Introduction
Quaternary ammonium hydroxides are obtained as deliquescent hygroscopic solids which may be obtained by treatment of an aqueous solution of a quaternary ammonium iodide with an excess of moist silver oxide.
Quaternary ammonium hydroxides are highly ionized in aqueous solutions and in alcoholic solution and thus are strong bases due to the high concentration of hydroxyl ion in these solutions. They are much stronger bases than the amines. They have a corrosive action on glass similar to that of sodium and potassium hydroxide.
Fig 1. Structure of Quaternary ammonium hydroxide
Formation of Quaternary Ammonium Hydroxides
Quaternary ammonium hydroxides may be obtained by treatment of an aqueous solution of a quaternary ammonium iodide with an excess of moist silver oxide. Silver iodide is precipitated. This and the excess silver oxide are removed by filtration and the solution contains the quaternary ammonium hydroxide. The hydroxide may be obtained pure by concentrating the aqueous solution in a vacuum. Quaternary ammonium hydroxides are obtained as deliquescent hygroscopic solids, often as hydrates. They have a corrosive action on glass similar to that of sodium and potassium hydroxide.
However, it will be seen below that these compounds are unstable when heated and decompose either to give an alcohol and a tertiary amine, or, more often, heating of a quaternary ammonium hydroxide gives an olefin, water and a tertiary amine.
Thermal Decomposition of Quaternary Ammonium Hydroxides
Thermal decomposition is the most common reaction of quaternary ammonium hydroxides to form an alkene, an amine and water.
Quaternary ammonium hydroxides are used as reagents in pyrolysis/alkylation techniques. Pyrolysis of pure tetramethylammonium hydroxide (TMAH) generates mainly methanol and trimethylamine, as shown in the following reaction:
Fig 2. Thermal decomposition of TMAH
In some pyrolysis reactions, instead of the formation of an alcohol, the result is the formation of water and an alkene. One example of this reaction is that of tetraethyl ammonium hydroxide, which generates by pyrolysis triethylamine, C2H4, and H2O. The general reaction taking place with a bimolecular elimination mechanism can be written as follows:
Fig 3. Thermal decomposition of tetraethyl ammonium hydroxide
In cases where different alkenes can result from the quaternary salt, the alkene with the smallest number of substituents at the double bond is typically preferred (Hofmann rule), as shown below for trimethyl(methylpropyl) ammonium hydroxide:
Fig 4. Thermal decomposition of trimethyl(methylpropyl) ammonium hydroxide
For quaternary ammonium hydroxides with different substituents at the nitrogen atom, the reaction leading to an alkene is also common. This reaction is shown below for phenylethyl trimethyl ammonium hydroxide, which by thermal decomposition forms styrene, trimethylamine, and water:
Fig 5. Thermal decomposition of phenylethyl trimethyl ammonium hydroxide
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