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Current time:0:00Total duration:7:54

Video transcript

sometimes when you're trying to synthesize a molecule you have to use a protecting group and in this video we're going to talk about how to protect alcohols using a tri alkyl silyl group and so let's say our goal is to make this target compound over here on the right and we need to start with this compound over here on the left and so you might think that this organolithium compound right here would function as a nucleophile alright so we have a negative 1 formal charge in this carbon so this lone pair of electrons right it's going to be our nucleophile and attack this carbon which is a little bit partially positive right here and so these electrons kick off onto your bromine and you would end up adding this carbon and this carbon on to form your target molecule so like that unfortunately this is not the reaction that occurs because not only can organolithium compounds be strong nucleophiles right they can also be strong bases and so what would actually happen is this lone pair of electrons here would function as a base right take this proton leaving these electrons behind on the oxygen to form an alkoxide so really you would form it would form this product over here so we would have an oxygen the oxygen would have now three lone pairs of electrons around it all right giving it a negative 1 formal charge and we'd have lithium plus so we would form an alkoxide product instead and so the point of a protecting group is we need to protect this hydroxyl group to prevent it from reacting right so if we can somehow protect this group we can allow our reaction to to occur at this portion of the molecule and then we could remove our protecting group to form our target compound and so that's the idea behind it so let's go ahead and show how we can use a protecting group so over here on the right we have this would be T butyl dimethyl shylo chloride right so there's a tert-butyl group attached to a silicon and then there's two methyl groups attached to the silicon and also a chlorine so this would be T butyl or Church butyl die methyl Sai little chlorides so tbdms CL and if you think about this silicon right this silicon is bonded to a carbon here carbon here a carbon here and a chlorine and all of those the carbon and the chlorine are more electronegative than the silicon so they're going to withdraw some electron density from the silicon making the silicon partially positive and so the silicon can function as an electrophile right it's electrophilic and we can get some electrons from the oxygen so the alcohol over here is going to function as a nucleophile and the lone pair of electrons is going to attack the silicon and then these electrons would kick off on to your chlorine and so we would lose HCl on the product in any process and the emitters all one of the things the the emitters all does is help to remove the HCL and the mechanism is a little more complicated than what I've shown but this is just a simple way of thinking about it right so you have nucleophile electrophile and you're going to put your protecting group on to your alcohol so let's go ahead and draw the product of this reaction right we would have now have our oxygen bonded to the silicon and our oxygen would have two lone pairs of electrons around it and the silicon is bonded to two methyl groups and also a tert-butyl group like that so we've put on our protecting group and sometimes you might see instead of drawing out all of that stuff around the silicon you might just see an oxygen alright and then you might see t be d ms for our protecting group which is our t-butyl dimethyl sie little protecting group like that so this would be another way of representing that portion of the molecule and so now that we've added now that we've added our protecting group we can go ahead and react with our organolithium compounds let me go ahead and draw in our organolithium compound again right so we had a carb anion here alright which now can function as a nucleophile so this lone pair of electrons right could attack this carbon right here and these electrons would kick off onto the bromine and so we can go ahead and and draw what we would get from that so now we would have all right we would add on our triple bond right here so once again let's highlight some carbons alright this carbon would have added on to here this carbon is right here and then we have these electron fun's right formed this bond right here like that and so we still have our protecting groups let's go ahead and draw that too right we have our oxygen and bonded to our oxygen we have our silicon with our methyl groups and also our tert-butyl group like that and so now that we've done the the desired reaction now we can take off our protecting group right so we can remove it to form our target compound and so we need to have something that reacts selectively with the silicon here and so we're going to use tetra butyl ammonium fluoride so tetra butyl ammonium fluoride which is really just a good source of fluoride anion so I'm going to go ahead and draw in fluoride anion here which is normally a extremely poor nucleophile so but it's actually selected for for silicon so if the fluoride functions as a nucleophile right it's going to attack the silicon here and it can do this for for a couple of reasons so let's let's talk about let's talk about those reasons here so first of all this silicon is bonded to some carbons right and silicon is bigger than carbon right if you look at where it is in the periodic table and so the silicon carbon bonds are longer than we're used to seeing and that means that there's decreased steric hindrance right so the silicon is a little bit more exposed and that allows a fluoride anion to attack it a little more so another factor that allows this is is silicon is in the third period on the periodic table so it has vacant D orbitals and so we can go ahead and show a bond forming right between the fluorine and the silicon so let me go ahead and draw what we would get after the fluoride attacks the silicon so we would have this portion of the molecule all right and we would have our oxygen oxygen bonded to our silicon and this in this intermediate and now we could show the fluorine bonded to the silicon like that and the silicon is still wanted to two methyl groups and also a tert-butyl group like that this will give these to look on a negative one formal charge and it looks a little bit weird because we see silicon half has five bonds to it but that's a and once it's okay because of where silicon is on the PRI table right it has those has those d-orbitals and so forming forming five bonds for an intermediate is okay it's okay for it to have an expanded octet another reason why fluoride can attack the silicon very well is because the bond that forms between fluorine and Silicon happens to be very strong all right so it's a very strong a single bond here and we can we can finish up by by kicking these electrons back on to the oxygen and protonating and forming our target compound so we would go ahead and form our target compound here so we would get back our alcohol like that and we also successfully added on this portion of the molecule on the right and then we would also form right we now have the Florent the fluorine bonded to the silicon like that so we selectively removed our protecting group and we formed our target compound and so that's the idea of of a protecting group it allows you to to protect one area of the molecule and react with another area of the molecule and it's also nice to have it easily removed to get back your target molecule