Oxocarbenium

In organic chemistry, an oxocarbenium ion (alternatively spelled oxacarbenium) is a chemical species characterized by a central sp²-hybridized atom of carbon, a substituent atom of oxygen, and an overall positive charge that is delocalized between the central carbon and oxygen atoms (R₂[CO]⁺R). An oxocarbenium ion is represented by two limiting resonance structures, one in the form of a carbenium ion with the positive charge on carbon (>C⁺−O−) and the other in the form of an oxonium species with the formal charge on oxygen (>C=O⁺−). As a resonance hybrid, the true structure falls between the two.

Compared to neutral carbonyl (C=O) compounds like ketones (>C=O) or esters, the carbenium ion form is a larger contributor to the structure. They are common reactive intermediates in the hydrolysis of glycosidic bonds, and are a commonly used strategy for chemical glycosylation. These ions have since been proposed as reactive intermediates in a wide range of chemical transformations, and have been utilized in the total synthesis of several natural products. In addition, they commonly appear in mechanisms of enzyme-catalyzed biosynthesis and hydrolysis of carbohydrates in nature. Anthocyanins are natural flavylium dyes, which are stabilized oxocarbenium compounds. Anthocyanins are responsible for the colors of a wide variety of common flowers such as pansies and edible plants such as eggplant and blueberry.