Dehydration, of alcohols

Dehydration of alcohols over solid catalysts can yield alkenes by intramolecular dehydration, whereas ethers are the product of an intermolecular process. The catalysts used can be acidic or basic solids or bifunctional acid-base materials. Although selective synthesis of any desired product is possible, complications can arise as a result of side-reactions-dehydrogenation and decomposition of the starting alcohol, decomposition and consecutive transformations of intermediates and products (j9-cleavage of carbocations, oligomerization of alkenes). 

In a similar fashion, diols and polyols can be transformed selectively to important chemicals, e. g. carbonyl compounds, dienes and cyclic ethers. Numerous reviews give adequate treatment of the dehydration both of monohydric alcohols [1-11] and of diols and polyols [12-14]. 

An additional specific dehydration, the transformation of methanol to gasoline-range hydrocarbons (MTG process [15,16]) has been running on a commercial scale for a short time. A partial conversion, the methanol-to-olefin process (MTO process [15,16]), might become more important as selective means of obtaining lower olefins. 

Dehydration of alcohols to olefins or ethers can be effected with most solid acid catalysts as well as with solid base catalysts. Solid acids are usuttlly more active than bases. Among acid catalysts, alumina is the most versatile. Metal phosphates, metal 

For metal oxides, Batta et al. noted that dehydration increases with the covalent character of metal-oxygen bond of the catalyst, whereas dehydrogenation is enhanced by increasing the ionic character. Thus, the catalysts can be placed in the following selectivity sequence.  

As described below, dehydration over acidic catalysts generally yields Saytzeff products, while dehydration over basic oxides such as Th02 and Z1O2 yields Hofmann elimination products. Dehydration over strongly basic catalysts such as MgO and CaO is always accompanied by appreciable dehydrogenation. 

Heteropoly acids are highly active for dehydration. The activity of dodecatung-stophosphoric acid is much higher than that of Y-type zeolites. The dehydration with heteropoly acids is unique, since the reaction proceeds not only on the surface of the solid, but also in the bulk of the solid. Because of the pseudo-liquid nature of the dehydration, every proton in the solid heteropoly acids can participate in the reaction. This explains the very high activity of the acids. 

Dehydration is very often accompanied by the subsequent isomerization of primary products. Isomerization may be avoided by poisoning acidic sites with alkali meted ions, ammonia, or organic bases. Cyclohexanol dehydrates to cyclohexene over alumina containing 0.4% sodium or potassium ions, but gives a large amount of methylcy-clopentenes over pure alumina. The cyclopentenes do not arise direcdy from cyclohexanol, but by the isomerization of cyclohexene, the primary product. The selectivity to 3-methyl-l-butene is significantly improved by adding small amounts of base to 7-alumina in the dehydration of 3-methylbutanoL 

Many reactions classified as dehydrogenations occur within the cells of living systems at 25 C. H2 is not one of the products, however. Instead, the hydrogens are lost in separate steps of an enzyme-catalyzed process. The enzyme indicated in the reaction  

Dehydrogenation of alkanes is not a practical laboratory synthesis for the vast majority of alkenes. The principal methods by which alkenes are prepared in the laboratory are two other P eliminations the dehydration of alcohols and the dehydrohalogenation of alkyl halides. A discussion of these two methods makes up the remainder of this chapter. 

A quote from a biochemistry text is instructive here. This is not an easy reaction in organic chemistry. It is, however, a very important type of reaction in metabolic chemistry and is an integral step in the oxidation of carbohydrates, fats, and several amino acids. G. L. Zubay, Biochemistry, 

In the dehydration of alcohols, the H and OH are lost from adjacent carbons. An acid 

Before dehydrogenation of ethane became the dominant method, ethylene was prepared by heating ethyl alcohol with sulfuric acid. 

As illustrated in Equations 10.6-10.8, each of the steps along the reaction pathway is reversible, so an alkene may undergo acid-catalyzed hydration to form an alcohol. In practice, reversal of the dehydration may be avoided by removing the alkene, whose boiling point is always lower than the parent alcohol, from the reaction 

The side reaction depicted in Equation 10.9 is avoided when the formation of the substitution product is reversible. For this reason, sulfuric acid is often used to effect the dehydration of alcohols in the imdergraduate laboratory because the intermediate alkyl bisulfate (Nu = HSO3O ) substitution product readily reionizes to the intermediate carbocation under the reaction conditions (Eq. 10.10). Subsequent loss of a proton then gives the desired alkene. 

