Alkenes, dehydration alcohols

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... 

Dehydration Alcohols undergo dehydration (removal of a molecule of water) to form alkenes on treating with a pro tic acid e.g., concentrated H2SO4 or H3PO4, or catalysts such as anhydrous zinc chloride or alumina (Unit 13, Class XI). 

Dehydration is reversible, and in most cases the equilibrium constant is not large. In fact, the reverse reaction (hydration) is a method for converting alkenes to alcohols (see Section 8-4). Dehydration can be forced to completion by removing the products from the reaction mixture as they form. The alkene boils at a lower temperature than the alcohol because the alcohol is hydrogen bonded. A carefully controlled distillation removes the alkene while leaving the alcohol in the reaction mixture. 

For dehydrating alcohols, a concentrated dehydrating acid (such as H2SO4 or H3PO4) is used to drive the equilibrium to favor the alkene. Hydration of an alkene, on the other hand, is accomplished by adding excess water to drive the equilibrium toward the alcohol. 

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. 

You saw in Chapter 19 that elimination reactions can be used to make alkenes from alcohols using acid or from alkyl halides using base. The acid-catalysed dehydration of tertiary butanol works well... 

Phosphorus oxychloride, POCI3 Reacts with secondary and tertiary and alcohols to yield alkene dehydration products (Section 17.6). 

Acids (hydrogen ion) and bases (hydroxide ion) act as homogeneous catalysts for many important organic chemical reactions in solution. These include esterification, ester hydrolysis (see Box 9.2), hydration of alkenes, dehydration of alcohols, and condensation reactions. 

You saw in Chapter 19 that elimination reactions can be used to make alkenes from alcohols using acid or from alkyl halides using base. The acid-catalysed dehydration of tertiary butanol works well because the double bond has no choice about where to form. But the same reaction on s-butanol is quite unselective—as you would expect, the more substituted alkene is formed (almost solely, as it happens) but even then it s a mixture of geometrical isomers. 

Despite the high temperatures required for acetate and xanthogenate pyrolysis, the accessibility of these compounds from the corresponding alcohols provides for a useful route to alkenes. One advantage is that as no acidic or basic reagents are required, rearrangement reactions, which can interfere with alcohol dehydration, are avoided. For example, pyrolysis of lactol acetates has been used to prepare 4,5-dihy-drofurans, as illustrated in equation (58), - and pyrolysis of the corresponding thionocarbonate was used to dehydrate alcohol (128 Scheme 38). 

Dehydration. a-Substituted cinnamic esters are obtained stereoselectively from the benzylic alcohols [an/t-alcohols -> (E)-alkenes iyn-alcohols -> (Z)-alkenes]. 

The relatively harsh conditions (acid and heat) required for alcohol dehydration and the structural changes resulting from carbocation rearrangements may result in low yields of the desired alkene. Dehydration, however, can be carried out under milder conditions by using phosphorus oxychloride (POCI3) and pyridine. 

One of the most common types of elimination is the dehydration of an alcohol. Formally, these reactions are the microscopic reverse of the hydration of an alkene, and therefore we have already covered these reactions (see Section 10.2). However, several points should be stressed here, because dehydrations are commonly used in synthesis. Furthermore, dehydrations and hydrations are important biosynthetic reactions, as many natural products possess alkenes and alcohols. The Connections highlight at the end of this section discusses how enzymes catalyze these reactions. 

Dehydration reactions are important for the synthesis of commodity ethers, such as tetrahydrofuran, which was synthesized from 1,4-butanediol at low yield in NCW. It is also important to note that the reverse reaction can be performed in NCW, although at a greatly reduced yield. For example, the conversion of alkenes to alcohols in NCW has been reported to proceed to < 10% equilibrium conversion. ... 

With the exception of implications regarding solubility, a feature not yet apparent is any recognized trend in the emissions from sulphur cures with variations in the base polymer. This is not the case with peroxide cures, where the reactivity of the polymer can influence both the quantity and type of emissions. A well-studied example is that of NR which carries an abundance of abstractable allylic hydrogens to favour alcohol formations (eqn (29)). Thus when DTOP (R = Me) is the peroxide, fert-butanol (BP 82°C) is obtained, whilst cumyl alcohol (2-phenyl-2-propanol BP 202°C) is obtained from Dicup (R = Ph). Ketone formation (eqn (30)) competes with hydrogen abstraction and can predominate in the presence of a different polymer emissions from formulations based on EPDM, silicone and a fluoroelastomer have been characterized. Other by-products include alkenes from alcohol dehydration, although numerous other reactions can occur. 

In the mechanism of alkene hydration, all the steps are reversible. The proton acts only as a catalyst and is not consumed in the overall reaction. Indeed, without the add, hydration would not occur alkenes are stable in neutral water. The presence of acid, however, establishes an equilibrium between alkene and alcohol. This equilibrium can be driven toward the alcohol by using low reaction temperatures and a large excess of water. Conversely, we have seen (Section 11-7) that treating the alcohol with concentrated acid favors dehydration, especially at elevated temperatures. 

Dehydration. Alcohols have been dehydrated to alkenes with Eaton s reagent (eq 8). In a formal dehydration, a cyclopen-tenone has been transformed into a diene (eq 9). ... 

Having considered the incorporation of alcohols into polyoxoanion frameworks as alkoxide groups, we now turn to their transformation into aldehydes or ketones (oxidation) and alkenes (dehydration). As above, the selection of the polyoxoanion system to be studied was dictated in part by analogies with molybdenum trioxide chemistry. Molybdenum trioxide has a layer structure [21], and the structure of a single M0O3 layer is shown in 16. 

At temperatures of 220-240 C it functions as an efficient, neutral dehydrating agent, amides yielding nitriles and alcohols yielding alkenes. 

Dehydrogenation of alkanes is not a practical laboratory synthesis for the vast majority of alkenes The principal methods by which alkenes are prepared m the labo ratory are two other (3 eliminations the dehydration of alcohols and the dehydrohalo genation of alkyl halides A discussion of these two methods makes up the remainder of this chapter... 

Except for the biochemical example just cited the stractures of all of the alcohols m Section 5 9 (including those m Problem 5 13) were such that each one could give only a single alkene by p elimination What about ehmmahon m alcohols such as 2 methyl 2 butanol m which dehydration can occur in two different directions to give alkenes that are conshtutional iso mers Here a double bond can be generated between C 1 and C 2 or between C 2 and C 3 Both processes occur but not nearly to the same extent Under the usual reachon con dihons 2 methyl 2 butene is the major product and 2 methyl 1 butene the minor one... 

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... 

Write a structural formula for the carbocation intermediate formed in the dehydration of each of the alcohols in Problem 5 14 (Section 5 10) Using curved arrows show how each carbocation is deprotonated by water to give a mixture of alkenes... 

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