Dehydration benzylic alcohols

Section 1115 The simplest alkenylbenzene is styrene (C6H5CH=CH2) An aryl group stabilizes a double bond to which it is attached Alkenylbenzenes are usu ally prepared by dehydration of benzylic alcohols or dehydrohalogena tion of benzylic halides... 

A typical phenol plant based on the cumene hydroperoxide process can be divided into two principal areas. In the reaction area, cumene, formed by alkylation of benzene and propylene, is oxidized to form cumene hydroperoxide (CHP). The cumene hydroperoxide is concentrated and cleaved to produce phenol and acetone. By-products of the oxidation reaction are acetophenone and dimethyl benzyl alcohol (DMBA). DMBA is dehydrated in the cleavage reaction to produce alpha-methylstyrene (AMS). 

Acid-catalyzed dehydration of benzylic alcohols is a useful route to alkenylbenzenes, as is dehydrohalogenation under E2 conditions. 

Acid-catalyzed dehydration (Section 5.9) This is a frequently used procedure for the preparation of alkenes. The order of alcohol reactivity parallels the order of carbocation stability R3C" > R2CH " > RCH2 ". Benzylic alcohols react readily. Rearrangements are sometimes observed. 

Dehydration reactions catalysed by NHC complexes have been reported where a new C-C bond is formed. Peris has used [Ir(OTf)2Cp (NHC)] complexes including compound 35 to benzylate arenes with alcohols and other reagents [14]. For example, the dehydrative C-C coupling of benzyl alcohol 8 with toluene 33 is catalysed by 0.1 mol% of 35 to give a mixture of benzylated products 34 (Scheme 11.8). 

There was also a polymerisation reaction during the sulphuric treatment of benzyl alcohol. The reactor in which the reaction of alcohol dehydration was... 

The procedure given here is essentially that described previously by the submitters2 and is based on the early work of Knoevenagel.8 2-Phenylindazole has been prepared by reduction of N-(o-nitrobenzyl)aniline with tin and hydrochloric acid,4 by reduction of N-(o-nitrobenzyl) -N-nitrosoaniline with tin and hydrochloric acid,5 by dehydration of 2-(phenylazo)benzyl alcohol,6 by elimination of acetic acid from 2-(phenylazo)benzyl acetate,7 by dehydrogenation of 3,3a,4,5,6,7-hexahydro-2-phenyl-indazole with sulfur,8 and by thermal decomposition of o-azido-benzalaniline.9... 

Izumi and Urabe [105] found first that POM compounds could be entrapped strongly on active carbons. The supported POMs catalyzed etherization of ferf-butanol and n-butanol, esterification of acetic acid with ethanol, alkylation of benzene, and dehydration of 2-propanol [105], In 1991, Neumann and Levin [108] reported the oxidation of benzylic alcohols and amines catalyzed by the neutral salt of Na5[PV2Mo10O40] impregnated on active carbon. Benzyl alcohols were oxidized efficiently to the corresponding benzaldehydes without overoxidation ... 

The reactor effluent is distilled and unreacted EB is recycled. The EB hydroperoxide is then reacted with propylene at 250°F and pressure in the range of 250-700 psi in the presence of a metal catalyst to produce propylene oxide and methylbenzyl alcohol B in Figure 8-7). The reactor mixture is separated by multiple fractionators. Unreacted propylene and EB are recycled. PO is recovered overhead. The methyl benzyl alcohol is easily dehydrated in the vapor stage at 450—500° F and 500 psi pressure over a titanium dioxide or silica gel catalyst to form styrene. Acephenone is one of the by-products. 

Benzylic alcohols were also converted to hydrocarbons by sodium borohy-dride [621], by chloroalane [622], by borane [623], by zinc [624], and by hy-driodic acid [225,625], generally in good to excellent yields. Hydrogenolysis of benzylic alcohols may be accompanied by dehydration (where feasible) [622],... 

Problein 14.9 Although benzyl alcohol, C HjCHjOH, is a T alcohol, it undergoes intermolecular dehydration by both S 2 and S l mechanisms. Explain. M... 

The p-acetoxystyrene monomer, precursor of polymer III, is prepared from p-hydroxyacetophenone using the procedure of Corson et al. (14) which involves acetylation of the phenolic group followed by catalytic hydrogenation of the ketone and dehydration of the resulting benzylic alcohol as shown in Scheme 3. 

Benzyl alcohol readily undergoes the reactions characteristic of a primary alcohol, such as esterification and etherification, as well as halide formation. In addition, it undergoes ring substitution. In the presence of acid, polymerization is observed, and the alcohol can be thermally dehydrated to toluene [108-88-3], Catalytic oxidation over copper oxide yields benzaldehyde benzoic acid is obtained by oxidation with chromic acid or potassium permanganate. Catalytic hydrogenation of the ring gives cyclohexylmethanol [100-49-2]. 

