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Current time:0:00Total duration:10:23

Video transcript

let's look at a few nucleophilic substitution reactions of alcohols and I'm assuming you've seen an sn1 and sn2 mechanism before let's start with a primary alcohol so this is ethanol it's a primary alcohol because the carbon bonded to the O H is bonded to one other carbon and primary alcohols react with hbr to form an alkyl bromide via an sn2 process and so we have HBR our strong acid and we have ethanol which is going to function as a base so actually the first step is to approve eight the oxygen alright so we get a proton transfer here so lone pair of electrons on the oxygen pick up this proton and these electrons are left behind on the bromine two E's so we form our bromide anion here so we get a bromide anion negative one formal charge so these electrons in here in blue all right let's say that these are these electrons right here on our bromide aniline so we're going to protonate the oxygen so let's go ahead and show that oxygen being protonated here so there's still one lone pair of electrons on this oxygen which gives us oxygen a plus one formal charge so these electrons in magenta right here on this oxygen pick up this proton to form this bond right here and so now we need to think about our sn2 type mechanism so in an sn2 type mechanism we need a nucleophile and we need an electrophile right we're also going to have our leaving group here and the reason why we protonate the oxygen is to form a much better leaving group so if we just have a nucleophile attack ethanol right here we'd have hydroxide as a leaving group that is not in that is not a good leaving group here we would have water as our leaving group which is what we're going to see in a second and so that that helps this process occur so let's think about this oxygen here withdrawing some electron density from this carbon that I just marked in red so this carbon in red is the electrophilic portion right so the nucleophile is going to attack the electrophile so the bromide anion is going to function as a nucleophile and it's going to attack our electrophile and going to form a bond between the bromine and the carbon at the same time these electrons in here are going to come off onto the oxygen so an sn2 type mechanism you remember is concerted so the bond the bond that forms right this happens at the same time that this bond breaks and the electrons come off on to our leaving group so when we draw our final product right we would form ethyl bromide or a bromo ethane let's put in our electrons right here and so these electrons in blue right formed a bond between the carbon and the bromine so this is the carbon in red down here and so we form bromomethane we'd also form water so let me go ahead and draw in water so h2o which is a good leaving group right because water is so stable so we go ahead and think about these electrons in green here coming off onto the oxygen and that gives us water so in an sn2 type mechanism sometimes you to think about stereochemistry but not for this example because this carbon in red right here so this carbon in red right here it's not a chiral center right there are two hydrogen's attached to that carbon in red and so this is our only product this is the only thing that we have to think about so bromo ethane as our final result all right let's look at another sn2 reaction this time we're doing a secondary alcohol and so here we have our secondary alcohol the carbon bonded to the O H is bonded to two other carbons so a secondary alcohol and we saw in an earlier video that alcohols will react with tosyl chloride and pyridine to form at oscillate so let's go ahead and draw the product alright at the first step so we form at oscillate we retain the stereochemistry so we had a wedge over here and we have a wedge down here as well so we have an oxygen right and then we'll just go ahead and put TS here for our abbreviation so we talked in great detail about this in an earlier video so once again this gives us a better leaving group so in the previous reaction right we protonated the oxygen to give us a much better leaving group and here we have an excellent leaving group so that's one of the reasons for using tossa lates here so in the second step we're going to add sodium bromide and we're going to get again an sn2 type mechanism so a nucleophile is going to attack our electrophile so we can identify our electrophile right it's the carbon into the oxygen so the oxygens withdrawing some electron density from this carbon so this is our the electrophilic portion of the molecule and then once again our bromide anion is going to function as a nucleophile so here's our bromide and I'm going to highlight this lone pair of electrons right here in blue and so our bromide anion attacks our electrophile at carbon at and forms a bond at the same time these electrons come off on to the oxygen so once again a concerted sn2 type mechanism and this time we do have to worry about stereochemistry right so we have this wedge in here we have this wedge so this part is coming out at us in space and so the bromide anion has to attack from the