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Conjugated dienes

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The focus of this chapter is on conjugated pi systems and so we’re going to start off by looking at the simplest conjugated pi system a conjugated dying alright so what is a dying well this is just a fancy way of saying an alkene with two double bonds when we named alkenes we end the name in ane and so a dying two double bonds there are three types of dying’s that

We might be interested in there’s cumulated dines or the pi bonds are directly adjacent to one another there’s conjugated dying’s which is what we’re going to be primarily interested in and that’s where the double bonds are separated by exactly one single bond and then isolated iines are when the double bonds are separated by two or more single bonds so in this

Chapter we’re interested in conjugated pi systems and what we mean by conjugated is when the double bonds are separated by exactly one single bond we actually have a continuous network of p orbitals and the pi electrons the electrons that are in these pi orbitals are spread out over the entire network they’re delocalized among all these carbons accumulated dining’s

There is no conjugation because the pi bonds are perpendicular to one another so we don’t have this conjugated system and when we have at least two single bonds separating the pi bonds they’re too far away right there’s a gap and so they don’t form this delocalized system and these conjugated systems these delocalized systems have some special properties and that’s

Why we have a chapter devoted to those all right so as i’ve mentioned the focus of this chapter is on conjugated systems and the special properties and reactivity associated with them in the simple examples i’ve given so far we’ve only had carbon atoms but we can also have hetero atoms involved in these conjugated systems again as long as the double bonds are just

Separated by one single bond it’s fine to have a nitrogen or an oxygen as part of the conjugated system all right so how do we make dines well we make them these would make them is the same way that we learned to make alkenes last chapter and that’s through elimination reactions so if we have an alkyl halide we hit it with a strong base we can get an elimination

Reaction that will give us our dye and we can either start with where we already have one double bond and we have one halogen or we can start with two halogens as long as they’re on adjacent carbon atoms we can make a conjugated dyeing now the choice of using the tert butoxide as the base this helps encourage e two reactions and it eliminates the possibility of

Getting an sn2 reaction if we use something like any o on this molecule right we get primarily e2 but we get some sn2 competing so by choosing a bulky base that will eliminate even more of the potential sn2 products so one of the first features that you notice about conjugated dying’s is that the length of the single bond separating the two double bonds is shorter

Than we would expect for a typical carbon-carbon single bond one way of explaining that is the fact that these two carbons when we have the conjugated diene are sp2 hybridized right in in this case of ethane they’re sp3 hybridized well why does that matter so the more s character there is in these hybrid orbitals the shorter the orbitals are and so the shorter

Of the sigma bonds that they make will be the p orbitals are significantly longer than s orbitals and so when we have an sp3 orbital that mixes three p orbitals with 1s orbital so it’s only 25% s character when we go to an sp2 orbital now we’re mixing two p orbitals with 1s orbital so now it’s up to 33% s character and so it gets a little bit shorter and so we’d

Expect the sigma bonds that it would make would be shorter than the sigma bonds made with an sp3 orbital in the next video or i guess in probably two videos from now we’ll talk about molecular theory and give another explanation for why these sigma bonds and the conjugated dining’s are shorter than we might expect so probably an even more important property of

Conjugated dying’s is their special stability conjugated dying’s are more stable than non-conjugated alkenes one way we can measure that is through the heats of hydration so how much heat is given off when i convert the alkenes into an alkane if i take 2 1 beautiness that just have one single bond versus 1 3 butadiene we can see that the energy released when we

Hydrogenate the conjugated dying is less than the energy that is given off when we hydrate two of the 1-butene and so the difference in those hydration energies tells us how much more stable the conjugated diene is and so you don’t have to worry too much about this hydrogenation i think the main take-home message from this slide is that conjugated designs are extra

Special stable compared to regular alkenes alright the next thing we want to consider is the confirmations of these conjugated dying’s so we know that it’s possible to freely rotate around sigma bonds so these two confirmations don’t represent different molecules they’re not different isomers the same molecule is free to rotate around this single bond so then these

Are just two different arrangements of 1 3 butadiene and we call these s sis and s-trans so they’re not like sis and trans when we have sis and trans in regular alkenes right because that is how things are arranged around a double bond which we can’t rotate around so the s sisson s trains sort of like pseudo cysts and pseudo trans right these are the same molecule

Just different confirmations but these are the two most stable conformation of 1 3 butadiene and the riesz that these are the two most stable conformation because in both of these we can have the p-orbitals in this same plane and so we get conjugation when the molecule absorbed ops one of these two confirmations when we twist and so when we’re when they’re not

Totally flat then we can’t line up the p orbitals like this and so we lose the conjugation and so that’s less stable and in fact the energy that is required to switch back and forth between the two confirmations is fifteen kilojoules per mole and interestingly you may note that this is the same energy as the stabilization energy of the conjugated diene regular

Relative to a regular alkene so what this really telling us when we go from the assist to the s trans right we have to break up that conjugation in order to convert from one conformation into the other and it makes sense that it’s the same as the stabilization energy of the of the conjugated diene and you’ll also notice that the s trans is more stable than the

Assists and that’s due the fact that has less steric hindrance right that the groups are able to kind of get away farther away from each other or not bumping into each other as much and if we look at this reaction energy diagram again for converting from one conformation into the other the top of this reaction energy diagram the transition state is when the the

Two pi groups are exactly perpendicular to one another so there’s absolutely zero conjugation at the transition state here

Transcribed from video
Conjugated dienes By Benjamin Shepler