Anhydrous copper(II)sulfate: an efficient catalyst for the liquid-phase dehydration of alcoholsJ. Org. Chem. 45(5), 917-919 (1980)
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https://www.thevespiary.org/rhodium/Rhodium/pdf/cuso4.dehydration.pdf)
A fundamental organic transformation for which a great number of methods have been reported is the conversion of alcohols to olefins. New procedures and reagents appear regularly, each of which offers certain advantages over other methods.
[2] We wish to report that anhydrous copper(II)sulfate, prepared by heating copper(II)sulfate pentahydrate at 200-300°C for 2 days, serves as an effective catalyst for the dehydration of secondary, tertiary, benzylic, and allylic alcohols to the corresponding olefins. We find that it is a valuable alternative to other dehydration reagents and works for a variety of alcohol types.
Of greatest note is the extreme simplicity of the method. The neat alcohol and the solid catalyst are heated at an appropriate temperature (vide infra), and the olefin distills from the reaction mixture. Dehydration occurs in the liquid phase so that gas-phase flow systems are unnecessary, the conditions are fairly mild (100-160°C), and the time required is relatively short (0.5-15 h). Furthermore, the olefin product is uncontaminated with byproducts other than the evolved water; filtration through a cotton pad gives pure product.
Experimental Guidelines. Anhydrous copper(II)sulfate can be prepared in large batches and stored indefinitely in a desiccator. It was found that an alcohol:catalyst ratio of 1:0.75 gave the best results. The temperature required for smooth dehydration varies with the alcohol structure - the reaction temperatures found to be best were about 120-130°C for the tertiary, benzylic, and allylic types and 160-180°C for secondary alcohols. Higher temperatures lead to somewhat decreased yields in cases of easily polymerized olefins.
The olefin product was collected at atmospheric pressure or under reduced pressure for olefins with boiling points greater than ~120°C. The pressure was chosen so that the reaction temperature was at or near the boiling point of the starting alcohol, allowing smooth distillation of the olefin from the reaction mixture.
In addition, the dehydrations of 1-phenylethanol and 1-phenylpropanol lead to side products uniquely indicative of carbenium ion intermediates. Heating either of these alcohols with anhydrous copper(II)sulfate yields first the bis ether
1, which then undergoes smooth cracking to the olefin. Furthermore, examination of the reaction residue reveals that the major byproduct is a styrene dimer,
2.
The exact catalytic species is not known at this time, nor is the role of copper understood. When very small amounts of pyridine are added to the reaction mixture, nearly complete destruction of the catalytic activity follows. It is therefore possible that in the preparation of anhydrous copper(II) sulfate, small amounts of strong, protic acids are formed which are responsible for the catalytic activity.
What is notable is that anhydrous copper(II)sulfate is a general dehydration catalyst that equals or surpasses other common catalysts in yields and is often more efficient (lower temperatures and/or shorter times).[6] For example, beta-methylstyrene is produced in 69% yield in the present work while potassium bisulfate gives 55-60% yields.[7]Experimental SectionAnhydrous copper(II) sulfate was prepared by heating CuSO
4.5 H
2O (ACS grade) in a porcelain dish at 275°C for 2 days. The off-white solid wm stirred several times during heating to help break up the lumps and was stored in a desiccator.
General Dehydration Procedure The alcohol to be dehydrated was placed in a round-bottomed flask containing a magnetic stirring bar and anhydrous copper(II)sulfate (0.75 molar equiv). A 7-cm Vigreux column and condenser was attached, and the mixture was immersed with vigorous stirring in an oil bath preheated to the desired reaction temperature. Heating was continued until the distillation of volatile products into an ice-cooled receiver stopped. The majority of the water layer of the product was removed by pipet, and the olefii product was filtered through a plug of cotton. In all cases the olefin product was pure as determined by NMR spectroscopy, which was also used to calculate ratios of isomeric products. Where the olefin product was higher boiling (>120°C), aspirator vacuum (25 mm) was applied to the same experimental setup.
Thus 1-Phenyl-1-propanol, at 125°C/25mmHg for 2h, gave beta-methylstyrene (trans/cis, 91:9) in 69% yield.References[2] See for example:
(a) Ashby, E. C.; Williard, G. F.; Goel, A. B.
J. Org. Chem. 44, 1221 (1979) (b) Brieger, G.; Watson, S. W.; Barar, D. G.; Shene, A. L.
J. Org. Chem. 44, 1340 (1979)[6] For a good recent review of alcohol dehydrations, see: Askani, R.
Methoden Org. Chem. (Houben-Weyl), 4th Ed. 1972, 5(1b), 44.[7] Wittig, G.; Harborth, G.
Chem. Ber. 77, 315 (1944)