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oxocarbenium
An oxocarbenium ion is a canonical form of a carbocation, real or hypothetical, which places a formal positive charge on an oxygen atom.〔(【引用サイトリンク】 IUPAC Gold Book )〕 They are common transition states for the hydrolysis of glycosidic bonds, and are a commonly used strategy for chemical glycosylation. These ions have since been proposed as transition states of 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.
== Structure ==
The general structure of an oxocarbenium ion contains an oxygen-carbon double-bond, with the oxygen atom attached to an additional group. The ion has a formal positive charge, which is placed on the oxygen atom. The oxocarbenium cation is stabilized by resonance, which places the positive charge on the carbon. Compared to a ketone, the oxocarbenium ion is more polar, and more reactive with nucleophiles. A common example of this is the activation of a ketone by the addition of a Lewis Acid to the oxygen. An average dipole moment for a general ketone R2CO is δ = 0.51. With the addition of an acidic hydrogen to the oxygen atom to produce ()+, the dipole moment increases to δ = 0.61, making the oxocarbenium ion more susceptible to nucleophilic addition than the ketone.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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