Two or more isomeric alkenes may be formed in El dehydration reactions. For example, consider the unsymmetrically substituted carbocation 17, formed from alcohol 16 by protonation and subsequent loss of water (Eq. 10.11). Loss of pro- 

4-Methyl-2-pentanol 4-Methyl-1 -pentene tra i-4-Methyl-2-pentene 

The acidotalyzed dehydration of 4-methyl-2-pentanol (22), which is one of the experiments in this section, is shown in Equation 10.12 and nicely illustrates the preceding principles. Dehydration of 22 produces a complex mixture of isomeric alkenes, including 4-methyl-l-pentene (23), frans-4-methyl-2-pentene (24), cis-4-methyl-2-pentene (25), 2-methyl-2-pentene (26), and 2-methyl-l-pentene (27). 

Other alcohols behave similarly. Secondary alcohols undergo elimination at lower temperatures than primary alcohols. 

HS04 and H3PO4 are very similar in acid strength. Both are much weaker than H2SO4, which is a strong acid. 

In the dehydration of alcohols, the H and OH are lost from adjacent carbons. An acid catalyst is necessary. Before dehydrogenation of ethane became the dominant method, ethylene was prepared by heating ethyl alcohol with sulfuric acid. 

Reaction conditions, such as the acid used and the temperature, are chosen to maximize the formation of alkene by elimination. Sulfuric acid (H2SO4) and phosphoric acid (H3PO4) are the acids most frequently used in alcohol dehydrations. Potassium hydrogen sulfate 

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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... 

The dehydration of alcohols resembles the reaction of alcohols with hydrogen halides (Section 4 7) m two important ways... 

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... 

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... 

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... 

In 1869 Berthelot- reported the production of styrene by dehydrogenation of ethylbenzene. This method is the basis of present day commercial methods. Over the year many other methods were developed, such as the decarboxylation of acids, dehydration of alcohols, pyrolysis of acetylene, pyrolysis of hydrocarbons and the chlorination and dehydrogenation of ethylbenzene." ... 

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. 

The dehydration of alcohols is an important elimination reaction that takes place under acidic rather flian basic conditions. It involves an El mechanism." The function of the acidic reagent is to convert the hydroxyl group to a better leaving group by protonation ... 

Ehminations of HX to give double bonds offer considerable scope for selectivity and choice of reaction conditions. The dehydration of alcohols is the most common example of this class and may be achieved directly or through intermediate derivatives. In most cases, such derivatives are transient species formed in situ, but sometimes e.g. sulfonates, certain other esters and halides) they are isolated and characterized. Eliminations from jS-substituted ketones are very facile. The dehydration of jS-hydroxy ketones has been covered in section V. 

The acid catalyzed elimination of acylated amines has recently been described and is said to resemble the acid dehydration of alcohols in character ... 

Alcohol A (CioHi O) is converted to a mixture of alkenes B and C on being heated with potassium hydrogen sulfate (KHSO4). Catalytic hydrogenation of B and C yields the same product. Assuming that dehydration of alcohol A proceeds without rearrangement, deduce the structures of alcohol A and alkene C. 

Hydrogenation of alkynes to alkenes using the Lindlai catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In tenns of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the sane place where the triple bond was. In tenns of stereoselectivity, only the cis alkene forms. Recall that dehydration and dehydrohalogenation normally give a cis-trans mixture in which the cis isomer is the minor product. 

From 5 the formation of alkene 2 is possible through loss of a proton. However, carbenium ions can easily undergo a Wagner-Meerwein rearrangement, and the corresponding rearrangement products may be thus obtained. In case of the Bamford-Stevens reaction under protic conditions, the yield of non-rearranged olefins may be low, which is why this reaction is applied only if other methods (e.g. dehydration of alcohols under acidic conditions) are not practicable. 

Evidence for the intermediate carhocations in the acid-catalvzed dehydration of alcohols comes from the observation that rearrangements sometimes occur. Propose a mechanism to account for the formation of 2,3-dimethyl-2-butene from 3,3-dimethyl-2-butanol. ... 

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). 

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