Formation and reactivity of gaseous acetylium ions from the methylation of carbon monoxide.

Free acetylium ions have been prepared in the diluted gas state at atmospheric pressure, via two different routes, i.e., the methylation of carbon monoxide by CT3+ ions from the decay of fully tritiated methane, and the nucleophilic displacement by CO on CH3M+ (M = HF, CH3F) ions, produced from the y-radiolysis of methyl fluoride. Independently of the preparation methods, the acetyl cation displays a much higher reactivity toward n-type than toward r-type nucleophiles. The reaction of thermalyzed CHjCO+ ions with alcohols or phenols yields the corresponding acetates, while no acylation of benzene and toluene was detected. Low yields of ring-acetylated products were obtained from the attack of acetylium ions on anisole and phenol, the reactivity being, however, strongly biased (83.8%) in favor of the oxygen atom of the latter substrate. From these results it appears that the intrinsic intramolecular selectivity of the free acetylium ion, measured in the gas phase, in the absence of the complicating phenomena, such as solvation and ion pairing, invariably affecting solution-chemistry studies, is characterized by a high ortho/para ratio in the acylated products.