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Conjugated Dienes | Organic Chemistry Lessons

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In this video lesson, we introduce conjugated dienes: compounds with alternating single and double bonds. Conjugated systems cause their pi electrons to be delocalized, which leads to many interesting properties and reactivities. We see how reactions of alkenes relate to conjugated dienes and the mixtures of products that can result. Finally, we study the topics of kinetic and thermodynamic control and how we can favor the production of kinetic and thermodynamic products.

Hello everyone and welcome back to another episode of total organic chemistry in this video we’ll be looking at conjugated dienes molecules with two conjugated double bonds and also some types of reaction control the questions that you should be able to answer at the end of this video are what is the structure of a conjugated diene how do these dienes react

In electrophilic additions just like regular alkenes what are kinetic and thermodynamic products and how do i favor those products one over the other if you’d like some more review on the reactions of just regular alkenes or also an introduction to delocalized pi systems please go ahead and subscribe to my channel and take a look at those videos before you

Continue on let’s start this video out with the simplest conjugated diene and this is called 1 3 butadiene so we have these two double bonds that are separated by one carbon and if we take a look at sort of a diagram of the orbitals here we can draw the carbon backbone where we have those four carbons and then we can draw the sigma bonds between those carbons

I’m going to leave out the hydrogens here just for clarity and we also know that the pi bonds from the double bonds are going to be formed from the perpendicular p orbitals to this plane so we can draw a p orbital from each carbon and that’s perpendicular to the plane of the molecule but now we can notice that instead of the two electrons in each pi bond being

Localized between two carbons the electrons in these pi bonds are actually delocalized because these four p orbitals are adjacent to each other so instead of having two discrete double bonds we actually have what is called a conjugated system where these electrons can move freely between all four of these carbons okay so what does this conjugation actually do

For us well we can measure the stability of alkenes using their heats of hydrogenation so if we take two different alkenes first this is one four hexadiene so we have six carbons and these two double bonds but we can notice that these are not conjugated so they’re separated by more than one single bond and if we put this in a reaction with hydrogen gas along

With a palladium on carbon catalyst we will produce the hydrogenated alkane just hexing then if we look at a different diene so this is 1 3 hexadiene where now we have these two double bonds and they are conjugated so they’re separated by only one single bond and if we perform the same reaction with this compound hydrogen gas with palladium on carbon this will

Form the same alkane the same hexane compound and if we measure the heats of reaction of each of these reactions we will find that the heat of hydrogenation of this first compound is going to be more negative than the second one and remember a more negative heat of reaction means it’s more exothermic and more exothermic means the products are going to be more

Stable than the reactants so this means that the non-conjugated alkene is much less stable than the conjugated alkene so although these two compounds are the same in all other respects so for each molecule we have one mono-substituted alkene and one we can see that the conjugation in the second one lends an extra stability to that molecule overall okay so we’ve

Seen how conjugation affects stability how does it affect reactivity of these alkenes well if we go back to our simple conjugated diene this 1-3 butadiene and we treat it with some hydrobromic acid hbr this reaction will be similar to that of regular alkenes and if you’d like some review on the reaction of hbr with regular alkenes you can go ahead and click on

That video at the top of the screen so what products will this produce well we’ll figure that out by drawing the mechanism so we start with our conjugated diene here and also our hbr and the first step of this reaction is going to be the electrons in that pi bond attacking the hydrogen on the hbr and that will cleave the hbr bond to give us this carbocation

Here and in this case because it’s a symmetrical diene it doesn’t matter which of the double bonds we choose for this first step and remember that because this is forming a carbocation we’re going to have markovnikov addition so this carbocation is going to be forming on the secondary position instead of the primary carbon and then the bromide ion ion that we

Get back from the first step we’ll attack this carbocation producing this compound here where we have the terminal alkene and the bromine on the secondary position with this hydrogen from the first step on the primary carbon but if we go back to this intermediate carbocation you might notice that this is an allylic carbocation and if you remember from my video

On allylic systems this carbocation can be delocalized so we can have this pair of electrons from the terminal alkene come over to form and double bond between these two carbons here and that will leave the positive charge on the primary carbocation so from this intermediate we can have the bromide anion come in to attack the primary carbocation giving us yet

