Dehydration of butyl alcohol

Butyl Ether. -Butyl ether is prepared by dehydration of -butyl alcohol by sulfuric acid or by catalytic dehydration over ferric chloride, copper sulfate, siUca, or alumina at high temperatures. It is an important solvent for Grignard reagents and other reactions that require an anhydrous, inert medium. -Butyl ether is also an excellent extracting agent for use with aqueous systems owing to its very low water-solubiUty. 

Although theoretical and computational advances now afford powerful insights into the mechanisms of heterogeneous catalysis, especially on acidic, zeolitic solids (6a-d), experimental studies (7, 8) still hold sway. This we hope to demonstrate here by reference to the wide range of techniques—spectroscopic, kinetic, and analytical—that we have brought to bear in our studies of the catalytic dehydration of butyl alcohols. 

Dehydration of butyl alcohols over HZSM-5 and AAS acid catalysts provides unique opportunities to elucidate experimentally how confinement of the reagents, intermediates, and products inside the pores of the catalyst influences the reaction pathways. Indeed, in both HZSM-5 and AAS the dehydration reaction proceeds over the same active site, viz. 

Fig. 6. Rate constants for dehydration of butyl alcohols into butene over HZSM-5 under steady-state conditions and subsequent desorption of butene when the helium-alcohol flow is switched to pure helium flow, (a) rec-Butyl alcohol at 369 K ( ) sample I (A) sample 2 (O) sample 4. (b) Isobutyl alcohol at 397 K ( ) sample 2. The horizontal lines on the extreme right denote the reaction rate constants after returning to the previous helium-butyl alcohol flow.

Thus, reaction intermediate r/// for dehydration of butyl alcohols can exist in two forms, i.e., butyl silyl ether (BSE) and adsorbed butyl carbenium ion (BC1). Our NMR and kinetic data imply the existence of reversible transformations between BSE, BCI, and adsorbed butene (BuadJ that ar shown in Scheme... 

Zamaraev and Thomas provide a concise summary of work done with a family of classic catalytic test reactions—dehydration of butyl alcohols—to probe the workings of acidic molecular sieve catalysts. This chapter echoes some of the themes stated by Pines and Manassen, who wrote about alcohol dehydration reactions catalyzed by solid acids in the 1966 volume of Advances in Catalysis. 

Oxirane Process. In Arco s Oxirane process, tert-huty alcohol is a by-product in the production of propylene oxide from a propjiene—isobutane mixture. Polymer-grade isobutylene can be obtained by dehydration of the alcohol. / fZ-Butyl alcohol [75-65-0] competes directly with methyl-/ fZ-butyl ether as a gasoline additive, but its potential is limited by its partial miscibility with gasoline. Current surplus dehydration capacity can be utilized to produce isobutylene as more methyl-/ fZ-butyl ether is diverted as high octane blending component. 

Since approximately 2.2 lb of /-butyl alcohol would be produced per 1 lb of propylene oxide, an alternative reactant in this method is ethylbenzene hydroperoxide. This eventually forms phenylmethylcarbinol along with the propylene oxide. The alcohol is dehydrated to styrene. This chemistry was covered in Chapter 9, Section 6 as one of the syntheses of styrene. Thus the side product can be varied depending on the demand for substances such as /-butyl alcohol or styrene. Research is being done on a direct oxidation of propylene with oxygen, analogous to that used in the manufacture of ethylene oxide from ethylene and oxygen (Chapter 9, Section 7). But the proper catalyst and conditions have not yet been found. The methyl group is very sensitive to oxidation conditions. 

Addition of water to an unsymmetrical alkene follows Markovnikov s rule. The reaction is highly regiospecific. According to Markovnikov s rule, in the addition of water (H—OH) to alkene, the hydrogen atom adds to the least substituted carbon of the double bond. For example, 2-methylpro-pene reacts with H2O in the presence of dilute H2SO4 to form t-butyl alcohol. The reaction proceeds via protonation to give the more stable tertiary carbocation intermediate. The mechanism is the reverse of that for dehydration of an alcohol. 

The steric requirements of the surface during the formation of the adsorption complex or transition state also manifest themselves in the dehydration of rigid alcohols with fixed conformations, e.g. of cyclic alcohols. Cis- and trans-2- and 4-alkylcyclohexanols differ markedly in their rate of dehydration on alumina (see Table 5). Most significant are the data on 4-tert-butylcyclohexanols where the bulky ferf-butyl group is in an equatorial position, and thus the differences in the reactivity of the cis and trans isomers indicate the differences in the reactivity of axial and equatorial hydroxyls. The high reactivity of cis-2-terf-butylcyclohex-anol is caused most probably by steric acceleration of the elimination, which is, however, absent in the case of 2,2-dimethylcyclohexanol. 

