkcnes. The reaction is accomplished by the formation of a carbocation intermediate. Thus there is the possibility of rearrangement in the process. First, we will take a look at the mechanism of this reaction. 

Because of the instability of 1° carbocations and the conclusion that 1° alcohols undergo acid-catalyzed oxygen exchange by an Sn2 process, there may be some question whether 1° carbocations are true intermediates in elimination reactions. An alternative mechanistic possibility for dehydration of 1° alcohols is an acid-catalyzed E2 mechanism. Narayan and Antal proposed such a mechanism for the specific acid-catalyzed dehydration of 1-propanol with sulfuric acid in supercritical water (Figure 10.44). ... 

Although the dehydration of most alcohols is acid catalyzed, base-catalyzed eliminations can be observed if the elimination of water leads to a conjugated product. For example, equation 10.61 shows a concerted mechanism for the dehydration of 7-methyl-3-methyleneoct-6-en-l-ol (34) to myr-cene A carbanion (Elcb) mechanism has been proposed for the... 

An alcohol can be converted to an alkene by dehydration—that is, by the elimination of a molecule of water from adjacent carbon atoms. In the laboratory, the dehydration of an alcohol is most often brought about by heating it with either 85% phosphoric acid or concentrated sulfuric acid. Primary alcohols are the most difficult to dehydrate and generally require heating in concentrated sulfuric acid at temperatures as high as 180 °C. Secondary alcohols undergo acid-catalyzed dehydration at somewhat lower temperatures. The... 

In this chapter, we looked at alkenes, a compound class characterized by the carbon-carbon double bond. In Chapters 7 and 9, we learned that alkenes are prepared synthetically by elimination reactions of haloalkanes and alcohols, hi this chapter, we examined these reactions in more depth. We saw that the structure of the base determines what products will form in E2 elimination from haloalkanes. Similarly, the stracture of an alcohol undergoing acid-catalyzed dehydration determines what mechanism takes place and how easily it occurs. 

The alkene can be made from an alcohol, via acid-catalyzed dehydration. 

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... 

FIGURE 5 6 The El mecha nism for the acid catalyzed dehydration of tert butyl alcohol... 

You may have noticed that the acid catalyzed hydration of an alkene and the acid catalyzed dehydration of an alcohol are the reverse of each other... 

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... 

As a method for the preparation of alkenes a weakness in the acid catalyzed dehydration of alcohols IS that the initially formed alkene (or mixture of alkenes) sometimes isomenzes under the conditions of its formation Write a stepwise mechanism showing how 2 methyl 1 butene might isomenze to 2 methyl 2 butene in the presence of sulfuric acid... 

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... 

Acid catalyzed dehydration of benzyhc alcohols is a useful route to alkenylben zenes as is dehydrohalogenation under E2 conditions... 

Addition of phenylmagnesium bromide to 4 tert butylcyclohexanone gives two isomeric ter tiary alcohols as products Both alcohols yield the same alkene when subjected to acid catalyzed dehydration Suggest reasonable structures for these two alcohols... 

Citral readily forms acetals by acid-catalyzed addition of alcohols or by the use of trialkoxyorthoformates. Citral dimethyl acetal [7549-37-3] is stable under alkaline conditions, whereas citral is not. Neryl and geranyl nitriles can be made by oximation of citral and dehydration of the intermediate oxime. For instance, geranonitrile [31983-27-4] is made as follows ... 

The acid catalyzed elimination of acylated amines has recently been described and is said to resemble the acid dehydration of alcohols in character ... 

A rather special procedure for the preparation of 21-hydroxy-20-ketopreg-nanes starts with the 17a-ethoxyethynyl-17 -hydroxy steroids described earlier. Free radical addition of ethanethiol to the triple bond, followed by acid-catalyzed hydrolysis and dehydration gives the 20-thioenol ether 21-aldehyde. This can be reduced with lithium aluminum hydride to the C-21 alcohol and then hydrolyzed to the C-20 ketone in the presence of mercuric chloride. The overall yield, without isolation of intermediates, is in the order of 50% ... 

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