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Introduction to Conjugated Dienes

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In this video we’ll start to learn about conjugated dienes when we’re looking at our dienes there are a few different classes we can look at there’s the accumulated diene where the two double bonds are adjacent to each other so the carbon in the middle has a double bond to one side and a double bond to the other side there’s the conjugated dienes where you have

Double bond single bond double bond so you have each one of your pi bonds is separated by one single bond or we also have our isolated system where we have our pi bond separated by two or more single bonds so the pi bonds are no longer close to each other and these different types of dienes have different atomic orbital structures in our accumulated diene where

They’re adjacent to each other one of the double bonds is perpendicular to our plane here and the other is parallel and these different types of dienes have different atomic orbital structures if we first look at the accumulated one we can see that one of the double bonds is perpendicular to the plane and the other is parallel so our double bonds in each one of

These are considered isolated from each other since they are not in a row near each other the conjugated diene you can see the double bond and the electrons from the double bond extend across all four carbons so we have a double bond here a pi bond there and a pi bond here and the electrons can extend across all four of these carbons so it said that the pi bond

Overlaps the entire system with our isolated system the pi bonds are separated by too great of a distance to have any overlap since they don’t have any electron overlap that’s why they’re considered an isolated system and a quick review about atomic orbitals the more s character carbon has the shorter its bonds will be so remember when we have our pi bonds

Those occur when you have an sp2 hybridized carbon and so you can see the sp2 hybridized carbon creates a shorter bond than the sp3 hybridized carbon sp3 hybridized carbons are the carbons that have single bonds so our sp3 is typically with our sigma bonds and sp2 typically has a pi bond and you can see that the shorter bond occurs because the sp3 only has 25

Percent s character while the sp2 has 33 s character this means the bonds that are in a conjugated system actually have the carbon atoms a little bit closer together and so now let’s look at how this conjugation actually affects its stability so if we have just a single bond and we hydrate it and turn it into an alkane the change in enthalpy will be a negative

254 kilojoules however if we do the same reaction we did a hydrogenation on a conjugated diene it only requires 239 kilojoules so you can see there’s a difference of about 15 kilojoules by adding the second double bond so the more conjugated system is the more stable it is so the lower in energy it is and then if we look at two possibilities for a conjugated

Diet one of them can have the two double bonds cis to each other or in what we call an s cis configuration and the other can have the two double bonds trans to each other what we call it an s trans configuration so in the s cis our single bond here is rotated so two double bonds are on the same side in the s trans the two double bonds are on opposite sides so

They’re trans to each other and we can look at the atomic orbitals seen up here where the s cis allows for full overlap as does the s tran this has full overlapping of the orbitals as well any other sort of confirmation besides s cis or s trans is actually a higher energy state the question is why so if we have a double bond and it’s only about halfway rotated

So if we have our conjugated double bond and it’s only about halfway rotated which means i can take this single bond here and rotate it just 90 degrees if i rotate it just 90 degrees then what happens is these orbitals no longer overlap in the middle here because this one would be more like accumulated system where it’d be parallel with our plane here well this

Side would be perpendicular so any other confirmation besides having the s cis or s trans will result in a higher energy confirmation so our conjugated dienes exist in either one of these states and so now you guys should know a little bit about conjugated dyeings and at least what their structures look like so remember a conjugated diene has double bonds that

Are connected to each other through one sigma bond and the conduction conjugated dying can go on further and further and as many double bonds as we wish

Transcribed from video
Introduction to Conjugated Dienes By Stephanie Stepp