we're going to talk about the 1 2 and the 1 4 addition products with dienes but let's begin our discussion with dienes so here is a typical alkene it has one double bond a diene is basically an alkene but with two double bonds now there's different types of dienes this particular diene is known as a conjugated diene the reason why it's conjugated is you have alternating double and single bonds here it's a double bond this is a carbon-carbon single bond and that's a double bond conjugated dienes have special characteristics now there are other type of dienes that you need to be familiar with so this is an isolated diene because the double bonds are too far apart from each other and then you also have accumulated dienes so that's where the double bonds are very close to each other of these three the most stable is the conjugated ion due to resonance an isolated diene reacts in the same way as a regular alkene because the double bonds are far apart but a conjugated diene it reacts in a different way and we're going to focus on that in our discussion today so let's begin so let's start with this common diene this is called one three butadiene if we count the number of carbons we have a total of four carbons so think of butane and we have a double bond on carbon one and three so it's called one three buta diene now what we're gonna do is we're gonna react this dyeing with hydrobromic acid hbr but we're going to react it under two different conditions we're going to react it at low temperature let's say negative 40 degrees celsius and at high temperature which we'll say at 60 degrees celsius what will be the major product for this reaction what would you say well at low temperature you're going to get something called the kinetic product the hydrogen is going to add on carbon 1. this is let's call this carbon one two three and four by the way due to the symmetry of this diene these two alkenes are equally reactive so the hydrogen is going to go on carbon one the bromine is going to go on carbon two we're going to have a double bond between three and four so this is called the one two addition product because hbr added to carbon to one and two now this is also called the kinetic product because it forms faster now at high temperatures the thermodynamic product will be the most stable product it's going to be the major product the hydrogen is going to add to carbon 1 but the bromine atom is going to add to carbon 4. and we're going to have the double bond between carbons 2 and three so this is called the one four addition product as you can see on carbon one we have the hydrogen and on the carbon four we have the bromine atom now this is also called the thermo dynamic product it is the product with the most stable alkane i mean the most stable alkene I take that back so this is the product that forms at high temperature now the kinetic product the reaction associated with the kinetic product is an irreversible reaction so we're going to use one arrow to represent it the reaction associated with the thermodynamic product is reversible so we're going to have two arrows but typically the one pointing to the right is going to be the bigger arrow because this product is the most stable product now let's talk about why this product is the most stable product the answer has to do with the alkene stability notice the number of r groups that are attached to this alkene this particular alkene has two r groups so it's a disubstituted alkene this alkene only has one r group that are attached to the two carbon atoms that are double bonded so this is a mono-substituted alkane so it is the least stable of the two so make sure you understand that difference the kinetic product is the product that forms faster ideally at low temperatures the thermodynamic product is the most stable alkene product that forms as the major product at high temperatures now let's briefly review alkene stability so this is a tetrasubstituted alkene it has four alkyl groups attached to the two carbon atoms that are double bonded this is a tri-substituted alkane it has three r groups a tri-substituted alkene is more stable than a di-substituted alkane here we have a trans dye substitute alkene and that's usually more stable than a cis disubstituted alkene and here we have a mono substitute alkane which is more stable than ethene which i'm just going to write like that so the more r groups that you have around an alkene the more stable it is and that's going to help you to identify the thermodynamic product but now let's go back to the reaction of one three butadiene with hbr and this time let's talk about the mechanism so let's react one three butadiene with hbr at low temperature conditions so as was mentioned before we're going to get the kinetic product as the major product even though both products can form this reaction is under kinetic control so the one that forms faster is going to be the major product when the temperature is low now in this reaction the diene is going to behave as the nucleophile hbr the acid is going to be the electrophile so we're going to write up a mechanism and to show the arrow just remember when when drawing the arrow it's going to start from a region of high electron density to a region of low electron density this double bond is electron rich hydrogen being partially positive is electron poor so the arrow basically describes the direction of electron flow so to speak so when the double bond interacts with the hydrogen the bond between hbr is going to break those two electrons is going to be pulled towards the more electronegative bromine atom now here's a question for you why should we put the hydrogen on carbon 1 and not on carbon 2 notice what happens if we put the hydrogen on carbon 2. if we were to do that we would get a primary carbocation but if we were to put the hydrogen on carbon 1 the plus charge is going to be on carbon 2. so we get a more stable secondary carbocation but notice that it's adjacent to a double bond so what we get is a secondary allylic carbocation and so that's the reason why hydrogen is going to add on carbon 1. it's because we get a more stable carbocation intermediate so that's the first thing you need to consider where will the hydrogen go now what will the bromine atom do or more specifically the bromide ion now by the way this structure has a resonance structure we could move the alkene here if we take those two pi electrons and move it to the left we can get another intermediate so this is going to be a less stable primary allylic carbocation so this is secondary and this is primary so just looking at the carbocation stabilities this particular resonance structure is more favorable than this one so because the secondary allylic carbocation is more stable it's going to form faster because there's less energy that's required to generate this particular resonance structure now which carbocation will the bromide ion interact with is it going to be the secondary carbocation or the primary allylic carbocation well as was mentioned before the fact that this is more stable means that bromide is more likely to interact with this secondary carbocation but that is not the only reason there's also something called the proximity effect because bromide is so much closer to the secondary carbocation it's easier for it to interact with that particular carbocation this positive charge is further away from the bromide ion so it's less likely that the bromide ion is going to interact with the primary carbocation so therefore there are two factors that are favoring the one-two kinetic product the first most important factor is the proximity effect and for this specific example the second thing is we have a more stable carbocation intermediate so once the bromide ion combined with the carbocation we're going to get the one two addition product so at low temperature conditions this is going to be the major product and the driving force is the proximity effect and the fact that we get a more stable carbocation intermediate now let's draw the resonance structure for the formation of the 1 4 product so let's react 1 3 butadiene with hydrobromic acid at high temperature conditions so let's use positive 80 degrees celsius so at a high temperature this reaction is reversible so the most stable alkene product will be the major product so like before we're going to react the alkene with hbr and we're going to put the hydrogen on the primary carbon so that we can put a positive charge on the secondary carbon now to get the 1 4 product we need to draw the resonance structure where this double bond is going to move here and now we have the plus charge on the primary allylic carbocation and then the bromide ion is simply going to interact with the carbocation and so that's how we can get the 1 4 thermodynamic product so that's the mechanism that you can write to show it and so for this one the driving force is alkene stability the thermodynamic product is the one with the most stable alkene product