Double Bond

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double bond

[¦dəb·əl ′bänd]
(physical chemistry)
A type of linkage between atoms in which two pair of electrons are shared equally.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Double Bond


a covalent four-electron bond between two neighboring atoms in a molecule. A double bond is usually designated by two valence lines: >C═C<, >C═N─, >C═O, >C═S, ─N═N ─, ─N═O, and so on. It is understood that one pair of electrons with sp2- or jp-hybridized orbitals forms a σ-bond (see Figure 1), whose electron density is concentrated along the interatomic axis; a σ-bond is similar to a single bond. The other pair of electrons with p-orbitals forms a σ-bond, whose electron density is concentrated outside the interatomic axis. If atoms of Groups IV and V of the periodic system participate in the formation of a double bond, then they and the atoms directly bonded to them lie in a single plane; the valence angles equal 120°. In the case of asymmetrical systems, deformations of molecular structure are possible.

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Figure 1. Diagram of a >C═C< double bond

A double bond is shorter than a single bond and is characterized by a high energy barrier of internal rotation; therefore, the positions of the substituents when there are double-bonded atoms are not equivalent, and this causes the appearance of geometric isomerism. Compounds containing a double bond are capable of addition reactions. If the double bond is electron-symmetrical, the reaction is accomplished by means of radical mechanisms (homolysis of the π-bond), as well as ionic mechanisms (as a result of the polarizing

Figure 2. Systems of conjugated bonds (view from above)

action of the medium). If the electronegativities of the double-bonded atoms differ, or if different substituents are bound to them, the σ-bond is strongly polarized. Compounds containing a polar double bond tend toward addition by the ionic mechanism: nucleophilic reagents combine readily with an electron-acceptor double bond; electrophilic reagents combine readily with an electron-donor double bond. The direction of electron displacement in double-bond polarization is indicated conventionally in formulas by arrows, and the excess charges formed are shown by the symbols δ- and δ+. This facilitates understanding of the radical and ionic mechanisms of addition reactions:

In compounds with two double bonds that are divided by one single bond, there occurs conjugation of the π-bonds and formation of a single π-electron cloud, whose lability is manifested along the entire chain (Figure 2, left). The capacity for reactions of 1,4-addition is the result of such conjugation:

If three double bonds are conjugated in a six-membered cycle, the sextet of π-electrons becomes common to the entire cycle, and a relatively stable aromatic system is formed (see Figure 2, right). The addition of both electrophilic and nucleophilic reagents to such compounds is difficult in terms of energy.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
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