14.1: Introduction
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Objective
After completing this section, you should be able to determine whether or not a molecule contains a conjugated system, given its Kekulé, condensed or shorthand formula.
Key Terms
Make certain that you can define, and use in context, the key terms below.
- conjugated diene
- conjugated double bonds
- diene
- enone
- polyene
Study Notes
Conjugated double bonds are double bonds which are separated by one carbon-carbon single bond. Thus the double bonds in butadiene, $\ce{\sf{CH2=CH-CH=CH2}}$, are conjugated, and this compound is an example of a conjugated diene .
Just as the term diene indicates the presence of two carbon-carbon double bonds in a compound, so the term polyene is used to describe compounds containing many carbon-carbon double bonds.
An enone is a compound containing a carbon-carbon double bond (ene) and a carbonyl group (one). A conjugated enone contains the structural unit:
Conjugated Dienes
A diene is a hydrocarbon chain that has two double bonds that may or may not be adjacent to each other. This section focuses on the delocalization of \(\pi\) systems by comparing two neighboring double bonds. The arrangements of these double bonds can have varying affects on the compounds reactivity and stability.
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:
Conjugated vs. Nonconjugated vs. Cumulated Dienes
Conjugated dienes are two double bonds separated by a single bond
Nonconjugated (Isolated) dienes are two double bonds are separated by more than one single bond.
Cumulated dienes (allenes) are two double bond connected to a carbon atom.
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.