Dehydration alcohols, mechanism

Maleic acid, 338 Maionic ester, 380 Markovnikoff s rule, 96 Mass spectroscopy, 247/T Mechanism, alcohol dehydration, 92 alkane halogenation, 56 benzyne, 217 El and E2, UOff SnI and Sn2, 122 ... 

There is a strong similarity between the mechanism shown m Eigure 5 12 and the one shown for alcohol dehydration m Eigure 5 6 The mam difference between the dehy dration of 2 methyl 2 butanol and the dehydrohalogenation of 2 bromo 2 methylbutane IS the source of the carbocation When the alcohol is the substrate it is the correspond mg alkyloxonmm ion that dissociates to form the carbocation The alkyl halide ionizes directly to the carbocation... 

Like alcohol dehydrations El reactions of alkyl halides can be accompanied by carbocation rearrangements Eliminations by the E2 mechanism on the other hand nor mally proceed without rearrangement Consequently if one wishes to prepare an alkene from an alkyl halide conditions favorable to E2 elimination should be chosen In prac tice this simply means carrying out the reaction m the presence of a strong base... 

These common features suggest that carbocations are key intermediates in alcohol dehydrations, just as they are in 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 desaibe the generation of tert-butyl cation by a process similar- to that which led to its formation as an intermediate in the reaction of tert-butyl alcohol with hydrogen chloride. 

As noted eaiTier (Section 4.10) primary caibocations aie too high in energy to be intennediates in most chemical reactions. If primaiy alcohols don t fonn primary caibocations, then how do they undergo elimination A modification of our general mechanism for alcohol dehydration offers a reasonable explanation. For primary alcohols it is... 

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). 

The Mechanism of Dehydration of Alcohols over Alumina Catalysts Herman Pines and Joost Manassen Complex Adsorption in Hydrogen Exchange on Group VIII Transition Metal Catalysts... 

When proton acids catalyze alcohol dehydration, the mechanism is El. ° The principal process involves conversion of ROH to ROHj and cleavage of the latter to R and H2O, though with some acids a secondary process probably involves conversion of the alcohol to an inorganic ester and ionization of this (illustrated for H2SO4) ... 

An alkene can accept a proton to generate a carbocation in a process that is essentially the reverse of the deprotonation step in the El mechanism for dehydration of an alcohol. 

The Mechanism of Dehydration of Alcohols over Alumina Catalysts... 

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. 

Before discussing the mechanism of dehydration of primary alcohols, it might be worthwhile to consider some of the published results on the dehydration of ethyl alcohol. Chiefly, two products result ethyl ether and ethylene. Most of the discussions over the years have centered around the problem whether ether is formed simultaneously, in-... 

Before discussing the problem of dehydration of primary alcohols over aluminas it is helpful to review what is known about the mechanism of dehydration of alcohols in solution. 

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