Dehydration of alcohols to alkenes

Alcohols can be dehydrated by heating them with a strong acid. For example, when ethanol is heated at 180°C with a small amount of concentrated sulfuric acid, a good yield of ethylene is obtained. 

This type of reaction, which can be used to prepare alkenes, is the reverse of hydration (Sec. 3.7.b). It is an elimination reaction and can occur by either an El or an E2 mechanism, depending on the class of the alcohol. 

Tertiary alcohols dehydrate by the El mechanism, t-Butyl alcohol is a typical example. The first step involves rapid and reversible protonation of the hydroxyl group. 

Ionization (the rate-determining step), with water as the leaving group, occurs readily because the resulting carbocation is tertiary. 

Proton loss from a carbon atom adjacent to the positive carbon completes the reaction. H CH, 

Because the pK of (ROH2) is —2, protonation of an alcohol occurs only with very strong acids—namely, those having a pKa -2. 

If the OH group of an alcohol is made into a good leaving group, alcohols can undergo P elimination and nucleophilic substitution, as described in Sections 9.8-9.12. 

Like alcohols, ethers do not contain a good leaving group, which means that nucleophilic substitution and P elimination do not occur direcdy. Ethers undergo fewer useful reactions than alcohols. 

The dehydrohalogenation of alkyl halides, discussed in Chapter 8, is one way to introduce a n bond into a molecule. Another way is to eliminate water from an alcohol in a dehydration reaction. Dehydration, like dehydrohalogenation, is a P elimination reaction in which the elements of OH and H are removed from the a and P carbon atoms, respectively. 

Dehydration is typically carried out using H2SO4 and other strong acids, or phosphorus oxychloride (POCI3) in the presence of an amine base. We consider dehydration in acid first, followed by dehydration with POCI3 in Section 9.10. 

Use of benzene as solvent in the above reaction gave much lower yields (65-75%). The dehydration proceeded much faster by using CljCCOjH as catalyst. 

The dehydrohalogenation of alkyl halides, discussed in Chapter 8, is one way to introduce a 71 bond into a molecule. Another way is to eliminate water from an alcohol in a dehydration 

Alcohols undergo dehydration in the presence of strong acid to afford alkenes, as illustrated in Equations [1] and [2], Typical acids used for this conversion are H2SO4 orp-toluenesutfonic acid (abbreviated as TsOH). 

Recall from Section 2.6 that p-toluenesulfonic acid is a strong organic acid (pK = -7). 

More shbstirated alcohols dehydrate more readily, giving rise to the following order of reactivity  

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Dehydration of alcohol to alkenes usually does not require superacidic conditions. Surprisingly, however, perfluroalkylcyclopentanols could not be dehydrated with para-toluenesulfonic acid, but alkenes could be isolated with the use of the stronger acid Nafion-H albeit in low yields737 [Eq. (5.282)]. 

Alcohols and phenols are also weak bases. They can be protonated on the oxygen by strong acids. This reaction is the first step in the acid-catalyzed dehydration of alcohols to alkenes and in the conversion of alcohols to alkyl halides by reaction with hydrogen halides. Alkyl halides can also be prepared from alcohols to alkyl halides by reaction with hydrogen halides. Alkyl halides can also be prepared from alcohols by reaction with thionyl chloride or phosphorus halides. 

A1203 (mostly y, in special cases a or q) support, catalyst Claus process, dehydration of alcohols to alkenes and ethers, support of hydrotreating catalysts, support for three-way catalyst... 

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 of alcohols to alkenes may also occur under add-catalysed conditions. Tertiary alcohols undergo elimination more readily than secondary alcohols, which are in turn dehydrated more easily than primary alcohols. Furthermore, where several elimination products are possible, the hydrogen atom that is lost tends to come from the carbon atom bearing the smaller number of hydrogen atoms. However, in more complex molecules this type of reaction may be accompanied by rearrangements. 

Indeed, there have been very few fundamental studies of the fairly important hydration of alkenes and most references to the latter are to be found in the patent literature, which is not too revealing in terms of in depth information. For these reasons, therefore, the greater part of this review is concerned with the dehydration of alcohols to alkenes. 

In the acid-catalyzed dehydration of alcohols to alkenes, which of the following aretrae ... 

A study of the stereochemistry of the dehydration of alcohols to alkenes over metal oxide catalysts utilized the compounds ( )-f/irco-2-butanol-3-di (51) and ( )-en/f/iro-2-butanol-3-di (52). Predict the products expected from each of these reactants for both syn and anti elimination pathways. 

Breaking Carbon-Heteroatom Bond Dehydration. The dehydration of alcohols to alkenes is a particular example of a wide range of fS (or 1,2-) eliminations (103-105). The reaction in the homogeneous phase can be characteristically promoted by Br0nsted acids, but certain Lewis acids (iodine, ZnCl2) may also be used. 

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