A conjugated diene follows a pattern of double single double bond. An unconjugated diene is a molecule where the double bonds are separated by more than one single bond. Conjugated dienes are extra stable when compared to the unconjugated dienes. This stability is due to electron density being delocalized over the conjugated diene.
Electrophilic Addition to Conjugated Dienes
A conjugated diene can undergo 1,2 and 1,4 electrophilic addition. The product will depend on the temperature at which the reaction was performed. Under low temperature, we will get kinetic product. Under high temperature, thermodynamic product will be observed.
1,2 product is achieved when the double bond gets protonated by HX, carbocation is formed. Then X- attaches to the carbocation. In another scenario, when the carbocation is formed, resonance can occur, shifting the carbocation and double bond. The end result is 1,4 product.
Mayya's Trick
Kinetic product (under low temperature) is always the one before resonance (1,2).
Thermodynamic product (under high temperature) is always the one with the most substituted double bond. It could be 1,2 or 1,4
Please look at your professor's notes or textbook for the range of low and high temperatures.
The same reaction can happen with Br2.
How can your professor trick you? He/She can have an unsymmetrical alkene molecule where two double bonds are different. How do we know which double bond will react?
Mayya's Trick
React both double bonds, one at a time, and show the resonance for each one as well. Make sure to always put carbocation next to the double bond so that resonance could happen.
Compare the carbocations made and assign them as primary, secondary and tertiary. The pathways with the more stable carbocations will be the winner. That must have been the double bond that reacted. Cross out the other pathway.
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