Reaction Review 15 - Reactions of Conjugated Dienes with H-X and X2 PDF

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Download Reaction Review 15 - Reactions of Conjugated Dienes with H-X and X2 PDF

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Rxn Review #15 – Reactions of conjugated dienes with H-X and X2 These reactions are similar to the electrophilic additions seen earlier for isolated alkenes. The conjugation of the diene now makes the possibility of both a 1,2 and 1,4 product an important consideration. Here is a generic example and mechanism:

In cases where no temperature is given, show both the 1,2 and 1,4 product. Temperature (low or high) can influence which product forms predominantly. Lower temperatures (approx. room temp and below) favor the KINETIC product. Higher temperatures (approx. 40o C and higher) favor the THERMODYNAMIC product by introducing “reversibility” into the equation. What does this mean? Consider this important text diagram:

Initially, the 1,2 product on the right forms most quickly (KINETIC) when H-Br is reacted with 1,3-butadiene. When the temperature is turned up, both reactions become reversible with the carbocation (middle intermediate). That means the Br is “popping” on and off the compounds. The 1,4 product is more stable (THERMODYNAMICALLY), lower overall energy. At high temperatures it is “selected” for since when it forms, the energy barrier for it to return to carbocation is higher than coming from the 1,2 product side (the right). It’s harder for it to get back over the barrier…and hence more of it forms and remains as product. Other examples:

Halogenations (X2) of conjugated dienes. Also similar to reactions with isolated alkenes. In this case, the 1,4 product is preferred (at all temps). Consider this example (mechanism):

Once again, in halogenations (X2) of conjugated dienes, the 1,4 and E product is favored. Not really mentioned in the Bruice text....