Chapter 20: Dienes, Conjugated Systems, and Pericyclic Reactions
Table of Contents:
Stability of Conjugated Dienes
Conjugated dienes are extra stable when compared to the unconjugated dienes.
Electrophilic Addition to Conjugated Dienes
1,2- and 1,4-Addition to a Conjugated Diene
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.
When we are given a conjugated diene with two different double bonds, we need to figure out which double bond would get protonated. To do this, we protonate both double bonds, one at a time, and show the resonance structures for both as well. Then, we check for the stability of carbocation intermediates in both pathways. The double bond that gave the most stable carbocation is the one that had to react.
Let's try an example.
Addition of one mole of HBr to 2,4-hexadiene gives a mixture of 4-bromo- 2-hexene and 2-bromo-3-hexene. No 5-bromo-2-hexene is formed. Account for the formation of the first two bromoalkenes and for the fact that the third bromoalkene is not formed.
The same reaction can happen with Br2.
Conjugated molecules absorb energy in the UV- visible region. Beer-Lambert Law shows the relationship between absorbance (how much radiation of a particular wavelength a compound absorbs), concentration, and length of the sample cell.
Important: The greater the number of double bonds in conjugation,
the higher the wavelength of ultraviolet radiation absorbed.
Pericyclic Reaction Theory
Pericyclic reactions are reactions that occur in one step without any intermediates and occur due to the favorable overlap of molecular orbitals.
The Diels-Alder Reaction
In Diels Alder reaction, two reagents: diene and dienophile react together under heat to make a cyclohexene.
Diene must be in s-cis confirmation. Both double bonds on the same side of the single bond. If it is not, molecule needs to be rotated . If the molecule can't turn into s-cis conformation, diene can not participate in Diels Alder.
The Effect of Substituents on Rate Of Dials-Alder Reaction
The rate of Diels Alder increases when electron withdrawing groups (EWG) are placed on the dienophile and electron donating groups (EDG) are placed on the diene.
Configuration of both diene and dienophile is retained.
Diels Alder can also form bridged compounds
Under kinetic conditions, ENDO is preferred!
What this means is that when you draw the product of Dials-Alder reaction that is a bridged compound, make sure to draw the groups on the side going DOWN, as shown in the picture below.
To figure out the diene and dienophile that gave the Diels-Alder product, we must first find a cyclohexene. We then, number it according to the prototype shown above. We can show the products of the prototype with the same numbering as above and attach groups that are missing looking at the starting molecule.
Cope rearrangement can be recognized if you see a 1,5 diene as a substrate under heat conditions.
If you see a molecule that has 1,5 diene, you have to think Cope or Claisen rearrangement!
Draw out the prototype
Number atoms 1 through 5 in the molecule (same as prototype) and then follow the arrows in the prototype.
Learn more about Organic Chemistry Tutoring Online