The role of silica gel in the solid-phase-catalyzed friedel-crafts tert-butylation of hydroquinone.

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Davis, Arthur
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Middle Tennessee State University
The use of silica gel as an acidic catalyst for the Friedel-Crafts alkylation of hydroquinone with tert-butyl bromide affords 2,5-di-tert-butylhydroquinone in high yield and purity if sodium carbonate is also present. Drying the sodium carbonate in a furnace (275{dollar}\sp\circ{dollar}C) effectively stops this Friedel-Crafts alkylation. Hence, a trace amount of water serves a necessary role as cocatalyst. Isobutene and its olefinic dimers are formed in small amounts from tert-butyl bromide during the reaction; however, isobutene is not the source of the electrophilic intermediate in the Friedel-Crafts tert-butylation because the extent of alkylation in a closed reaction vessel is the same as that obtained when the stirred and heated reaction was conducted in an open system whose thermostated reflux condenser was kept at temperatures above the boiling point of isobutene.
A proton NMR technique of multicomponent, trace-organic analysis was developed to distinguish the numerous, minor byproducts and also to quantify the product distribution. It is expected that application of this analytical method in kinetic and other experiments will contribute to a better understanding of the role of silica gel in the mechanism of the reaction. Although the reaction mixture has a somewhat complicated NMR spectrum with many peaks in the aromatic, olefinic, hydroxyl, and alkyl regions, the spectra of all components were discernible, and most of them have now been distinguished by comparison with the spectra of 16 reference compounds.
Since the Doctor of Arts is fundamentally a college teaching degree, an extra chapter entitled "Educational Implications" provides a detailed review of the microscale literature. The greater environmental awareness of current students, the expense of operation, and the force of law brought about by inclusion under OSHA's regulations have caused concerned academic chemists to seek alternatives to standard laboratory practices and have raised doubts as to the viability of undergraduate laboratories. Microscale experimentation may have diverted the conversation from more radical solutions, but the ultimate fate of lower-division laboratories in chemistry is not yet certain.