Various dehydrating agents—concentrated sulphuric acid, zinc chloride, phosphorus pentoxide—can be used. Sulphuric acid, although perhaps the most convenient, has the disadvantage that it tends to sulphonate the aromatic substances employed. At a low temperature, however, diphenylmethane can be obtained from benzyl alcohol and benzene. At 140° phosphorus pentoxide condenses benzene and diphenylcarbinol to triphenylmethane (see B., 7,1204). Not only substituted benzyl alcohols, but even mandelic acid can be brought within the scope of the reaction, while in place of benzefte its nitro, amino or phenolic derivatives may be used. 

A benzylic alcohol is transformed with active MnC>2 in a benzaldehyde that condenses in situ with ammonia, in the presence of MgS04 as dehydrating agent, delivering an imine that is oxidized to a nitrile with active Mn02. 

Most experimental data are reported on the use of Pd-Ti02 catalysts in the hydrogenation. As equation 24 shows, product distribution is considerably affected by the para substituent. The formation of benzyl alcohols is favorable on nonacidic supports while acidic supports promote hydrogenolysis. Hydrogenolysis can also be avoided under strongly acidic conditions in the presence of ethanol. In this case, the product benzyl alcohol readily undergoes dehydration to form benzyl ethyl ether. 

An improved route to the key intermediate 326 was also developed (165). Namely, 322 was converted to the monoprotected 1,4-dione 327 by sequential addition of the Grignard reagent derived from 2-(2-bromoethyl)-2-methyl-l,3-dioxolane followed by oxidation of the resulting benzylic alcohol with pyridin-ium dichromate (PDC). The ketone 327 was then smoothly transformed to the 2-azadiene 328 by olefination with BAMP. The regioselective addition of n-butyllithium to 328 as before followed by alkylation of the resulting metalloenamine with benzyl A-(2-bromoethyl)-A-methylcarbamate and acid-catalyzed hydrolysis furnished 325, which was converted to the cyclohexenone 326 by base-induced cycloaldolization and dehydration. 

The synthesis is straightforward with available PhLi being used instead of a Grignard. The acylation of the tertiary benzylic alcohol 23 needs mild conditions to avoid dehydration. 

Alkenes can be made by the dehydration of alcohols 2, usually under acidic conditions, the alcohol being assembled by the usual methods. This route is particularly good for cyclic alkenes 3 and those made from tertiary and/or benzylic alcohols as the El mechanism works well then. The same alkene is formed1 from 2 regardless of which side eliminates but 4 gives a 76% yield of an 80 20 mixture of 5 and 6. 

An alternative method for the manufacture of styrene (the oxirane process), uses ethylbenzene that is oxidized to the hydroperoxide and reacts with propylene to give phenylmethylcarbinol (or methyl benzyl alcohol) and propylene oxide. The alcohol is then dehydrated at relatively low temperatures (180 to 400°C) by using an acidic silica gel (Si02) or titanium dioxide (Ti02) catalyst. 

Where rhenium-catalyzed deoxydehydration has attracted a lot of interest, only two reports concerning dehydration catalyzed by rhenium complexes are noteworthy in view of their application on biomass-derived substrates. The first was published in 1996 by Zhu and Espenson and uses MTO as catalyst for the dehydration reaction of various alcohols, either aliphatic or aromatic, to obtain the corresponding olefins. Using MTO in benzene or in the alcohol itself at room temperature after 3 days gives reasonable turnovers and, in the case of benzylic alcohols, good yields. In the same paper, MTO is used for the amination, etherification, and disproportionation of alcohols, which are all reactions interesting in the viewpoint of biomass transformation [123]. 

Chem Systems Inc. proposed a process in which benzyl alcohol obtained by an undisclosed direct oxidation of toluene is homologated with synthesis gas to yield 2-phenylethyl alcohol, which is then readily dehydrated to styrene (57). This process eliminates the intermediate formation of methanol from synthesis gas but does require the independent production of benzyl alcohol. 

The transformation was called an homologation reaction because essentially it consisted in going from one alcohol to an alcohol containing one carbon atom more than the starting material (Wender, Levine, and Orchin, 14). Tertiary alcohols reacted most rapidly, secondary alcohols less rapidly and primary alcohols only very slowly. It was of considerable importance to ascertain whether the olefin intermediate was essential and for this purpose, methanol and benzyl alcohol, neither of which can dehydrate to an olefin, were used in the reaction. Both compounds, contrary to other primary alcohols, reacted quite rapidly and gave the homologous alcohol of the methanol converted, about 40 mole per cent went to ethanol and with benzyl alcohol, a 30% yield of 2-phenylethanol was secured. In both examples, however, reduction products were also present of the methanol converted, 8 mole per cent went to methane and from benzyl alcohol, a 50 to 60% yield of toluene was secured. The conversion of methanol to methane appears to be the only case in which an appreciable quantity of hydrocarbon is secured from a purely aliphatic alcohol. The behavior of benzyl alcohol and its derivatives will be discussed later. 

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