Before we can talk about dienes you must understand the concept of resonance. The classic definition is on slide #1. Resonance explains why when we react an alkene with X2 and heat we get an “allylic” product.
In the first step of the mechanism we generate a free radical that has an important resonance form, slide #3. Notice that when drawing the resonance form there is no movement of atoms only electrons.
Look at the two resonance structures in the “Dienes” link on the web site. In both structures all of the atoms are in the exact same places, the only thing that moves between the two structures is electrons. Try drawing one of the resonance structures, then draw the other.
Resonance will be important in our study of dienes and benzene.
So dienes are basically alkenes with two double bonds in them. The double bonds can be conjugated, isolated or cumulated. Name the following dienes and label them as isolated or conjugated.
Cumulated dienes are rare and isolated dienes will undergo the same addition reactions as regular alkenes. Conjugated dienes have some unique chemistry because they are considerably more stable than isolated or cumulated dienes.
First they are formed from dehydrohalogenation of halo alkenes. Predict the major product from the dehydrohalogenation of:
4-bromo-1-hexene + KOH (alcohol) ---->
Second they yield 1,2 plus 1,4 addition products, slide #7. Please study the “Dienes” link on the web site and make sure you understand how the 1,2 and 1,4 products arise given the resonance structures of 1,3-butadiene.
Focus on the first half of the Dienes I work sheet and the Dienes II work sheet. Note that if you are adding more than one mole of something say, HX, the first products will be 1,2 and 1,4 addition followed by the “regular” Markovnikov addition. i.e.
H2C=CHCH=CH2 + 2 moles HBr ---->
CH3CH(Br)CH=CH2 (1,2 addition) + CH3CH=CHCH2Br (1,4 addition) then the second mole of HBr
CH3CH(Br)CH(Br)CH3 + CH3CH(Br)CH2CH2Br + CH3CH2CH(Br)CH2Br
These are the Markovnikov addition products.