Let’s first take a look at the molecule on the top here and i can see that this molecule has two double bonds in it so we’ve already covered how to name molecules that have two double bonds in its right i could start numbering from the left or from the right since it doesn’t matter for this molecule i’ll just start from the right so i get 1 2 3 and 4 and so we have
A 4 carbon molecule so bute would be the root and we have 2 double bonds so it’d be a dying alright so this would be butadiene right here so butadiene like that the first double bond starts at carbon one and the second double bond starts at carbon 3 so 1 3 butadiene 1 3 butadiene is an example of a conjugated dyeing and the term conjugation refers to the alternating
Single and double bonds present in the molecule right so let’s go ahead and write conjugation here and we can see that 1 3 butadiene is conjugated and right you have a double bond followed by a single bond followed by a double bond and conjugated molecules are special because if i think about each carbon each carbon in 1 3 butadiene right this carbon right here is
Sp2 hybridized this carbon is sp2 hybridized this one and this one are also sp2 hybridized so i have i have a 4 sp2 hybridized carbon so let’s go ahead and draw those carbons over here on the right since each one is sp2 hybridized each of those carbons has ap orbital alright so i’m going to go ahead and draw ap orbital on each of those 4 carbons there and we can see
That we have p orbitals right next door to each other right so 4 p orbitals next to each other means that we can get some of those pi electrons to spread out throughout the molecule here so because we’re going to get d localizations of d localization of some of those pi electrons that allows for a little bit extra stability for this conjugated dying so in terms of
What exactly is happening i’m going to leave that for the for the video on molecular orbital theory so we’re gonna do a video on 1 3 butadiene mo theory so for right now we just know it’s a little bit more stable due to the fact that it has p orbitals and sp2 hybridized carbons right next door to each other this carbo cation down here is similar because if i think
About this carbon right this carbo cation is sp2 hybrid this carbon is sp2 hybridized and this carbon is also sp2 hybridized so each of those carbons is going to have ap orbital and we can also draw a resonance structure for this carbo cation all right so i can go ahead and put in my residence bracket and arrows here and i can think about these pi electrons moving
Over here to the right and so let’s go ahead and draw the results of that resonance structure here so the the pi the pi electrons moved over to the right which means the carbo cation is now on the carbon on the left like that and so this carbo cation it’s a primary carbo cation which normally doesn’t happen in organic chemistry but this carbo cation is resonance
Stabilized we call this an allylic carbo cation so this is an allylic carbo cation so an allylic carbo cation which is more stable than a regular carbo cation you could draw a resonance structure for it and of course you could think about each carbon being sp2 hybridized alright so once again we go ahead and draw our carbons we think about each carbon being sp2
Hybridized hybridize meaning each carbon has ap orbital which means that those electrons are those those two pi electrons are spread out or delocalized over all three of those carbons here and that positive charge is spread out over those two carbons on the end as well in the resonance hybrid here so an allylic carbo cation is more stable than a regular carbo
Cation and now we’re going to take a look at an addition reaction to a conjugated dying and you’re going to see that an allylic carbo cation plays a major role in the mechanism so let’s look at the mechanism for an addition reaction to a conjugated die in here so we’re going to start with with with 1 3 butadiene so let’s go ahead and draw that and 2 1 3 butadiene
We’re going to add a hydrogen halides so i’m gonna go ahead and put h x right here as our generic hydrogen halide let me go ahead and number my 1 3 butadiene again somebody that’s 1 2 3 & 4 so we’ve seen the reaction of alkenes with hydrogen halides before and in that reaction the pi the pi electrons functioned as a base and picked up a proton from my hydrogen
Halide so it’s going to be a the exact same art here so i’m gonna say that the pi electrons between one and two are going to function as a base so these pi electrons in here are going to pick up a proton and these electrons would kick off onto your halogen like that so now the question becomes where does the proton add the proton could add to carbon one or the
Proton could add to carbon two let’s go ahead and show the proton adding to carbon one to start with so i’m going to show the proton adding to carbon one like that i still had a double bond between three and four i just took a bond away from carbon two so that’s where my carbo cation is going to go and immediately you should see that that carbo cation is an allylic
Carbo cation right so if i if i look at this carbon that has the positive charge on it now at carbon two that’s right next door to an sp2 hybridized carbon and over here is an sp2 hybridized carbon as well so that’s an allylic carbo cation it’s also a secondary allylic carbo cation if i think about what would happen if the proton added on to carbon two let’s go
Ahead and show that over here so i could think about the proton adding to carbon two and so i’m going to show the proton i on the carbon two right here which means i took a bond away from carbon one which means my carbo cation is at carbon one that’s a primary carbo cation and i’d say think about this carbon is having the positive charge on it that’s a primary
Carbocation it’s attached to one only one other carbon and that one other carbon does not have a p-orbital right that carbon is not sp2 hybridized and so this is not an allylic carbo cation it’s a primary carbocation that is not stabilized so this does not form right you’re not gonna see the proton add on to carbon 2 you’re gonna see the proton add on to carbon 1
In the mechanism which creates a secondary allylic carbo cation here which is resonance stabilized so if we think about the next step of the mechanism right we had a halogen with three lone pairs of electrons was just picked up another lone pair so we now have a halide anion and so are our halide anion is negatively charged which means that’s going to function as
Our nucleophile so we have we have a nucleophile and we have an electrophile right so opposite charges attract a lone pair of electrons on my halide anion are going to attack my carbo cation to form a new bond all right so if i go ahead and show the the product right of nucleophilic attack at carbon 2 i’m going to still have my proton at carbon 1 i’m going to show
My halogen has just added on to carbon 2 now and i still have a double bond between carbon 3 and 4 so let me just go ahead and number my carbons again 1 2 3 & 4 so i just showed my halogen adding to carbon 1 and sorry my proton adding the carbon 1 my halogen adding to carbon 2 this is the one to add up alright so an addition reaction to a conjugated dying this
Is one of the possible products that can form it one two addict where the proton adds on to carbon 1 and the halogen adds on to carbon 2 if i go back up to your here to my allylic carbo cation i know i can draw a resonance structure for my allylic carbo cation so let’s go ahead and see what happens all right if i draw a resonance structure for this which would mean
I would take these pi electrons and move them in here so if i were to draw the results the resulting resonance structure my pi electrons now moved between carbons 2 and carbon 3 i took a bond away from carbon 4 which means that is where my carbo cation is now like that okay so let’s go ahead and think about what’s going to happen next well of course my still have
My halide anion alright which with a negative 1 formal charge so this time my halide anion functioning nucleophile it attacks my electrophile which is at carbon for now so if i were to draw the result of that nucleophilic attack all right i added a proton onto carbon 1 i have a double bond between carbon 2 and carbon 3 and my halogen added on to carbon 4 so let me
Go ahead and put in those lone pairs of electrons like that and if i number find number my molecule 1 2 3 and 4 i can see that i added a halogen a sorry a proton onto one and a halogen on to force this is the one for adduct this is the one for as a product here and let me go ahead and put this other resonance bracket in here as well so you can see that when you
Form an allylic carbo cation right it’s possible to get two different products because of the resonance structure you can get a one two addict and you can get a one four addict and it’s necessary to think about both of these as as being the products in your reaction so in the next video where we’re going to do an example of this addition reaction to conjugated ions
Transcribed from video
01 Addition reactions of conjugated dienes 01 Addition reaction of conjugated dienes I Mechan By L Hub