Dehydration reactions of alcohols

We studied the mechanism for dehydration of alcohols to alkenes in Section 7-10 together with other syntheses of alkenes. Dehydration requires an acidic catalyst to proton ate the hydroxyl group of the alcohol and convert it to a good leaving group. Loss of water, followed by loss of a proton, gives the alkene. An equilibrium is established between reactants and products. 

Dehydration results from E1 elimination of the protonated alcohol Step I Protonation converts the hydroxyl group to a good leaving group. 

Let s review the utility of dehydration and give guidelines for predicting the products  

Dehydration usually goes by the E1 mechanism. Rearrangements may occur to form more stable carbocations. 

Dehydration works best with tertiary alcohols and almost as well with secondary alcohols. Rearrangements and poor yields are common with primary alcohols. 

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The formation of the more stable alkene is the general rule (Zaitsev s rule) in the acid-catalyzed dehydration reactions of alcohols. 

Various reviews have appeared in the past dealing with the dehydration reaction of alcohols (27, 28, 337-339). The elimination of water from aliphatic alcohols on alumina is known to proceed through two possible routes, namely, monomolecular olefin formation,... 

Reactions of Alcohols with Phosphorus Halides 484 Reactions of Alcohols with Thionyl Chloride 485 11-9 Mechanism 11-3 Reaction of Alcohols with PBr3 485 11-10 Dehydration Reactions of Alcohols 487... 

As pointed out above, many studies on TOP "decomposition" emphasize the influence of water formed in a dehydration reaction of alcohol present either as solvent or as traces. Such a chain mechanism involving a first alcohol molecule producing one water molecule which, by the hydrolysis reaction, gives two additional alcohol molecules has been shown by Bradley and al. studying zirconium alkoxides [11]. Curiously, such a "hydrolytic" decomposition of titanium derivatives has not been taken into account in the most recently reported studies on CVD experiments from TOP [8-10]. 

Xu Q, Li Q (2013) Recent advances of transition metal-catalyzed aerobic dehydrative reactions of alcohols and amines and related researches. Chin J Org Chem 33(l) 18-35... 

The acid catalysis of heteropoly compounds in the solid state is classified into bulk-type and surface-type reations. The former type reactions proceed in the catalyst bulk and the latter only on the surface. Dehydration reactions of alcohols belong to the former and isomerization of butene to the latter. So the classification is closely related to the adsorption property of reactants. The activities for the surface-type reactions are more sensitive to pretreatment. 

In some reactions in which either the C—O or the O—H bond breaks, a C—H bond also breaks. That C—H bond may be on the P carbon atom or on the carbon atom bearing the hydroxyl group. The dehydration reaction of alcohols results in the cleavage of both the C—O bond and the C—H bond of the P carbon atom. We discussed this process, a P-ehmination reaction that results in net dehydration, in Section 9.20. 

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... 

The dehydration of alcohols resembles the reaction of alcohols with hydrogen halides (Section 4 7) m two important ways... 

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... 

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... 

Dehydration to olefins, which sometimes accompanies the reaction of alcohols with DAST [95, 108], is seldom as extensive as with a-fluoroamines (FAR and 1,1,2,3,3,3 hexafluoropropyldiethylamine) but occurs in a few cases to the exclusion of fluonnation, thus, 9a-fluoro-11-hydroxysteroids give 9a fluoro-A -steroids [127, 128] Dehydration accompanied by Wagner-Meerwein rearrangement occurs during the fluonnation of testosterone [129] Intermolecular dehydration to form ethers in addition to fluorides is observed in the reaction of benzhydryl alcohols [104] (Table 6)... 

We ve already seen several reactions of alcohols—their conversion into alkyl halides and tosylates in Section 10.6 and their dehydration to give alkenes in Section 7.1—although without mechanistic details. Let s now look at those details. 

A third important reaction of alcohols, both in the laboratory and in biological pathways, is their dehydration to give alkenes. The C-0 bond and a neighboring C—H are broken, and an alkene tt bond is formed. 

These experiments demonstrate that the surface-catalyzed 2 reaction is far more effident than either the or C pathway for the dehydrative coupling of alcohols over the solid add catalysts tested. High selectivity to configurationally inverted chiral ethers ensues, espedally in the case of the HZSM-5 catalyst, in which the minor C or C paths were further suppressed by "bottling" of 3-ethoxypentane by the narrow zeolite chaimels. 

There was also a polymerisation reaction during the sulphuric treatment of benzyl alcohol. The reactor in which the reaction of alcohol dehydration was... 

In another example, a catalytic amount of a Bronsted acid such as H3PO4 was dissolved in [NRR 3][NTf2] (R = hexyl, R = butyl) 119). The catalyst was applied for the condensation reaction of alcohols, which usually requires strongly acidic media and dehydrating conditions. The condensation of veratryl alcohol was facilitated because the water that formed was continuously removed as vapor, which assisted in driving the reaction to high yields. The product (cyclotriveratrylene) separation, however, required the addition of a co-solvent. 

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,... 

Alkenes are obtained hy dehydration of alcohols via elimination reactions (see Section 5.4.3), and esters are prepared conveniently by the acid-catalysed reaction of alcohols and carboxylic acids (see Section 5.5.5). 

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