Another product where we have now the bromine on the primary carbon and the double bond has shifted to be between carbons 2 and 3 instead of terminally so the first product we formed here the one without any rearrangement of the double bond we call this the kinetic product or also a one-two addition because the hydrogen and the bromine have added along the

One and two carbons so this is called the kinetic product because we form the most stable intermediate so in this case we have a secondary carbocation and that’s going to be more stable than the primary carbocation in the other intermediate the second product however is called the thermodynamic product or also 1 4 edition because those h and br atoms have added

Along the 1 and 4 carbons instead of on adjacent carbons so this is the thermodynamic product because it forms the most stable final product so in this case we have a disubstituted alkene which is going to be more stable than the monosubstituted alkene for the kinetic product so the existence of these kinetic and thermodynamic products is a very important thing

To watch out for when we’re performing these reactions with conjugated dienes let’s look at another example where the diene is not symmetric so we’re going to have a couple other products we can draw this alkene here where almost like the first one but now we have a methyl group on one of these carbons here and we can perform another reaction that we’re pretty

Comfortable with from alkenes this is with aqueous sulfuric acid so h2so4 in h2o and what products is this going to form well let’s find this out from drawing the intermediates so we know that for this reaction the sulfuric acid is going to protonate one of the double bonds so first of all let’s look at the double bond on the left being protonated and this

Is again going to be markovnikov addition so we know the carbocation is going to form on this carbon the secondary carbon and then we know that the water will come in as the nucleophile and we’ll end up with this product where the oh is bonded on the secondary carbon and we have this terminal alkene on the right hand side we can also think about protonating the

Other double bond so that will give us this intermediate here where we now have a tertiary carbocation and just as before we’re going to add oh to that carbocation giving us this tertiary alcohol where now the double bond is on the left hand side okay so now for the other products we’re going to have to think about the rearrangement of that allylic carbocation

So let’s think back to the first protonation where we end up with this secondary carbocation and the alkene on the right hand side of the compound we know that that carbocation is delocalized so the electrons in the double bond will come down to form the tri-substituted alkene and the carbocation will be on the right-hand side here in a primary position and

That means we can add the o-h to that carbocation giving us this final product and finally let’s look at the second carbocation again so this is the tertiary carbocation that we formed earlier just like before it is delocalized so the electrons in the pi bond can come up to form the tri-substituted alkene again leaving us with this primary carbocation and as

Always the oh can capture that carbocation giving us this final product so we can label these products as kinetic for the first two because we have a more stable carbocation whereas the second two products are thermodynamic products because we have less stable intermediates so those carbocations are primary rather than the more stable secondary and tertiary

Carbocations in the kinetic products and also because that alkene is more substituted and more substituted alkenes are more stable so again these are the thermodynamic products so if i’m performing this reaction and i get a mixture of four different products how can i favor the creation of one product over another so this gets into what we call kinetic and

Thermodynamic control and this is incredibly useful when we’re performing reactions with these conjugated systems so as a rule of thumb kinetic control is when we perform our reaction at lower temperatures and with shorter reaction times so performing either of the previous two reactions at a relatively low temperature much lower than room temperature and only

For a short amount of time maybe an hour or two that will favor the creation of the kinetic products and this is because those intermediates are more stable and there’s not really enough time for the products to equilibrate with those more stable more substituted alkenes on the other hand thermodynamic control is where we perform the reaction at relatively high

Temperature with longer reaction times and that will favor the thermodynamic products over the kinetic ones so if we did either of these addition reactions at elevated temperature for maybe several days as a reaction time that will give us the thermodynamic products over the kinetic ones and again that’s because the elevated temperature and the long reaction

Time are allowing those intermediates to equilibrate and we end up with the more stable final product even though the intermediates of the kinetic products are going to be more stable kinetic and thermodynamic control are incredibly useful in organic synthesis and we will be revisiting those topics in later videos for now i hope this video helped you understand

Conjugated dienes and some of the simpler reactions that they undergo if you like this video please like and subscribe to my channel take a look at my page on facebook and visit my website on the screen finally if you’re willing and able please consider donating to my patreon page which allows me to continue creating this content for all of you thanks for watching

Transcribed from video
Conjugated Dienes | Organic Chemistry Lessons By Total Organic Chemistry