professor Dave here, let's talk about leaving group derivatives
let's go back to the SN2 reaction. we know that we have say, an alkyl
bromide, this might react with a nucleophile and do SN2, we're gonna
get inversion of stereochemistry and that's gonna work just fine. however we
we might, let's say we have a hydroxyl group here, if we wanted to do SN2 on this
alcohol
well that's not going to work because a hydroxyl group is not a good leaving
group in a substitution reaction and so let's say we really had our
heart set on SN2 at this hydroxyl position for some reason in our synthetic pathway
we really want to do SN2 right there
well what could we do we know that we could probably do that with some acid
have it try to leave, we could try to do an SN1 reaction but there's a lot of
problems there because number one you're gonna get competing elimination
reactions, you get some some alkene, you're gonna have a mess in your product
mixture and number two
SN1 reactions are not stereospecific so you're gonna get a racemic mixture and
if you have a lot of stereospecific steps on your pathway it's not gonna be very good so what we
want to say is what if there was something that we could do to this
alcohol to make it receptive to an SN2 reaction
well actually there is, and we could we could turn it into something called a
leaving group derivative by reacting it with one of these compounds. so let's say
we react with tosyl chloride so this is called tosyl chloride and this right
here is the tosyl group, so this is tosyl and then chloride, or mesyl
chloride, this is the mesyl group right here and then this is the trifyl group
right here and reaction of an alcohol with any of these will turn this into a
leaving group derivative so let's go ahead and take a look at the mechanism
for that and then see why exactly it works that way
ok let's take a look at how this works so let's take our alcohol and remember
we really want to do SN2 with this alcohol so let's react with
tosyl chloride, this is the tosyl group over here and then the chloride so what
can happen is that the oxygen is going to attack this sulfur atom and pop that up
here let's just pretend that we're gonna get rid of that proton on the way, save
ourselves a step, so now we have this, we have oxygen attached to all that
remember this pi bond went up here so we have the oxyanion there, now that
oxygen can go, sorry that lone pair can come back to regenerate that double
bond but now it might kick off the chloride, so that kicks off the
chloride instead of the original part of the molecule right here, now we're here. now
this is O-Tosyl so we would abbreviate that as O-Ts because O and then
there's our tosyl, the chloride is gone so here's an interesting thing is that we have
completely changed the reactivity of this center by turning this from OH into
O-Tosyl because before OH was a terrible leading group for the SN2
reaction but now we'll look at what happens if we have O-Tosyl, bromide is
actually gonna have no problem doing an SN2 reaction on O-Tosyl right
there and we will get our SN2 product with inversion of
stereochemistry, again SN2 is stereospecific and anything
stereospecific is very useful for synthesis and then the reason that's
going to work is because when O-Tosyl leaves, this is O-Tosyl look at all that
resonance, we are going to be able to put resonance up there resonance down there
and so remember that the leaving group ability of some group has to do with its
stability once it has left, and so if that tried to leave and we had OH
minus, now that's not very stable that's a strong base that's why OH is a
terrible leaving group but O-Tosyl is a great leaving group because it's got
resonance for days all over the tosyl group and so basically we have learned that
leaving group derivative, whether using a tosyl group, mesyl group, trifyl group
which may be useful for very specific circumstances whatever the case may be
a leaving group
derivative is a great way to turn a hydroxyl group into a good leaving group
for an SN2 reaction. thanks for watching guys, subscribe to my channel for more
tutorials and as always feel free to email me