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A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that is between two carbon atoms, can be found in alkenes. Many types of double bonds exist between two different elements. For example, in a carbonyl group with a carbon atom and an oxygen atom. Other common double bonds are found in azo compounds (N=N), imines (C=N) and sulfoxides (S=O). In skeletal formula the double bond is drawn as two parallel lines (=) between the two connected atoms; typographically, the equals sign is used for this.〔.〕〔''Organic Chemistry'' 2nd Ed. John McMurry.〕 Double bonds were first introduced in chemical notation by prominent Russian chemist Alexander Butlerov. Double bonds involving carbon are stronger than single bonds and are also shorter. The bond order is two. Double bonds are also electron-rich, which makes them reactive. ==Double bonds in alkenes== The type of bonding can be explained in terms of orbital hybridization. In ethylene each carbon atom has three sp2 orbitals and one p-orbital. The three sp2 orbitals lie in a plane with ~120° angles. The p-orbital is perpendicular to this plane. When the carbon atoms approach each other, two of the sp2 orbitals overlap to form a sigma bond. At the same time, the two p-orbitals approach (again in the same plane) and together they form a pi-bond. For maximum overlap, the p-orbitals have to remain parallel, and, therefore, rotation around the central bond is not possible. This property gives rise to cis-trans isomerism. Double bonds are shorter than single bonds because p-orbital overlap is maximized. With 133 pm, the C=C bond length is shorter than the C−C length in ethane with 154 pm. The double bond is also stronger, 636 (KJ/mol) versus 368 kJ/mole but not twice as much as the pi-bond is weaker than the sigma bond due to less effective pi-overlap. In an alternative representation, the double bond results from two overlapping sp3 orbitals as in a bent bond.〔''Advanced Organic Chemistry'' Carey, Francis A., Sundberg, Richard J. 5th ed. 2007.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Double bond」の詳細全文を読む スポンサード リンク
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