Double Bond

(redirected from Double bonds)
Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
Related to Double bonds: conjugated double bonds

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.

C-C< double="" bond""="" height="239" width="298">
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.

G. A. SOKOL’SKII

References in periodicals archive ?
Purging was found to have a minor but nevertheless detectable effect on the conversion of the acrylic double bonds, which clearly demonstrates the sensitivity of the NIR method.
The reaction speed of ozone with double bonds of rubber components is dependent not only on environmental influences, but also on the structure of the respective polymer.
To investigate the reactivity of the double bond in the AF material and the possibility of interaction with the peroxide molecules C-NMR analysis was performed.
For example, if the average number of double bonds per triglyceride for a given plant oil is "X," then the number of double bonds for a corresponding polymer derived from that same plant oil would be DPX/3 where "DP" is the degree of polymerization for the polymer.
This work has investigated the melt-phase hydrosilylation of terminal double bonds in PP by a non-catalytic, free radical method.
It was due to the reason that the molecular chain of PEGDA575 between two double bonds is shorter than that of PEGDA700.
whose C = C double bonds are different in polymerization reactivity.
Various chemical reactions can induce one of the double bonds to shift so that it is separated from the other by only one single bond.
In both, FAME and SO, we can find peaks related to the double bonds, in particular to --CH-- protons of the double bonds at 5.
NMR revealed that only ~70% of the double bonds had been epoxidized, so the reaction mixture was again cooled to 10[degrees]C and 75 g of m-chloroperbenzoic acid was added portion-wise over 30 min (14[degrees]C exotherm).
The composition is achieved by subjecting an acrylic-modified polyester resin obtained by graft polymerizing a polyester resin having ethylenic double bonds that has a number average molecular weight of 2,000 to 100,000.