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.

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.


References in periodicals archive ?
The experimental results of silane incorporation and terminal double bond (vinyl) formation are depicted in Figures 3 and 4.
FTIR measurements and iodine number titration have indicated the disappearance of double bonds as a result of the radical reaction.
The Double Bond is in many ways an instructive contribution to Levi studies, if only for the amount of meticulous research that went into its composition and the light it shines on the milieus in which Primo Levi lived and wrote.
As a research tool, The Double Bond offers a wealth of annotations (113 pages worth), a bibliography broken down by categories, and a conceptual index.
In the numerical designation, the number to the left of the colon indicates the number of carbons in the fatty acid; the number immediately to the right of the colon indicates the number of double bonds in the fatty acid.
Elastomers, especially those with residual double bonds, show various kinds of aging due to environmental influences:
cis-polybutadiene rubber is provided containing 1,2-polybutadiene and a polymer substance of a melting point lower than that of the 1,2-polybutadiene and with at least one unsaturated double bond per repeating unit, where the 1,2-polybutadiene and the polymer substance are dispersed at physically and/ or chemically adsorbed states in the cis-polybutadiene rubber as the matrix component of the vinyl.
Comparing the spectra of the PNIPA hydrogel without silica (b) and of the NIPA monomer (a), the stretching band of the C = C double bond at 1620 [cm.
has obtained a patent for a method for forming an acrylic emulsion comprised of emulsion copolymerization of monomers comprised of an acrylic monomer having an epoxy group, an acrylic monomer having a hydroxyl group using a reactive emulsifier having an unsaturated double bond in the molecule and a polymerization initiator having an imidazole group and/or an imidazoline group in the presence of a silane compound having an epoxy group and an alkoxy group in the molecule.
7-16) They consist generally of an aqueous dispersion or emulsion of a telechelic oligomer end-capped by the very reactive acrylate double bond.