Alcohols dehydration reaction

The mechanism for the dehydration of an alcohol must account for two experimental observations. First, the dehydration reaction requires an add catalyst. Second, the order of reactivity of alcohols decreases in the order 3° 2° 1°. These facts remind us of the substitution reaction of tertiary and secondary alcohols using hydrogen halides, which occurs by an S l process. Only primary alcohols react by an Sj 2 mechanism. 

Tertiary and secondary alcohols undergo add-catalyxed dehydration by an El mechanism primary alcohols are dehydrated by an E2 mechanism. In either mechanism, the first step is the rapid protonation of the lone pair electrons of the oxygen atom to produce an allqfloxonium ion. The acid is represented as HA in the reaction mechanism for the dehydration of tert-huty alcohol shown below. 

After the alcohol has been protonated, an El mechanism occurs in two steps. First, a tertiary alcohol loses water in a first order process to produce a tertiary carbocation. Second, a proton is then rapidly transferred to a Lewis base from a P-carbon atom to the tertiary carbocation. 

Elimination reactions of primary alcohols occurs by an E2 mechanism in an acid-cataly2ed reaction. First, the acid protonates the oxygen of a primary alcohol to give a primary alkyl oxonium ion. Then, water is lost by an E2 mechanism because a primary carbocation is too unstable to form in an El process. This concerted step resembles the reaction of primary alkyl hahdes with a base. The proton of the alkyl oxonium ion is deprotonated by a Lewis base, which is water in the dehydration of alcohols. The electron pair in the C—H bond moves to form a carbon-carbon double bond, and the electron pair of the C—O bond is retained by the oxygen atom. The reaction with ethanol illustrates the process. In both the El process and the E2 dehydration reaction, the acid serves only as a catalyst. It is regenerated in the last step of the reaction. 

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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. 

The prders of the relative atomic catalytic activities of the sites, log(/c,/k,), in the alcohol dehydration reactions were estimated from the data of Malysheva et al. (128) by means of the empirical relationship... 

Alcohol Dehydration Reactions as Chemical Precursors for Coke Formation and Acidity Probes in Tungstated Zirconia Catalysts... 

Electrosynthesis by anodic dissolution of the metal in absolute alcohol appears for many elements to be a promising, inexpensive way to obtain large amounts of metal alkoxides [54]. The process goes smoothly and has good current yields for metals, such as scandium and lanthanides, and early transition metals (Ti, Zr, and Nb) using tetrabutylammonium bromide as an electrolyte. Oxo or hydroxo compounds are obtained for more electropositive metals (Mg and alkaline-earth metals), however, probably as a result of alcohol dehydration reactions. 

In short, the sulfate catalyst has a dual function of intrinsic acid and polar medium. Undoubtedly this is partly responsible for the superior catalytic activity of NiS04, in many reactions, as compared with silica-alumina (covalent insulator) of higher acidity (see Section V). One other aspect which is often overlooked but receiving increased attention is the importance of base sites as well as acid sites on the surface in promoting the selectivity for many alcohol dehydration reactions. This was recently illustrated in the literature 28). This matter will be discussed in detail (see Section V). 

In Section 6.3B, we discussed the acid-catalyzed hydration of alkenes to give alcohols. In the present section, we discussed the acid-catalyzed dehydration of alcohols to give alkenes. In fact, both the alkene hydration and the alcohol dehydration reactions are reversible and represent different directions of the same process. The following equilibrium exists ... 

In addition to being regioselective alcohol dehydrations are stereoselective A stereo selective reaction is one m which a single starting material can yield two or more stereoisomeric products but gives one of them m greater amounts than any other Alcohol dehydrations tend to produce the more stable stereoisomer of an alkene Dehydration of 3 pentanol for example yields a mixture of trans 2 pentene and cis 2 pentene m which the more stable trans stereoisomer predominates... 

As noted earlier (Section 4 10) primary carbocations are too high m energy to be intermediates m most chemical reactions If primary alcohols don t form primary car bocations then how do they undergo elimination s A modification of our general mech amsm for alcohol dehydration offers a reasonable explanation For primary alcohols it is... 

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 reactions can be cataly2ed by bases, eg, pyridine, or by Lewis acids, eg, 2inc chloride. In the case of asymmetric alcohols, steric control, ie, inversion, racemi2ation, or retention of configuration at the reaction site, can be achieved by the choice of reaction conditions (173,174). Some alcohols dehydrate to olefins when treated with thionyl chloride and pyridine. 

Synthetic piae oil is produced by the acid-cataly2ed hydration of mainly a-piaene derived from sulfate turpentine, followed by distillation of the cmde mixture of hydrocarbons and alcohols. The predominant alcohol obtained is a-terpiueol, although under the usual conditions of the reaction, reversible and dehydration reactions lead to multiple hydrocarbon and alcohol components (Fig. 1). 

The reaction gives poor yields of ethers with secondary and tertiary alcohols dehydration to form the corresponding olefin is a more favorable reaction. The reaction fails for the production of diaryl ethers from phenols. 

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. 

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