Question

Treatment of gamma-pyran with the hydride (h-) acceptor triphenylmethyl perchlorate gives triphenylmethane and the perchlorate salt a, c5h5clo5. draw the cation of a as its most stable resonance form.

Answer 1

I have provided two images to help with this question. The first image is the reaction that is taking place. The γ-pyran is treated with the hydride acceptor triphenylmethyl perchlorate. A hydride is a hydrogen atom containing a lone pair of electrons giving it a negative charge. The triphenylmethyl cation is a positively charged carbocation that greatly wants to accept an electron pair to stabilize its charge. Therefore, it abstracts a hydride from the γ-puran starting material. It grabs one of the hydrogen atoms that is drawn in the reaction scheme. This results in the formation of triphenylmethane and a pyrylium perchlorate salt with the formula C₅H₅ClO₅. The important aspect of the structure is shown in the attached images. The most stable resonance form of the pyrylium cation is shown with a positive charge on the oxygen.

The reason this pyrylium ion is the most stable resonance form is because the formation of the oxonium ion (positive charged oxygen with 3 bonds) leads to an aromatic compound. There are 6 pi electrons in conjugation in this ring similar to a benzene ring and this results in the most stable structure.