Effects of Lewis acid catalysts on the cleavage of aliphatic and aryl-aryl linkages in coal-related structures

✍ Scribed by Newell D. Taylor; Alexis T. Bell

• Publisher: Elsevier Science
• Year: 1980
• Tongue: English
• Weight: 917 KB
• Volume: 59
• Category: Article
• ISSN: 0016-2361
• DOI: 10.1016/0016-2361(80)90177-5

✦ Synopsis

ZnClq and AICl3 catalyse the cleavage of aliphatic linkages between aromatic nuclei but not the cleavage of direct aryl-aryl bonds between such nuclei. The rate of alkyl-aryl bond breakage depends on the Bronsted acidity of the active catalyst (e.g. H+(ZnCl2X)-or H+(AlCl3X)-) and the Bronsted basicity of the aromatic portions of the reactant. AICl3 is significantly more active than ZnClq, and reactants containing hydroxyphenyl or napthyl groups are more reactive than those containing phenyl groups. The distribution of finafproducts is strongly affected by the reactions of the aryl-alkyl carbonium ion formed upon cleavage of an alkyl-aryl bond. If the alkyl portion of the carbonium ion contains three or more carbon atoms, the ion preferentially undergoes an intramolecular reaction to form a hydroaromatic product. With only one or two carbon atoms the carbonium ion reacts via either hydride abstraction or electrophilic substitution, the hydride abstraction producing an alkylaromatic product, which may in turn undergo dealkylation. Reactant and products produced by a Scholl condensation act as the principle sources of hydride ions. Molecular H2 also contributes some hydride ions. The reaction of carbonium ions by electrophilic substitution leads either to the regeneration of the initial reactant or to the formation of high-molecular-weight tars.