Dehydration of secondary alcohols

To circumvent the need for strong acid and allow the dehydration of secondary alcohols, reagents have been developed that are effective under mild, basic conditions. One such reagent, phosphorus oxychloride (POCI3) in the basic amine solvent pyridine, is often able to effect the dehydration of secondary and tertiary alcohols at 0 °C. 

Rate Constants and Activation Energies of Dehydration of Secondary Alcohols over... 

Fig. 3. Correlation of the slopes p for the dehydration of secondary alcohols on various catalysts (series 3-6) with independently measured heats of adsorption of water and diethyl ether, sensitivity to pyridine poisoning (41), and deuterium kinetic isotope effects (68). [Reprinted with permission from Berdnek and Kraus (13, p. 294). Courtesy Elsevier Scientific Company.]...

The relation between the acid strength of the catalysts and the mechanism has also been demonstrated by correlations [55,123] of the reaction parameter, p, of the Taft equation for the dehydration of secondary alcohols on A1203 + NaOH, Zr02, Ti02 and Si02 (see Table 4) with the sensitivity to pyridine poisoning, the heat of adsorption of water and diethylether and the kinetic isotope deuterium effects (Table 3) on the same catalysts (Fig. 5). The parameter p reflects the mechanism being... 

Fig. 5. Correlation of the Taft reaction parameter for the dehydration of secondary alcohols (see Table 4) on four different oxide catalysts with the heat of adsorption, A//ads> °f water and diethylether, with the sensitivity of the rate to pyridine poisoning 7> [55] and with the value of the deuterium kinetic isotope effect [123] for the same catalysts.

Oppenauer, R. V. Dehydration of secondary alcohols to ketones. I. Preparation of sterol ketones and sex hormones. Reel. Trav. Chim. Pays-Bas 1937, 56,137-144. 

Dehydration of secondary alcohols. Hutchins et a .1 attempted to convert irans-4-tm-butylcyclohexanol into the corresponding civ-iodide by treatment with MTPI in HMPT at room temperature. Instead, they obtained 4-ieri-butylcyclohexene in 88 % yield. They then found that secondary alcohols in general are dehydrated by treatment with a twofold excess of MTPI in HMPT at 25-75° for 0.25-25 hr. Primary alcohols are converted into the corresponding iodide in excellent yield under these conditions tertiary alcohols are practically inert. Dehydration apparently involves initial conversion into the corresponding inverted iodide followed by dehydrohalogenation induced... 

Verley, A., Bull. soc. chim. France, Sur 1 change de groupements fonctionnels entre deux mol cules. Passage des c tones aux alcohols et inversement. 37, 537,1925. Oppenauer, R. V., Dehydration of secondary alcohols to ketones. I. Preparation of sterol ketones and sex hormones. Reel. Trav. Chim. Pays-Bas, 56, 137,1937. 

Attempts to synthesize ethers by intermolecular dehydration of secondary alcohols are usually unsuccessful because alkenes form too easily. 

Poisoning effects on catalytic reactions have been frequently attempted to reveal the nature of the active sites. The results differ depending on the reactions and reaction conditions, and sometimes between investigators. However, the following may be stated in general. Both acidic and basic poisons exhibit retarding effects in dehydration of alcohols. This poisoning experiment as well as the presence of deuterium isotope effect and stereospecific anti elimination" led to the conclusion that dehydration of secondary alcohols proceeds concertedly on pair sites of acid and base. As for the acid sites in this mechanism, some consider Lewis acid sites and others assume surface OH sites. ... 

The effect of the substituents on the dehydration of secondary alcohols of the general structure over alumina was studied by Dautzenberg and Knozinger and the ratios of 2-olefins and 1-olefins in the products (S21) determined. ... 

It is generally accepted that degradation of epoxy resins starts by dehydration of secondary alcoholic groups followed by homolytic scission of the formed allylic bond [34, 35]. There have been various studies carried out for the brominated epoxy resin treatment. In the study of Balabanovich [33], brominated epoxy resin produces gases and oil as pyrolysis products at the temperature of about 100 °C. However, these pyrolysis volatiles are contaminated by brominated phenols, brominated alkanes, and HBr, which is a difficult point for pyrolysis of brominated epoxy resin. 

In addition, trifluoracetic anhydride has been a useful acid catalyst for the esterification of phenols. The use of heavy metal salts as effective esterification catalysts has been widely reported. For example, BuSnOjH and BujSnO are equal or superior to p-MeC6H4S03H as esterification catalysts and do not effect appreciable dehydration of secondary alcohols. It has been reported that catalytic activities generally descend in the order Sn, Co" , Fe" , Al, Bi, Cr, Sn", Cu, Co Pb, Fe+ Zn, Ni, Mn, Cd, Mg, Ba, K, and CIO4, SO4, PhSOj, Cl, Zr, I, NO3. The use of 1 wt% of Fe2(S04)3 is recommended as a noncorrosive catalyst in steel. 

Dehydration of secondary alcohols usually occurs by El, but it can occur by E2. Ethanol, a primary alcohol, undergoes dehydration by a concerted E2 reaction, as shown below ... 

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