Novel reaction protocol for dehydrating alcohols

Microwave Assisted Dehydration of Benzylic Alcohols Using Kaolin Catalysed Iodination and Catalytic Transfer Hydrogenation

Abstract
Microwave chemistry is a relatively new field, but the short reaction times, and in some cases the lack of need for specialised glassware makes it a very attractive area to study. In this paper i will describe the process and calculate the proper stochiometric ratios for optimal reaction. Having observed an initial test of this method for reducing benzylic alcohols, it seems quite likely to me that the iodination utilises an in-situ generation of alkali metal-aluminium hydrides and is thus on par with the conventional use of such reducing agents as Lithium Aluminium Hydride (LAH), except without the high cost of these reagents.

The Importance of Water of Hydration
An important point i would like to make about ionic reactions in general is that the formulas never contain the water. In the halogenation reaction, the amount of water is critical. Excess water re-hydrates the formed halides, returning them to the alcohol. Thus, the exact content of water is vital to achieving a quantitative yield. For this reason I chose the most readily available halide to demonstrate, because if a slight excess of halide is used, some of it will gas off, but excess water reduces yield. The amount of sulphuric acid should be matched to the amount of halide present in order to ensure it is gassed of. Careful calculations and weighing should eliminate any need for this, any excess of the halide salt increases the amount of hydride formed which will over-reduce the substrate, reducing purity.

The exact density of water added to the reaction, and the content of water occluded in the ions must be counted, to achieve quantitative yields. Every ion pair in dry salts added to the reaction contains water of hydration, and this level of hydration can be acheived by heating the salt at a little above water's boiling point, about 120 degrees C is enough to do this.

By weighing the water used in the reaction, and comparing with the density of totally pure water, one can establish the dissolved ions in the water, and the true molarity of water that will be added. In the case of concentrated acids such as phosphoric acid and sulphuric acid, the acid should be weighed and volumetrically measured, compared with the accurate density measured in laboratories by using special methods to eliminate water, and the count of water molecules added to the reaction calculations.

For this reason, water of hydration is noted for each initial reaction mixture, and the exact molarity of water needed to completely hydrate the salt mixtures is specified. If these guidlines are followed, the reactions should proceed quantitatively. The ion pairs indicate the minimum hydration, and water of hydration is the amount required by the product salts water of hydration.

Reaction Temperature control
The boiling point of the reacting substrate should be the terminal point of the reaction temperature. Concentrated ionic solutions also become dangerous when heated too close to the boiling point of water, causing the mixtures to bump, and risking damage to the microwave oven. Thus, the rule of thumb for performing these reactions is to run the reaction about 30 degrees lower than the boiling point of either water or the substrate, depending on which is lower.

For most benzylic alcohols this means the ideal reaction temperature is somewhere near 70 degrees as they all boil above the temperature of water. There is good reason to suggest that these reactions can also occur in aliphatic alcohols, such as methanol and ethanol and propanol and ispropanol. For these I suggest (respectively) 35, 48, 67 and 52. All higher alcohols boil above 100 degrees, and as such, do not need any lower temperature.

In order to use a domestic microwave, which has a fixed wattage magnetron, the use of microwave-sink water is reccommended. Before adding the reagents to the acidic substrate, place a vessel containing water with the reaction vessel, and monitor the temperature achieved with full power, and adjust the amount of water to achieve the reaction temperature stability during the expected peroid of irradiation. It is preferable to not use the switching-power control (high med low etc) on the microwave because this causes stop/start boiling which promotes bumping, and in most cases the substrate will float above the ionic solution, or at least tend towards rising above it, due to the fact that the water contains concentrated ions, meaning a bump will cause a loss of substrate. Placing a watch glass atop a beaker, or a loosely fitted glass stopper type device can further prevent this problem. Keeping the temperature from rising above the desired maximum goes a long way towards this goal too.


Description of Reaction System
The reaction system consists of mixing kaolin clay with sulphuric acid and the addition of an alkali metal halide such as sodium chloride, potassium iodide or any other combination of Li, Na, K with Cl, Br or I, and after this reaction with the benzylic alcohol, the use of a microwave assisted catalytic transfer hydrogenation following it to de-halogenate.

Formulas
Halogenation:
Initial mixing of reagents
H2SO4 + Al2Si2O4(OH)4 + 2(Na(+)Cl(-)) --> H2SO4 + 2(NaAl(2+)) + 2(SiO2(-)) + 2(Cl(-)) [4 mol H2O]
First microwave reaction
H2SO4 + 2(NaAl)(2+)) + 2(SiO2(-)) + 2(H(+)Cl(-)) --(Microwave radiation)--> H2SO4 + 2(SiO2(-)) + 2(NaAlH(2+)) + 2(Cl(+)) [4 mol H2O]
Second microwave reaction (this step causes the halogenation)
H2SO4 + 2(SiO2(-)) + 2(NaAlH) + 2(Cl(-)) + 2(PhOH) --(Microwave radiation)--> H2SO4 + 2(NaAl(+)) + 2(SiO2(-)) + 2(BzlCl)

Catalytic Transfer Hydrogenation:
Na(+)OEt + 2(Saturated Hydrocarbon) + BzlCl --(Microwave radiation)--> 2(mono-unsaturated hydrocarbon) + Toluene+ Na(+)OH(-)H(+)Cl(-) + EtOH
This reaction is catalysed by the still-present NaAl which oxidises (removing hydrogen) from the unsaturated hydrocarbons and then donates it to the benzylic chloride. The reaction occurs at the interface of the two phases. No water should be added, sodium ethoxide will perform the catalysis without adding water.

Ensuring Purity of Kaolin Clay
To account for the presence of iron oxide in the kaolin, measure the displacement of the kaolin in water of the batch of clay being used. The discrepancy will indicate the amount of iron oxide. It can be removed by some method, such a method should be found, possibly by heating with sulphuric acid and washing. otherwise, if it is negligable, and the amount is below .1% then it should be okay to omit, possibly account for it with extra sulphuric acid.

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