Sulfuric acid alcohol dehydration

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). 

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. 

Phosphoric acid (H3P04) often is used in place of sulfuric acid to dehydrate alcohols. This is because phosphoric acid is less destructive it is both a weaker acid and a less powerful oxidizing agent than sulfuric acid. Dehydration probably proceeds by mechanisms similar to those described for sulfuric acid (Section 15-5C) and very likely through intermediate formation of a phosphate ester ... 

Propose a mechanism for the sulfuric acid-catalyzed dehydration of tert-butyl alcohol. 

Predict the products of sulfuric acid-catalyzed dehydration of the following alcohols. 

Predict the products of the sulfuric acid-catalyzed dehydration of the following alcohols. When more than one product is expected, label the major and minor products. 

Why do you suppose only symmetrical ethers are prepared by the sulfuric acid-catalyzed dehydration procedure What producl(s) would you expect if ethanol and 1-propanol were allowed to react together In what ratio would the products be formed if the two alcohols were of equal reactivity ... 

The dilute sulfuric acid catalyzed dehydration of 2-methyl-2-butanol (r-amyl alcohol) proceeds readily to give a mixture of alkenes that can be analyzed by gas chromatography. The mechanism of this reaction involves... 

These common features suggest that carbocations are key intermediates in alcohol dehydration, just as they are in the conversion of alcohols to alkyl halides. Figure 5.6 portrays a three-step mechanism for the sulfuric acid-catalyzed dehydration of ieri-butyl alcohol. Steps 1 and 2 describe the generation of tert-butyl cation by a process similar to that which led to its formation as an intermediate in the reaction of ieri-butyl alcohol with hydrogen chloride. Step 3 in Figure 5.6, however, is new to us and is the step in which the double bond is formed. 

When hydrochloric acid or hydrobromic acid is the hydrolytic agent the replacement of the alcoholic hydroxyl of tropine respectively by chlorine or bromine has been observed to occur (186), while sulfuric acid causes dehydration to the unsaturated base, tropidine (192a). 

As noted earlier (Section 4.10) primary carbocations are too high in energy to be intermediates in most chemical reactions. If primary alcohols don t form primary carbocations, then how do they undergo elimination A modification of our general mechanism for alcohol dehydration offers a reasonable explanation. For primary alcohols it is believed that a proton is lost from the alkyloxonium ion in the same step in which carbon-oxygen bond cleavage takes place. For example, the rate-determining step in the sulfuric acid-catalyzed dehydration of ethanol may be represented as ... 

Sulfuric acid (H2SO4) and phosphoric acid (H3PO4) are the acids most frequently used m alcohol dehydrations Potassium hydrogen sulfate (KHSO4) is also often used... 

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... 

Perchloric acid Acetic acid, acetic anhydride, alcohols, antimony compounds, azo pigments, bismuth and its alloys, methanol, carbonaceous materials, carbon tetrachloride, cellulose, dehydrating agents, diethyl ether, glycols and glycolethers, HCl, HI, hypophosphites, ketones, nitric acid, pyridine, steel, sulfoxides, sulfuric acid... 

Bisa.codyl, 4,4 -(2-PyridyLmethylene)bisphenol diacetate [603-50-9] (Dulcolax) (9) is a white to off-white crystalline powder ia which particles of 50 p.m dia predominate. It is very soluble ia water, freely soluble ia chloroform and alcohol, soluble ia methanol and ben2ene, and slightly soluble ia diethyl ether. Bisacodyl may be prepared from 2-pyridine-carboxaldehyde by condensation with phenol and the aid of a dehydrant such as sulfuric acid. The resulting 4,4 -(pyridyLmethylene)diphenol is esterified by treatment with acetic anhydride and anhydrous sodium acetate. Crystallisation is from ethanol. 

In the three-step process acetone first undergoes a Uquid-phase alkah-cataly2ed condensation to form diacetone alcohol. Many alkaU metal oxides, metal hydroxides (eg, sodium, barium, potassium, magnesium, and lanthanium), and anion-exchange resins are described in the Uterature as suitable catalysts. The selectivity to diacetone alcohol is typicaUy 90—95 wt % (64). In the second step diacetone alcohol is dehydrated to mesityl oxide over an acid catalyst such as phosphoric or sulfuric acid. The reaction takes place at 95—130°C and selectivity to mesityl oxide is 80—85 wt % (64). A one-step conversion of acetone to mesityl oxide is also possible. 

Ma.nufa.cture. Mesityl oxide is produced by the Hquid-phase dehydration of diacetone alcohol ia the presence of acidic catalysts at 100—120°C and atmospheric pressure. As a precursor to MIBK, mesityl oxide is prepared ia this manner ia a distillation column ia which acetone is removed overhead and water-saturated mesityl oxide is produced from a side-draw. Suitable catalysts are phosphoric acid (177,178) and sulfuric acid (179,180). The kinetics of the reaction over phosphoric acid have been reported (181). 

The typical acid catalysts used for novolak resins are sulfuric acid, sulfonic acid, oxaUc acid, or occasionally phosphoric acid. Hydrochloric acid, although once widely used, has been abandoned because of the possible formation of toxic chloromethyl ether by-products. The type of acid catalyst used and reaction conditions affect resin stmcture and properties. For example, oxaUc acid, used for resins chosen for electrical appHcations, decomposes into volatile by-products at elevated processing temperatures. OxaUc acid-cataly2ed novolaks contain small amounts (1—2% of the original formaldehyde) of ben2odioxanes formed by the cycli2ation and dehydration of the ben2yl alcohol hemiformal intermediates. 

Etherification. Isopropyl alcohol can be dehydrated ia either the Hquid phase over acidic catalysts, eg, sulfuric acid, or ia the vapor phase over acidic aluminas to give diisopropyl ether (DIPE) and propylene (qv). 

The butanols undergo the typical reactions of the simple lower chain aUphatic alcohols. For example, passing the alcohols over various dehydration catalysts at elevated temperatures yields the corresponding butenes. The ease of dehydration increases from primary to tertiary alcohol /-butyl alcohol undergoes dehydration with dilute sulfuric acid at low temperatures in the Hquid phase whereas the other butanols require substantially more stringent conditions. 

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. 

Isopropyl Ether. Isopropyl ether is manufactured by the dehydration of isopropyl alcohol with sulfuric acid. It is obtained in large quantities as a by-product in the manufacture of isopropyl alcohol from propylene by the sulfuric acid process, very similar to the production of ethyl ether from ethylene. Isopropyl ether is of moderate importance as an industrial solvent, since its boiling point Hes between that of ethyl ether and acetone. Isopropyl ether very readily forms hazardous peroxides and hydroperoxides, much more so than other ethers. However, this tendency can be controlled with commercial antioxidant additives. Therefore, it is also being promoted as another possible ether to be used in gasoline (33). 

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