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16.6 Conjugated Dienes

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    28327
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    Conjugated vs. Non-conjugated Dienes

    Dienes are compounds which contain two double bonds. These dienes can be non-conjugated (the two double bonds are separated by at least one sp3 hybridized atom. Conjugated dienes have the two double bonds separated by a single bond. Conjugated dienes have properties and reactivity which are distinctly different than non-conjugated dienes. Determining if double bonds are conjugated represents an important skill in organic chemistry. 

     

    Conjgated Example.svg

     

    Cumulated dienes (allenes) are two double bond connected to a carbon atom.

    Bond line drawing of 2,3-hexadiene.

    The reactivity of these molecules is substantially different from that of alkenes which have isolated C=C. These molecules are thus considered a different class of organic molecule. Conjugated dienes, especially butadiene, are very important materials in the production of rubber, and thus for the tires of our cars.

    Dienes can adopt two possible conformations through rotation about the single bond joining the two double bonds: the s-cis and the s-trans conformations.

    To get from the s-trans to the s-cis configuration there is a rotation about 2-3 bond.

    The energy barrier to isomerization is normally low, and the s-trans conformer is often more stable than the s-cis conformer.

    Naming Dienes

    First identify the longest chain containing both carbons with double bonds in the compound. Then give the lowest possible number for the location of the carbons with double bonds and any other functional groups present (remember when naming alkenes that some groups take priority such as alcohols). Do not forget stereochemistry or any other orientation of the double bond such as (E/Z,cis or trans).

    Examples:

    Examples_naming.bmp

    Different conformations of Conjugated Dienes

    There are two different conformations of conjugated dienes which are s-cis and s-trans conformations. s-cis is when the double bonds are cis in reference to the single bond and s-trans is when the two double bonds are trans in reference to the single bond. The cis conformation is less stable due to the steric interation of hydrogens on carbon. One important use of the cis conformation of a conjugated diene is that it is used diels-alder cycloaddition reactions. Even though the trans conformation is more stable the cis conformation is used because of the molecule's ability to interconvert and rotate about the single bond.

    Conformations.bmp

    ContributorsEdit section


    This page titled 16.6 Conjugated Dienes is shared under a not declared license and was authored, remixed, and/or curated by Layne Morsch.

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