Ketenyl

Ketenyl is a chemical term that can refer to a few related concepts in organic chemistry. Most commonly, it describes a reactive intermediate species characterized by a carbon-carbon double bond adjacent to a carbonyl group, with the general formula R2C=C=O. This structure makes ketenyl species highly electrophilic at the terminal carbon atom. They are typically generated in situ and are not isolable due to their extreme reactivity.

Ketenyl can also be used as a substituent group, denoted as C=C=O, which is attached to another molecule. In this context, it is also known as a ketene group. The parent compound, ketene (CH2=C=O), is the simplest member of this class and is a highly toxic gas.

The generation of ketenyl intermediates often involves the dehydrohalogenation of acyl halides or the thermolysis of carboxylic acid derivatives. Their reactivity is exploited in various synthetic transformations, such as cycloaddition reactions, nucleophilic additions, and polymerization processes. For example, ketenes readily undergo [2+2] cycloaddition reactions with alkenes and alkynes to form cyclobutenes and cyclobutenones, respectively. They also react with alcohols and amines to form esters and amides. Due to their transient nature, their presence and reactions are often inferred through trapping experiments or spectroscopic analysis.