Alkenes, dehydration

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... 

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. 

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. 

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. 

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. 

The decarbonylation-dehydration of the fatty acid 887 catalyzed by PdCl2(Ph3P)2 fO.Ol mol%) was carried out by heating its mixture with acetic-anhydride at 250 C to afford the terminal alkene 888 with high selectivity and high catalyst turnover number (12 370). The reaction may proceed by the oxidative addition of Pd to the mixed anhydride[755]. 

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... 

Hydnde shifts often occur during the dehydration of primary alcohols Thus although 1 butene would be expected to be the only alkene formed on dehydration of 1 butanol It IS m fact only a minor product The major product is a mixture of cis and trans 2 butene... 

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... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

Acid catalyzed dehydration of 2 2 dimethyl 1 hexanol gave a number of isomeric alkenes including 2 methyl 2 heptene as shown in the following formula... 

You may have noticed that the acid catalyzed hydration of an alkene and the acid catalyzed dehydration of an alcohol are the reverse of each other... 

We now have a new problem Where does the necessary alkene come from Alkenes are prepared from alcohols by acid catalyzed dehydration (Section 5 9) or from alkyl halides by dehydrohalogenation (Section 5 14) Because our designated starting material is tert butyl alcohol we can combine its dehydration with bromohydrm formation to give the correct sequence of steps... 

As a method for the preparation of alkenes a weakness in the acid catalyzed dehydration of alcohols IS that the initially formed alkene (or mixture of alkenes) sometimes isomenzes under the conditions of its formation Write a stepwise mechanism showing how 2 methyl 1 butene might isomenze to 2 methyl 2 butene in the presence of sulfuric acid... 

Dehydration of 2 2 3 4 4 pentamethyl 3 pentanol gave two alkenes A and B Ozonolysis of the lower boiling alkene A gave formaldehyde (H2C=0) and 2 2 4 4 tetramethyl 3 pentanone Ozonolysis of B gave formaldehyde and 3 3 4 4 tetramethyl 2 pentanone Identify A and B and suggest an explanation for the formation of B in the dehydration reaction... 

When optically pure 2 3 dimethyl 2 pentanol was subjected to dehydration a mixture of two alkenes was obtained Hydrogenation of this alkene mixture gave 2 3 dimethylpentane which was 50% optically pure What were the two alkenes formed in the elimination reaction and what were the relative amounts of each" ... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

Alkenylbenzenes are prepared by the various methods described m Chapter 5 for the preparation of alkenes dehydrogenation dehydration and dehydrohalogenation... 

Addition of phenylmagnesium bromide to 4 tert butylcyclohexanone gives two isomeric ter tiary alcohols as products Both alcohols yield the same alkene when subjected to acid catalyzed dehydration Suggest reasonable structures for these two alcohols... 

The point was made earlier (Section 5 9) that alcohols require acid catalysis in order to undergo dehydration to alkenes Thus it may seem strange that aldol addition products can be dehydrated in base This is another example of the way in which the enhanced acidity of protons at the a carbon atom affects the reactions of carbonyl com pounds Elimination may take place in a concerted E2 fashion or it may be stepwise and proceed through an enolate ion... 

Addition and elimination processes are the reverse of one another in a formal sense. There is also a close mechanistic relationship between the two reactions, and in many systems reaction can occur in either direction. For example, hydration of alkenes and dehydration of alcohols are both familiar reactions that are related as an addition-elimination pair. 

Treatment of a- or P hydroxyacids with sulfur tetrafluoride leads to conver Sion of the carboxylic group into the tiifluoromethyl group, but the hydroxyl group undergoes either fluorination, fluorosulfination, esterification, or dehydration to form esters, ethers, or alkenes The ratio of the products depends on P substitution [209, 210] (equations 103 and 104)... 

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