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Conjugated Dienes: Introduction & Nomenclature

Compounds containing more than one double bond are said to be conjugated if they possess a series of adjacent sp2 centers. In a compound such as this, the adjacent p-orbitals overlap to form a continuous p-system, as seen in the graphic below.

Polyenes are named as are simple alkenes, using the multipliers di-, tri-, etc. to indicate the number of double bonds and numbers to show their positions.

As with simple alkenes:

1. Find the longest chain containing the double bonds; the name of the parent hydrocarbon is taken from the longest continuous chain of carbon atoms containing the double bonds.

2. Number the chain, giving the double bonds the lowest possible number; use multipliers to indicate the total number of double bonds (di, tri, tetra, etc.) Recall that in constructing the name, substituents are arranged in alphabetical order, without regard for these multipliers.

3. For cycloalkenes, begin numbering at one double bond and proceed through the double bond in the direction to generate the lowest number at the first point of difference.

4. If appropriate, assign stereochemistry using the E-Z designation.

The rules for assigning E-Z designations are as follows:

1. rank atoms directly attached to the double bond according to their atomic number

2. if there is a "tie" at any substituent, look at the second, third, etc., until a difference is found

3. multiple bonds count as multiples of that same atom

4. if the highest priority groups are on the same side of the double bond, the molecule is Z; if the highest priority groups are on opposite sides, the molecule is E

Examples:

Conjugated Dienes: Ionic Addition Reactions

When compounds containing conjugated double bonds undergo typical ionic alkene addition reactions (addition of HBr or Br2, for example), the products which are obtained are not those which would be expected for addition to the individual double bonds in the molecule. For the addition of HBr to 1,3-butadiene, two products are obtained, 3-bromo-1-butene and 1-bromo-2-butene. These products can be seen to arise from a "standard" 1,2-addition to one of the terminal double bond (Markovnikov-style), and from a 1,4-addition of HBr to the two terminal carbons, with relocation of the double bond onto the central two carbons.

The formation of these products can be readily understood by examining the mechanism of the addition reaction. Protonation of the conjugated diene on either terminal carbon will generate a carbocation on the adjacent secondary carbon. This carbocation, however, can be stabilized by resonance with the adjacent double bond to give a delocalized carbocation (an allylic carbocation) in which there is positive character on both a secondary center and on the terminal, primary carbon. Since both of these centers share positive character in the resonance hybrid, both are subject to nucleophilic attack by bromide anion; attack on the secondary carbon gives the 1,2-addition product, and attack on the terminal carbon gives the 1,4-addition product.

Some further observations on this reaction reveal the following:

* The 1,2-addition product forms rapidly at low temperatures;

* the 1,4-addition product is predominant at higher temperatures;

* even at low temperatures, 1,4-addition products will predominate if given enough time;

* the addition of HBr to butadiene is reversible and isolated 1,2-addition product will convert to the 1,4-product at higher temperatures or at longer times.

These data can be explained using the reaction coordinates shown below. The pathway to form the 1,2-product must have a lower activation energy, because it forms more rapidly than the 1,4-product. The 1,4-product, however, must be more stable than the 1,2-product because it accumulates at equilibrium (note that the reaction appears freely reversible, since isolated 1,2-product reverts to 1,4-, given enough time).

The 1,2-addition product is referred to as the kinetic product since it is formed faster. The 1,4-product is the thermodynamic product since it is thermodynamically more stable. A similar product distribution is observed for Br2 addition, through a similar mechanism.

Conjugated Dienes: Cycloaddition Reactions

Conjugated dienes react with alkenes to yield cyclohexene derivatives. The reaction is termed a 4+2 cycloaddition and is generally referred to as the Diels-Alder Reaction. The reactants in the cycloaddition are referred to, generically, as a diene and a dienophile. The reaction usually requires heat and pressure to give good yields and is promoted by electron withdrawing groups on the dienophile and electron donating groups on the diene.

The mechanism of the reaction is generally described as concerted involving an electrocyclic transition state in which the two new sigma bonds form simultaneously; this is usually represented by showing the electron movement with "curved arrows", as shown above. Since both bonds form at the same time, it is necessary for the diene to be in the proper conformation prior to the reaction, that is, the s-cis conformation is required, and dienes which cannot adopt this conformation will not react.

Examination of the animation shown above for the reaction of ethene with butadiene clearly shows that the initial product of the reaction is the boat cyclohexene. This can also be appreciated by examination of the sequence of images shown below for the reaction of butadiene with butenedinitrile. Lining up the reacting centers and allowing the cycloaddition to proceed generates the structure shown on the right. Rotating this along the X-axis (with the double bond remaining in the back) shows the compound in the "boat" conformation. Converting this to a "chair" by rotating one end down (and rotating the molecule slightly along the Z-axis) gives the middle image on the second row, which is an idealized cyclohexene "chair". In fact, the geometry of the double bond contorts