Alcohols, general dehydration

The dehydration of highly fluonnated alcohols generally requires phosphorus pentoxide or sulfuric acid, but hexafluoroisobutyl alcohol is easily converted to hexafluoroisobutylene by potassium hydroxide with or without solvent at 20-... 

Ions of the later transition metals such as Pt+ may not form [MO]+ ions with water and alcohols as shown in Table I for the reaction of Pt+ with methanol, where the formation of Pt+-CO or Pt+-H2 ions are preferred (102). As previously mentioned, Cr+ and Mn+ appear to be much less reactive than any of the other transition metals. The Cr+ ion was reported to be unreactive to primary alcohols but initiated dehydration of branched-chain alcohols it was also described as being unreactive toward propanal and acetone (9). The Mn+ ion has received scant attention due to its reduced activity. The reactions of Fe+, Co+, and Ni+ with alcohols, ethers, aldehydes, and ketones have been extensively covered (9). These ions are more reactive than Cr+ and Mn+ and generally react with alcohols causing dehydration. 

In general, dehydration means loss of water molecules from chemical substances, irrespective of their structure. Even if all cases where water is bonded in hydrate form are excluded, a number of reactions remain which also include formation of nitriles from amides, lactones from hydroxy acids etc. However, the present treatment will concentrate on the heterogeneous catalytic decomposition of alcohols in the vapour phase, which can be either olefin-forming or ether-forming reactions, and on the related dehydration of ethers to olefins. 

Tertiary alcohols are dehydrated instantaneously by sulfurane (1) at room temperature. Most secondary alcohols are also dehydrated rapidly at room temperature. There is evidence for a preferred (rnni-diaxial disposition of leaving groups. Thus cis-4-i-butylcyclohexanol is converted into 4-/-butylcyclohexene at least 150 times more rapidly than the /rons-isomer. Primary alcohols (ROH) are generally not dehydrated but react rapidly and quantitatively with (I) at - 50 to give ethers, R OR dehydration occurs only if the /S-proton is sufficiently acidic. 

Tertiary alcohols (with the exception of the ones containing strong electron withdrawing groups, such as CF3) generally dehydrate very fast in the acid media, and the intermediate protonated species cannot be observed, even at low temperatures before cleavage. 

The mechanism of the Leuckart-Wallach reaction has been the subject of much discussion and is generally accepted to proceed via the reaction of the amine with the carbonyl functionality to give an a-amino alcohol, which dehydrates to give an iminium ion.3 The reduction of this iminium is effected by formic acid.6-8... 

Water can be removed from alcohols by dehydrating agents either directly or indirectly, but the analogous removal of ammonia or an amine from primary, secondary, or tertiary amines is not generally feasible. The latter type of reaction succeeds only in individual cases when, occasionally, an olefin is formed by loss of, e.g., ammonium chloride from the salt of an amine with a hydrohalogen acid ... 

The conversion of alcohols on carbon catalysts has been studied most extensively [77-84]. In general, both dehydration and dehydrogenation products are formed simultaneously. With secondary alcohols, the dehydration activity has been found to result from the presence of carboxyl groups, while dehydrogenation depends on the simultaneous presence of Lewis acid and basic sites [80], as shown in Figures 6.4 and 6.5. 

In general, alcohols undergo dehydration to olefins and ethers over solid acid catalysts, and dehydrogenation to aldehydes and ketones over solid base catalysts. However, certain solid base catalysts promote dehydration in which the mechanisms and product distribution differ from those for acid-catalyzed dehydration. 

In general, these alcohol-based dehydrative A-alkylation reactions can be classified into two categories. One is the direct nucleophilic substitution of the hydroxy group by amines/amides mediated/catalyzed by Bronsted acids, Lewis acids, or TM complexes via formation of carbocation or coordinated cationic metal complexes under acidic conditions (Scheme 2) [16-20]. The famous Ritter reaction of nitriles and alcohols giving alkylated amides may be classified as one of these reactions, in which the nitriles serve as the A-nuleophile (Eq. 2) [21, 22]. 

TM-firee A(-aIkylation reactions were first reported more than 100 years ago [4]. More reports with other methods appeared later [5-9], but these methods require rather harsh reaction conditions. Hence TM-catalyzed methods attracted much more attention and developed rapidly thereafter. It had previously been generally held that TM catalysts and dehydrogenative alcohol activation via formation of [MH]/[MH2] species was indispensible in alcohol-based dehydrative alkylation reactions. The fact that TM-free catalysts can also be used in the same transformations seems to have been overlooked in recent decades. 

FIGURE 19.73 The acid-catalyzed aldol condensation of acetone. The first product, diacetone alcohol, is generally dehydrated in acid to give 4-methyl-3-penten-2-one. 

Dehydration of Alcohols. The title reagent (1) is useful for the dehydration of alcohols. In general, tertiary alcohols are dehydrated instantaneously at rt. Some secondary alcohols are dehydrated. In cyclohexane rings, a tran -diaxial orientation of the leaving groups significantly increases the rate of elimination (eq 1). Primary alcohols do not yield products of dehydration unless the (3-proton is sufficiently acidic. In most cases, the ether [(CF3)2PhCOR] is obtained. ... 

Amylene is a general name for the ethylenic hydrocarbons of the molecular formula CjHio. Two of these hydrocarbons are the main products of the dehydration of the appropriate amyl alcohols ... 

As noted earlier (Section 4 10) primary carbocations are too high m energy to be intermediates m most chemical reactions If primary alcohols don t form primary car bocations then how do they undergo elimination s A modification of our general mech amsm for alcohol dehydration offers a reasonable explanation For primary alcohols it is... 

In general, the reactions of the perfluoro acids are similar to those of the hydrocarbon acids. Salts are formed with the ease expected of strong acids. The metal salts are all water soluble and much more soluble in organic solvents than the salts of the corresponding hydrocarbon acids. Esterification takes place readily with primary and secondary alcohols. Acid anhydrides can be prepared by distillation of the acids from phosphoms pentoxide. The amides are readily prepared by the ammonolysis of the acid haUdes, anhydrides, or esters and can be dehydrated to the corresponding nitriles (31). 

Trifluoromethanesulfonic acid is miscible in all proportions with water and is soluble in many polar organic solvents such as dimethylformamide, dimethyl sulfoxide, and acetonitrile. In addition, it is soluble in alcohols, ketones, ethers, and esters, but these generally are not suitably inert solvents. The acid reacts with ethyl ether to give a colorless, Hquid oxonium complex, which on further heating gives the ethyl ester and ethylene. Reaction with ethanol gives the ester, but in addition dehydration and ether formation occurs. 

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