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Current time:0:00Total duration:12:17

Addition of water (acid-catalyzed) mechanism

Video transcript

anytime you're trying to come up with a mechanism for a reaction it's worthwhile to study a little bit of what you are starting with and then thinking about what you finish with and think about what is different so what we're starting with we could call this one two three four five so this is let's see we have a methyl group on the number two carbon it is a pentane and that's double bond between the number two number three carbons so this is two methyl two two methyl pent pent two E and two E so that's what we start with we're in the presence we're in a siddik environment we've got it's going to be catalyzed by our hydronium here and we end up with this and how is the product different from what we started with well the double bond is now gone the number three carbon gains this hydrogen and now the number two carbon gains a hydroxyl group so one way to think about this is in the presence of an acid its acid catalyzed we have gained two hydrogen's and an oxygen which is what we've gained that the built that what could be used to make a water and this is actually called an acid catalyzed addition of water there's the water isn't sitting on one part of the molecule but if you take the hydrogen we added and the hydroxyl we add it if you combine them that's that's what you need to make a water so let's think about how we can how this actually happens in the presence of our hydronium so let me redraw this molecule right over here so let me copy and paste it so and that's not exactly it yet that is just with the single bond so let me draw whoops wrong tool let me draw the double bond there and now let me put it in the presence of some hydronium alright so we have an oxygen bonded to so this would just be water as oxygen has two lone pairs but hydronium is a situation where the oxygen is sharing one of those lone pairs with a hydrogen proton thus making the entire molecule positive because the hydrogen you could view if the hydrogen proton was positive so there you go this now has a positive charge and this can be pretty reactive because we know that oxygen is quite electronegative it likes to keep its electrons so what if there was a way what if there is a way for the oxygen to take back the electrons in this bond right over here the two electrons in this bond well what if one of these carbons especially the ones that have the double bonds what if some of the electrons from this double bond could be used to snap to take that hydrogen proton and then oxygen can hog its electrons again and you might say well that's that's reasonable but which of these carbons would actually do it and to think about which of those carbons would do it we have to turn to markovnikov's rule markovnikov's rule tells us that look if you have a reaction like this an alkene reaction the carbon that already has that's already attached to more hydrogen's is more likely to gain more hydrogen's the carbon that's attached to more functional groups is more likely to gain more functional groups another way to think about it is think about well what what is the order of the carbons because the higher order of the carbon the more stable it will be if it forms some type of cation so if you look at this carbon right over here our number two carbon let me circle it our number two carbon is bonded to one two three carbons so this is a tertiary carbon this one right over here this carbon on the other side of the double bond it's only bonded to one two carbons so this is a secondary carbon so the tertiary carbon is going to be more stable as a carbo cation you can think of it as it can spread the charge a little bit so it would be more likely to lose the electrons in one of these in one of these bonds and so the way that we could think about this mechanism and it might be a little bit clearer when it we form the carbo cation is let's have I'll do this in blue let's have these two electrons that form this bond well now they form bond with that hydrogen and now the oxygen can take back these two electrons and what are we going to what is going to result and I'm drawing an equally remember all these things are going back and forth depending on how things bump in to each other but what are we left with so let me copy and paste this again and I'm copying and pasting it away that I've just so I kind of this is that if you come view this as the backbone and I'll add what I need to add so once this happens we have this carbon the number three carbon now whoops I keep using the wrong tool the number three carbon now forms a bond with this hydrogen just like that this carbon our number two carbon has lost an electron it's no longer sharing this bond and so now it is going to have a positive charge it is a carbo-cation and once again is a tertiary carbo cation it is bonded to one two three carbons that is stable more stable than if we did the other way around if this one grabbed the hydrogen somehow then this would be a secondary carbo cation it would be harder for it to spread that positive charge around and what about our what about our molecule up here well let's see what it looks like now we have our oxygen bonded to the two hydrogen's it had one of those lone pairs and now the electrons in this bond are now going to form another lone pair so it took back an electron or you could think of it it gave away a hydrogen proton and so this is now just neutral water and we see that we have a conservation of charge here this was positively charged now our original molecule is positively charged and what feels good about this is we're getting we're getting close to our end product at least on our number three carbon we now have we now have this hydrogen now we need to think about well how do we get a hydroxyl group added right over here well we have all this water we have all this water floating around let me I could use this water molecule but the odds of it being the same water molecule we don't know but there's all sorts of water molecules we're know aqueous solution so let me draw another water molecule here so they're the water molecules are all equivalent but let me draw another water molecule here and you can imagine it's just or if they just bump into each other in just the right way this is water as a as a polar molecule has a electro it has a partially negative end near the oxygen because the oxygen likes to hog the electrons and and then you have a partial positive end near the hydrogen's because they get their electrons hogged so you could imagine the oxygen end might be attracted to this tertiary carbo cation and so just bumping in in just the right way it might form a bond so let me let's say these two electrons right over here let's say they form a bond with this with that number two carbon and then what is going to result so let me draw so what is what is going to result let me scroll down a little bit and let me copy and paste whoops let me copy and paste our original molecule again so there we go so what could happen so let me construct it actually so we have the hydrogen there we have the hydrogen now this character so we have the water molecule so oxygen bonded to two hydrogen's you have this one lone pair that isn't reacting but then you have the lone pair that does do the reacting and so it now forms a bond oops let me do it in that orange color it now it now forms an actual bond and we're really close to our final product we have our hydrogen on the number three carbon we have more than we want on our number two carbon we just want a hydroxyl group now we have a whole water bonded to the number two carbon so somehow we have to get one of these other hydrogen's swiped off of it well and that could happen with just another water molecule so let's draw that so another water molecule someplace I'll do in a different color just to differentiate although as we know water it's hard to say even what color is water if you're looking at the molecular scale so here we go and we're really in the homestretch at this point you have another water molecule another water molecule let's say let me pick a color so let's say these electrons right over here maybe they form a bond with that hydrogen proton and then these the electrons in that bond can go back to form a lone pair on that oxygen and then what are we left with and this really is the home stretch so we are in equilibrium with so let me draw my five carbon so let's see I have a ch3 C carbon-carbon car a ch2 ch3 I have a ch3 I say a ch3 seed instead of ch3 I wrote it that way just so it's clear that the carbons are bonded to the carbons you have you have the original hydrogen right over there you have the one that we just added as a part of this mechanism you have this orange bond - now this hydroxyl group a hydroxyl group and it had one lone pair before it had one lone pair reformed but now took both of the electrons from this bond to form another lone pair to form another lone pair which I am depicting in pink and then this water is now or this water molecule is now a hydronium molecule so let's draw that so this is now oxygen hydrogen hydrogen had one lone pair that didn't react but it had one lone pair that I put in blue that is reacting with this hydrogen proton with that hydrogen just like this and so since it got the hydrogen proton it's giving its elector sharing its electrons now now this has a positive this has a positive charge just like that oh I have to I have to be very careful in the last step I forgot to draw the positive charge and you always want to make sure that your charge is being conserved we started off with a positive charge on the hydronium then we have the positive charge on a tertiary carbo cation right over here on a number 2 carbon and now we have the positive charge would be right over here because that oxygen what we saw before that oxygen which which this water molecule was neutral but you could say you could view it as well now it's going to be give it's now sharing these two electrons instead of keeping them so you could view it as maybe it's giving it away an electron and so now it becomes positive and then and then the positive charge finally gets transferred to that other water molecule when it becomes hydronium but just like that we are done we have added a hydroxyl group and a hydrogen combined that's a water so that's why we call it water addition of water and it was catalyzed by acid so it's acid acid catalyzed addition of water