Chemical properties of alkadienes

Chemical properties of alkadienes
Alkadienes- unsaturated hydrocarbons, which include two double bonds. General formula of alkadienes - C n H 2n-2.
If double bonds are in a carbon chain between two or more carbon atoms, then such bonds are called isolated... The chemical properties of such dienes do not differ from alkenes, only 2 bonds enter into the reaction, and not one. If double bonds are separated by only one σ - link, then this is a conjugate link: If diene looks like that: C = C = C, then such a bond is cumulated, and the diene is called - allen.
The structure of alkadienes.
π -electron clouds of double bonds overlap with each other, forming a single π -cloud. In a conjugated system, electrons are delocalized over all carbon atoms The longer the molecule, the more stable it is. Isomerism of alkadienes. For dienes isomerism carbon skeleton, isomerism of the position of double bonds and spatial isomerism. Physical properties of alkadienes. Butadiene-1,3 is an easily liquefied gas with an unpleasant odor. Isoprene is liquid.
The properties of alkadienes largely depend on the mutual arrangement of double bonds in their molecules. On this basis, three types of double bonds in dienes are distinguished:
1) isolated double bonds are separated in a chain by two or more s-bonds:
CH 2 = CH – CH 2 –CH = CH 2 (separated sp 3 -carbon atoms, such double bonds do not exert on each other mutual influence and enter into the same reactions as the double bond in alkenes; 2) cumulated double bonds are located at one carbon atom: CH 2 = C = CH 2 (similar dienes (alleles) are less stable than other dienes and rearrange to alkynes when heated in an alkaline medium; 3) conjugate double bonds are separated by one s-bond:
CH 2 = CH – CH = CH 2.
Conjugated dienes are of greatest interest. They differ characteristic properties due to electronic structure molecules, namely, a continuous sequence of four sp 2 - carbon atoms. All carbon atoms lie in the same plane, forming an s-skeleton. The unhybridized p-orbitals of each carbon atom are perpendicular to the plane of the s-skeleton and parallel to each other, mutually overlap, forming a single p-electron cloud. This special kind of mutual influence of atoms is called conjugation. Overlapping p-orbitals butadiene molecules takes place not only between C 1 - C 2, C 3 - C 4, but also between C 2 - C 3. In this regard, the term "coupled system" is used. The consequence of the delocalization of the electron density is that the lengths of the C 1 - C 2 (C 3 - C 4) bonds are increased in comparison with the length of the double bond in ethylene (0.132 nm) and amount to 0.137 nm; in turn, the length of the C 3 - C 4 bond is shorter than in ethane C - C (0.154 nm) and is 0.146 nm.