opposite side and so and so if it's attacking from the opposite side when you draw the final product you'd have to show you'd have to show this is a - right so the bromine it had to attack from the opposite side which gives us inversion of configuration inversion of absolute configuration here so when you assign your your absolute configuration for your starting alcohol right it's a B R so our chiral Center would be right here so this carbon is chiral and so this carbon is chiral for our products and for our product moved form the s and the antia mirror here so we've formed the S enantiomer and we started with the R and neon tumor so an sn2 type mechanism inversion of configuration because the nucleophile has to attack from the opposite side all right so so let's look at one more example here let's do an sn1 mechanism using a tertiary alcohol alright so let's say let's do that so here we have tert-butyl chloride or tert-butanol reacting with concentrated hydrochloric acid alright so concentrate hydrochloric acid is going to function as an acid our alcohol is going to function as a base and let me just go ahead and highlight the fact that is going to be an sn1 type mechanism because we have a tertiary alcohol right this carbon bonds to the O H is monitored three other carbons and so the alcohol functions is the base right and is protonated and we would form the chloride anion here so let's go ahead and draw that in so we'd have the chloride and - of negative one formal charge and let's go ahead and put those electrons in blue like we did before so these electrons in here alright let's say that those are these electrons so the chloride anion we protonate the oxygen so let's go ahead and draw that so if we protonate the oxygen we have our tert-butyl group over here alright protonate the oxygen so a +1 formal charge on this oxygen so let's go ahead and draw that in so +1 formal charge on the oxygen electrons in magenta right here pick up this proton let's say it's this bond right in here so once again we have an excellent leaving group like we talked about before water is a good leaving group and so in an sn1 type mechanism alright these electrons are going to come off on to the oxygen which gives us water and also gives us a carbo cation alright so let's go ahead and sketch in this carbo cation in a plus one formal charge on this carbon right here this carbon in red gets a plus 1 formal charge that's this carbon because it's losing its losing a bond alright so let me go ahead and highlight the electrons that it's losing so these electrons in here are coming off on to the oxygen to form water so we could go ahead and draw water over here so we lose water at this point so h2o and the electrons in green come off onto this oxygen right here so we form h2o taking a bond away from the carbon in red so we form a carbo cation so this is a stable carbo cation this is a tertiary carbo cation all right so that's why this is this one this tertiary alcohol reacts via an sn1 type mechanism right the stability of the carbo cation and so in our in our final step we have the nucleophile is going to attack our electrophile right so the nucleophile attacks our electrophile the chloride anion attacks our our carbo cation the text that carbon there and so we form our final product which is tert-butyl chloride so let me go ahead and draw on these electrons and let's highlight some again the electrons in blue and these electrons form the bond all right so they bonded right here and so let me go ahead and highlight this carbon in red so that's carbon in red is this one right here so we form church butyl chloride right and we lost water in the process so this is a very easy to reaction to do it occurs at room temperature and you just take tributon all and add some hydrochloric acid and just shake them together and and you could form your final product this way so thinking about stereochemistry in an sn1 type mechanism the carbon in red right here is not a chiral center and so we don't have to worry about about what what kind of stereochemical outcome that we would predict for the product so this is our final product all right there's no stereochemistry but just to refresh your memory right for an sn1 type mechanism because this formation of this carbo cation right this carbon in the carbo cation is sp2 hybridized and so it's planar and so when we draw out that carbon in red here let's say that's that carbon in red it's sp2 hybridized which means that the carbons that are directly bonded to it lined the same plane right so these all these carbons line the same plane sp2 hybridize with a p orbital right so there's a p orbital and sketch that in so there's a plus 1 formal charge in this carbon so when your nucleophile attacks alright your nucleophile could attack from either side of that plane so the nucleophile attack from this side or it could attack from this side and so if you if your final product has a chiral center you need to think about stereochemistry alright but not in this case in this case we don't have one so we lucked out this one was a little bit straightforward so that's a that's a couple of examples of sn1 and sn2 reactions of alcohols