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Conjugated Dienes Tutorial

Since you already know how to name alkenes, this tutorial will focus more on the concepts of dienes.

There are three types of conjugated dienes:

  • Isolated - these are double bonds that are isolated
    ncdi1.gif (1123 bytes)
  • Cumulative - these are double bonds that follow one another
    ncdi3.gif (1099 bytes)
  • Conjugated - these are double bonds that are separated by one single bond.
    ncdi2.gif (1168 bytes)
    As you will read later, conjugated dienes are very important.

Delocalization- existence of p electron cloud extends over four carbons (in a conjugated diene), opposed to two in a normal -ene. This is important because the delocalization means that the carbon has more electrons to share amongst itself, which lowers energy and makes the molecule more stable.

Resonance theory - states whenever a molecule or ion can be represented by two or more structures that differ only in the position of the electrons, there are three implications: (I shamelessly stole these off of my organic chem book)

  1. There are multiple structures to represent the same molecule
  2. The actual structure is a mixture (hybrid) of the different possible structures
  3. The molecule or ion is considerable more stable (that is, of lower energy) than would be expected based on any single structure

ncdi4.gif (1644 bytes)

This is a reaction where the bromine is removed from an alkene. The removal of bromine creates a carbocation (since the bromine stole the electron pair that originally kept the carbon neutral). A carbocation is a carbon with a positive charge. As the image above demonstrates, the p bonds from the neighboring carbon can "shift" over to the carbocation. However, this leaves the other carbon with a positive charge. Eventually, you will add something to get rid of that charge. This is resonance. Later on, in the aromatics chapter, you will see more examples of resonance.


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