Fourier-transform IR spectra show that the main products of butyl alcohol dehydration, when they are adsorbed on HZSM-5 in quantities smaller than or equal to the number of the active sites, are water and butene oligomers, the... 

For all four alcohols in the zeolitic catalysts with small enough crystallite sizes—when diffusion limitations also disappear—dehydration kinetics are well approximated by the exponental function, a fact that is explicable in terms of the unimolecular decay of molecules of butyl alcohol adsorbed on identical active sites. With isobutyl alcohol, for example, the rate coefficient k may be written... 

For instance, the activation energy for butene formation from n-butyl alcohol is 140 lOkJmol-1 on HZSM-5 and only 95 lOkJmol-1 on AAS. At 378 K, 94% ether plus 6% butene are formed over HZSM-5, whereas 43% ether and 57% butene are formed over AAS. Bearing in mind that butyl alcohol molecules, as well as those of intermediates and products of their dehydration, have dimensions closely similar to the diameter of the zeolite channels, we infer that a liquid-like packing of butyl alcohol molecules and other reaction participants occurs in the channels (as schematized in Fig. 5). We opine that some specific ordering of the adsorbed species in the catalyst channels may be induced by hydrogen bonding and hydrophobic interactions between them. 

The overall reaction scheme for dehydration is identical for all butyl alcohols and for both our catalysts, but the relative amounts of various reaction intermediates and the relative values of the rate coefficients for various reaction steps can be dramatically different. Thus, for a given temperature and given catalyst loading in the reactor, and for a given gas-flow rate through the reactor and concentration of butyl alcohol in the gas flow, the observed reaction rates and selectivities with respect to various reaction products can be crucially different for different butyl alcohols and different catalysts (i.e., crystalline HZSM-5 or AAS). 

Note that the rate coefficients k determined by our kinetic studies with the static FTIR reactor for all four butyl alcohols are the true rate coefficients for the forward step of stage II of Scheme 1, i. e., k = k+//. But under the steady-state conditions of the flow microreactor, the observed reation rate, Wbuoh, of butyl alcohol dehydration is less than or equal to the product (k+//N) of the rate... 

We have characterized (8e-i) this intermediate using solid-state 13C CP/MAS NMR, 2H NMR, and two dimensional (2D). /-resolved 3C NMR spectroscopy in the course of dehydration of isobutyl alcohol and /er/-butyl alcohol in HZSM-... 

It is interesting that, for reaction mixtures consisting of molecules with dimensions close to the cross-sections of the catalyst pores (as in the case of butyl alcohol dehydration in HZSM-5), the reacting mixture may be envisaged as a liquid with dimensions less than three. This, in turn, introduces additional factors with respect to the unanalyzed peculiarities of mass-transfer kinetics in the catalyst pores. 

The dehydration of ditertiary alcohols in the presence of hydrobromic acid may lead to dienes (e.g. pinacol to 2,3-dimethylbuta- 1,3-diene, cognate preparation in Expt 5.12), although in this case some concomitant rearrangement to t-butyl methyl ketone (pinacolone, Expt 5.98) occurs under the acidic conditions employed. 

Dehydration of either alcohol yields 4-ferf-butyl-l-phenylcyclohexene. 

Problem 17.1 (a) Give all steps of a likely mechanism for the dehydration of an alcohol to an ether, (b) Is this the only possibility Give all steps of an alternative mechanism. Hint See Sec. 14.16.) (c) Dehydration of w-butyl alcohol gives -butyl ether. Which of your alternatives appears to be operating here ... 

Isobutylidenediurea is used as a slow-release fertilizer. If there is a surplus of isobutyraldehyde, recent work has shown that it can be decarbonylated to propylene over a palladium on silica catalyst.220 There is also the possibility of dehydrogenation to methacrolein for conversion to methaciylic acid, then to methyl methacrylate. Dehydration of isobutyl alcohol would produce isobutylene for conversion to methyl-ferf-butyl ether, although this would probably be uneconomical. 

It is interesting to note that the cryoscopic factor of 6 for this reaction is close to the factor of 5.6 obtained (136) for J-butyl alcohol, indicating that with this compound a similar reaction may have occurred subsequent to dehydration of the alcohol. 

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