Dehydration, alcohol, elimination reactions

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 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 ... 

The use of mesyl chloride for the dehydration of C-11 alcohols has already been mentioned, and mesylates can certainly be intermediates at least in the a-series. The preference for a coplanar trans arrangement is demonstrated by the well-known elimination reactions of tosylates of epimeric 20-alcohols (ref. 185, p. 616), although this does not restrict the usefulness of the reaction, and in some cases (sulfonates of 1 la-alcohols, for example) cw-elimination occurs (ref. 216, p. 293 ref. 224, 225, 226). 

The two most common elimination reactions arc dehydroUalogenalion—the loss of HX from an alkyl halide—and dehydration—(he loss of water from an alcohol. Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base such as potassium hydroxide. For example, bromocvclohexane yields cyclohexene when treated with KOH in ethanol solution. 

Alkenes are generally prepared by an elimination reaction, such as dehydrohalo-genation, the elimination of FIX from an alkyl halide, or dehydration, the elimination of water from an alcohol. 

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

Alcohols form alkenes in an elimination reaction when reacted with dehydrating agents. 

The geometry of the cis-alkylcyclohexanol is favorable for trans elimination since the hydroxyl and the neighboring trans hydrogen are coplanar, but this is not true for the l,i-trans isomer hence the molecular conformation has to flip over, to set the hydroxyl group in the axial position for the trans elimination to occur. This would require a few kilocalories of energy and for frans-lert-butylcyclohexanol it would be more difficult to achieve than for IroMs-methylcyclohexanol. It is, therefore, possible that the trans elimination from a boat conformation, or possibly even an epimerization from the trans to the cis isomer which then undergoes a trans elimination reaction. Such an epimerization was found to occur under conditions of dehydration of certain alcohols over alumina, as will be seen under 1,4-cyclohexanediol. The more facile elimination of the cis-i-tert-butylcyclohexanol system as compared with the trans system in solution was also reported in the literature 63). 

There is a strong parallel between elimination reactions in solution and the dehydration of alcohols over alumina. The trans elimination reactions and the anchimeric assistance of alcohols over aluminas suggest that the dehydration must occur within either the submicroscopical pores, or crevices, or channels of the aluminas. The aluminas therefore must surround the alcohol molecules providing acid sites to act as proton donors or electron acceptors and basic sites to act as proton acceptors or electron donors. For that reason the aluminas seem to act as solvating agents and therefore may be considered as a pseudosolvents for dehydration reactions. 

As has been mentioned previously, one is most likely to find analogies to catalytic reactions on solids with acidic and/or basic sites in noncatalytic homogeneous reactions, and therefore the application of established LFERs is safest in this field. Also the interpretation of slopes is without great difficulty and more fruitful than with other types of catalysts. The structure effects on rate have been measured most frequently on elimination reactions, that is, on dehydration of alcohols, dehydrohalogenation of alkyl halides, deamination of amines, cracking of the C—C bond, etc. Less attention has been paid to substitution, addition, and other reactions. 

An inspection of Table II (26,40-64) shows that, in most cases, the slopes have negative values, some being highly negative. There may be two reasons for these unusual magnitudes. Elimination reactions bring the problem of the division of the reacting molecule into the reaction center and the substituent. When, for example, the dehydration of aliphatic alcohols is studied... 

You will recall that this reaction can also be described as an elimination reaction, i.e. a reaction In which the elements of a simple molecule, such as water, are removed from the organic molecule and not replaced. Concentrated phosphoric acid is preferred to concentrated sulfuric acid when dehydrating alcohols because more side reactions occur with the latter and it tends to lead to charring. 

Alkenes are obtained hy dehydration of alcohols via elimination reactions (see Section 5.4.3), and esters are prepared conveniently by the acid-catalysed reaction of alcohols and carboxylic acids (see Section 5.5.5). 

Alkenes are obtained by the transformation of various functional groups, e.g. dehydration of alcohols (see Section 5.4.3), dehalogenation of alkyl halides (see Section 5.4.5) and dehalogenation or reduction of alkyl dihalides (see Section 5.4.5). These reactions are known as elimination reactions. An elimination reaction results when a proton and a leaving group are removed from adjacent carbon atoms, giving rise to a tt bond between the two carbon atoms. 

The HDO and isomerization reactions were previously described as bimolecular nucleophilic substitutions with allylic migrations-the so-called SN2 mechanism (7). The first common step is the fixation of the hydride on the carbon sp of the substrate. The loss of the hydroxyl group of the alcohols could not be a simple dehydration -a preliminar elimination reaction- as the 3-butene-l-ol leads to neither isomerization nor hydrodehydroxyl at ion (6). The results observed with vinylic ethers confirm that only allylic oxygenated compounds are able to undergo easily isomerization and HDO reactions. Moreover, we can note that furan tetrahydro and furan do not react at all even at high temperature (200 C). 

Type of reaction elimination Reaction condition gas-solid Keywords alcohol, HC1 gas, dehydration, alkene... 

We seem not to be getting closer to 58, but 58 would actually result by dehydration of 62. In real life, the alcohol 62 is turned into the chloride and a double elimination with NaNth gives 58 after acidification.14 Though we saw elimination reactions used to make alkenes in chapter 15, this is the first we have seen to make alkynes. 

The dehydration of alcohols is an example of a wide range of elimination reactions having the following general form (Reaction XXIX). 

Ammonium ions have also been used as templates for creating pockets capable of stabilizing positively charged transition states. For example, antibodies raised against hapten 77 accelerate the dehydration of P-hydroxy ketone 75 to give enone 76. The ammonium group in 77 served as a surrogate for the protonated alcohol formed in the acid-catalyzed elimination reaction, and evidence obtained with... 

Alcohols, like alkyl halides, can undergo elimination reactions to form alkenes (Following fig.). Since water is eliminated, the reaction is also called a dehydration. 

This bimolecular dehydration of alcohols is a type of condensation, a reaction that joins two (or more) molecules, often with the loss of a small molecule such as water. This method is used for the industrial synthesis of diethyl ether (CH3CH2—O—CH2CH3) and dimethyl ether (CH3—O—CH3). Under the acidic dehydration conditions, two reactions compete Elimination (dehydration to give an alkene) competes with substitution (condensation to give an ether). 

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... 

The Nohr-McDonald elimination reaction is a high temperature tertiary alcohol dehydration performed using a high boiling point aromatic solvent with zinc chloride and is described by the author. A second transorber, (I), was prepared by the author using the Nohr-McDonald elimination reaction and subsequently used to prepare the /3-cyclodextrin graft polymer as illustrated in Eq. 1 ... 

Many steroid secondary alcohols are resistant to acid-catalysed dehydration, but can be converted into olefins by elimination reactions of their sulphonate esters, usually the toluene 3 -sulphonate ( tosylate ) or methanesulphonate ( mesylate ). These elimination reactions always accompany attempted nucleophilic substitution of the sulphonate (p. 41), and can be made to predominate by appropriate choice of reaction conditions. Activated alkaline alumina has been used to prepare A - and AH-olehns from the sulphonates of 3 - [131] and i2a-alcohols [132]. In the latter case the naphthalene-i-sulphonate was most effective, and the methanesulphonate distinctly less so, suggesting that steric acceleration occurs with the more bulky naphthalenesulphonate [ 132c]. This view is strengthened by the enhanced efficiency of elimination of a i2a-sulphonate in the presence of a 17a-methyl group, which increases the compression at C(i2) [132b]. The mechanism and stereochemical features of these heterogeneous reactions have not been described in detail. 

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