Double bond

A double bond, is a form of the atomic bond in the four bonding electrons are involved. The two atoms are thus connected to each other via two unshared electron pairs. In structural formulas double bonds as two parallel lines (=) are shown between the bonded atoms.

Many atoms may together form double bonds. The most common functional groups or classes of compounds with double bonds are alkenes (C = C), carbonyls (C = O), sulfoxides (S = O), imines (C = N) and the azo group (N = N).

Double bonds are stronger and shorter than single bonds. The bond order is two. Double bonds are electron rich, which makes them reactive.

History

The representation of the double bond by two double lines was introduced by Joseph Loschmidt. 1865 August Wilhelm von Hofmann led the endings -an, -en,- in the name of saturated and unsaturated alkanes one.

Detailed Description

Can be described by sp2 hybrid orbitals, the bonding of a molecule of the second period with a double bond. The neighboring atoms of such atom with sp2 hybridization are in a plane ( the molecule is planar). The angles between the bonded atoms are usually close to 120 ° (see VSEPR model ), the distance of the bonding atoms is about 1.34 Å. The remaining electron is in an orbital with pure p character perpendicular to the molecular level. Each of the two atoms connected by the double bond each have such a simple occupied p orbital. By combining these two results in a binding π - orbital and an antibonding π * orbital.

This type of double bond is not indeed limited to the second period, but it takes place at double bonds between elements of higher periods different binding patterns. This is due on the one hand generally lower homonuclear bond energies for larger atoms, and on the other hand a lower tendency hybridization of the s and p valence orbitals. The combination of these phenomena, it takes in some cases that the energy required to bring the molecular fragments to the double bond system is based in a bondable condition, and can not be applied by the subsequently liberated binding energy. In such cases, it is replaced by either of these fragments (i.e. Carbenhomologe or analogous molecules ) can be isolated as compounds or dimerization is via a double Lewis acid -base reaction and results in two donor -acceptor bonds ( a double dative bond ). Discoverable is such a double bond, which for example is observed at the higher carbon homologues, on a characteristic structure (trans- bent- bending of the substituents ) and the low energy that is necessary for binding separation. The dissociation energy may even be significantly smaller than a normal covalent bond. Typical examples of such double-bond systems are Distannenes ( R2Sn = snr2 ) and Diplumbene ( R2Pb = PBR2 ).

The energy gap between the bonding π and antibonding π * orbital is usually smaller than the energy gap between a corresponding σ and an antibonding σ * orbital. In the case of conjugated double bonds, the excitation energy is reduced so that this may be sufficient visible light to raise an electron to an empty orbital. In this case, the less energy ( longer wavelength ) is required, the greater the number of conjugated double bonds ( see also: particle in a box ). In the case of carotene with eleven conjugated double bonds, blue light is absorbed and the molecule appears orange ( the complementary color orange results from the subtraction of the blue component of the incident white light).

Bond lengths and bond angles

The bond lengths of double bonds between two atoms in the non-conjugated systems are shorter than those of the corresponding single bonds between the same atoms.

Nomenclature

The rotation about a double bond is not readily possible. If both binding partners of two different substituents, is therefore another discriminated with respect to their position between the cis and trans isomers.

Several double bonds, which are separated by just a single bond are as conjugated, directly adjacent as cumulative and called isolated on more than one single bond up to the next double bond.

Typical chemical reactions of molecules having a double bond

• Electrophilic addition to the double bond another ( electrophilic ) molecule is deposited either by attack on E (E = electrophile ) and ionic intermediate to breakage of double bond in the molecule or with electrophiles larger atoms (e.g. bromo ) to form an annular, ionic intermediate ( " onium ion", so here bromonium ion ) followed by backside attack of the radical (in the example Br ).

• A molecule whose bond easily homolytically (in two radicals) is cleavable (eg, chlorine) is the absence of light (UV ) is cleaved into two radicals, which then attack the double bond.

• Reduction of the C = O bond to alcohols with reducing reagents such as hydrogen or Grignard reactions.
• Oxidation of the C = O bond (only aldehyde) to a carboxylic acid.
• Passerini reaction.

• Addition of hydrocyanic acid to prepare α -amino nitriles.
• Ugi reaction

Further reactions see also named reactions, Cumulative double bond.

Detection of C = C double bonds

A rather non-specific detection of organic compounds with C = C double bonds, ie of alkenes is possible with bromine water. In the presence of an alkene, the yellow-colored by Br2 bromine water is decolorized by shaking with the fabric sample. Via an addition reaction arises from the alkene bromoalkane. For details, see Alkennachweis.