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  Dienes - Organic Compounds - Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Dienes

It is helpful to identify three different ways in which these functions may be oriented with respect to each other, when considering compounds having two or more double bonds in a molecule. First Through one or more sp3-hybridized carbon atoms, the double bonds may be separated, like in 1,5-hexadiene. In this condition each double bond behaves independently of the other and we denote them as isolated. Through a single bond,a second relationship has the double bonds connected to each other like in 1,3-hexadiene and we denoted this type of arrangement as conjugated. At last, two double bonds might share a carbon atom, like in 1,2-hexadiene. The central carbon atom in this type of a system is sp-hybridized and we describe that double bonds cumulated. These three isomers are displayed in the picture that is given below. In examples where stereoisomers are possible only the E-isomer is displayed.

Rotation about the single bond joining the two double bonds (colored blue) alters a trans-like s-trans conformation to its s-cis form. The energy barrier for this conformational isomerization is usually low and the s-trans conformer is frequently more stable than the s-cis conformer, as displayed in the picture below.

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These classifications are based on more than obvious structural variations. In the chemical properties of dienes depending on their structural type, we find major variations.For an instance, catalytic hydrogenation converts all the dienes displayed here to the alkane hexane, but heats of reaction (heat of hydrogenation) reflect characteristic variations in their thermodynamic stability. This is demonstrated in the figure on the right. As a reference, By Taking the heat of hydrogenation of 1-hexene (30.1 kcal/mole), we locate that the isolated diene, 1,5-hexadiene, as supposed, generates double this heat of reaction on conversion to hexane. The cumulated diene1,2-hexadiene, has a 6 kcal/mole high heat of reaction, that is indicating that it is less stable than the isolated diene by this magnitude. In contrast, conjugation of double bonds appears to stabilize a diene by about 5 kcal/mole. The increment in stability of 2,4-hexadiene on the 1,3-hexadiene (both are conjugated) is because of the increased double bond substitution of the former, a factor that is noted previously for simple alkenes. 

By conjugation the stabilization of dienes is less dramatic than the aromatic stabilization of benzene. however, identical molecular orbital and resonance descriptions of conjugation may be written. A resonance description, like the one displayed here involves charge separation, implying a comparatively small degree of stabilization.

CH2=CH-CH=CH2   ↔   (+)CH2-CH=CH-CH2:(-)

A molecular orbital model for 1,3-butadiene is displayed below. Note: In each pi-orbital the lobes of the four p-orbital components are colored in a different way and carry a plus or minus sign. This distinction refers to distinct phases, illustrated through the mathematical wave equations for this type of  orbitals. Nodes are the Regions in which adjacent orbital lobes undergo a phase change. Orbital electron density is zero in that type of regions. So, at the nucleus a single p-orbital has a node, and all the pi-orbitals displayed here have a nodal plane that is explained by the atoms of the diene. This is the single nodal surface in the lowest energy pi-orbital, π1. The Higher energy pi-orbitals have an increasing number of nodes.

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