Mechanism alcohol dehydration with acid

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... 

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. 

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). 

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) ... 

Also other Type B and C series from Table II are consistent with the above elimination mechanisms. The dehydration rate of the alcohols ROH on an acid clay (series 16) increased with the calculated inductive effect of the group R. For the dehydrochlorination of polychloroethanes on basic catalysts (series 20), the rate could be correlated with a quantum-chemical reactivity index, namely the delocalizability of the hydrogen atoms by a nucleophilic attack similar indices for a radical or electrophilic attack on the chlorine atoms did not fit the data. The rates of alkylbenzene cracking on silica-alumina catalysts have been correlated with the enthalpies of formation of the corresponding alkylcarbonium ions (series 24). Similar correlations have been obtained for the dehydrosulfidation of alkanethiols and dialkyl sulfides on silica-alumina (series 36 and 37) in these cases, correlation by the Taft equation is also possible. The rate of cracking of 1,1-diarylethanes increased with the increasing basicity of the reactants (series 33). 

This is the reverse of acid-catalyzed hydration of alkenes discussed previously (Section 10-3E) and goes to completion if the alkene is allowed to distill out of the reaction mixture as it is formed. One mechanism of dehydration involves proton transfer from sulfuric acid to the alcohol, followed by an E2 reaction of hydrogen sulfate ion or water with the oxonium salt of the alcohol ... 

In writing mechanisms for acid-catalyzed dehydration of alcohols, begin with formation of the carbocation intermediate ... 

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 pro-tonate 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. 

The dehydration reaction proceeds via a carbocation mechanism. The three step mechanism starts with the protonation of the alcohol oxygen with a hydrogen ion from sulfuric acid by a Lewis acid-Lewis base reaction. Water departs in the second step leaving a carbocation intermediate. In the final step, a hydrogen ion leaves the adjacent carbon and the double bond forms. 

There seems little doubt that 7r-complexes do not play a role in the ratedetermining step of acid-catalysed dehydration of 1,2-diphenylethanols and related substrates. However, in simple alkyl structures in which carbonium ion stability is lower, rr-complexes may offer an alternate pathway of lower energy. Certainly rr-complexes appear to account for the greater preference for the formation of c/5-but-2-ene rather than the thermodynamically more stable trans isomer from the dehydration of secondary butanol over solid catalysts ". Of course metal cations with a greater variety of orbitals do not really provide appropriate models for assessing the co-ordination properties of a proton. Considering all the available kinetic evidence the most plausible mechanism for dehydration can be described as (197), with route A for tertiary and secondary alcohols and route B for primary and secondary alcohols, viz. 

Let s illustrate all this with an example. The Z allylic alcohol below dehydrates in acid solution to the E diene. We have lots of data on this mechanism, all summarized in the diagrams. You may like to note as well that the product contains no deuterium after dehydration in D2O. 

---