Not only can coals be alkylated readily, they also can be used to alkylate aromatic compounds.
Both types of reaction have received limited study. With both types of reaction, soluble products are formed that can be analysed using techniques which are not suitable for unreacted coal because they require soluble material.
It is this factor which causes much of the interest in alkylation reactions of coal. Much structural information can be obtained using soluble, reacted, coals which is impossible to obtain using the non-soluble unreacted coal. Also, the two techniques give different and complementary products which in combination provide important information about coal structure that is difficult to obtain by other means. There is now enough information available for its consideration to yield insights into coal structure and reactivity and to suggest many new experiments. That is the purpose of this review.
COAL AS AN ALKYLATING AGENT
With great insight Heredy and Neuworth first realized that coal could be 'depolymerized' by using it as an alkylating agent.
First they used model compounds to show that methylene chains joining two aromatic groups could be cleaved at the ring and that the methylene group would then alkylate another aromatic substrate'.
It is generally agreed that the first step in this reaction is protonation of the aromatic moiety at the position of alkyl substitution', followed by nucleophilic bimolecular substitution of an aryl group by phenol.
A bimolecular mechanism is preferred to one involving an alkyl carbonium ion since it is known that primary alkyl substituents undergo intermolecular transfer from one aryl species to another with little or no rearrangement to secondary alkyl substituents2-4.