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double bond[¦dəb·əl ′bänd]
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.
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
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.
G. A. SOKOL’SKII