hey guys its professor Dave, let's talk about carbocation rearrangement
so to take a look at carbocation rearrangement
let's revisit some old reactions. we're gonna take a look at the E1 and
SN1 reactions but this time they're gonna be acid catalysed
remember that the E1 and SN1 reactions have carbocation
intermediates so let's take a look here
we have this secondary alcohol here and
if we treat with strong acid that will pick up this proton like this
now we have basically water
a water molecule and that is going to be a good leaving group remember
that if something is a good leaving group it's going to be stable once it has left
water's very stable so that's a great leaving group so that leaves us with this
secondary carbocation. now here's the thing
we remember that due to hyperconjugation a more
substituted carbocation is a more stable one and
if a substrate is going to be able to rearrange
so as to generate a more stable carbocation
it will do so and there's two ways they can do this
so let's say we have a secondary carbocation
next to either a tertiary or quaternary carbon
so here's our tertiary carbon next door
in this case we can have something called a hydride shift
now a hydride shift is H minus so
typically we're seeing hydrogen as an ion
see it as H plus, a proton, but we can also have H minus
this is basically a hydrogen nucleus with with a pair of electrons
so that's H minus and a hydride shift means that this
hydrogen atom with both of the electrons in this bond
can basically shift over so here's that hydrogen
that's literally just going to hop from one carbon to the other
and the reason it will do this is because
in doing so it will neutralize the cabocation over here
but generate a new one on the tertiary carbon so this is a more stable
carbocation intermediate
so basically
we also want to take a look at the transition state here where
the hydrogen atom is weakly
interacting with both these carbons so it's almost as though we have
a partial plus or almost like half a positive charge
on each of those carbons as it is shifted from one
right over to the next one so typically we're looking at this
as being able to happen between adjacent carbons so if we have a
secondary carbon
that's very far away from, sorry a secondary carbocation
that's rather far away from a tertiary or quaternary carbon
this probably isn't gonna happen but because there is a tertiary carbon next
door
this hydride is able to migrate one carbon over
so now we have that tertiary carbocation and let's say we're just gonna interact with
with a water molecule, a molecule of solvent
this oxygen can either go ahead and coordinate to the carbocation giving us
a racemic mixture of SN1 products or it can eliminate, grab a proton
there or grab the proton that shifted
and give us now one of these two different E1 products
in this case because water is pretty small we're probably going to expect
the Zaitsev product to dominate over the Hofmann
so that's the hydride shift. now we can also have a methanide shift
a methanide is CH3 minus
so it's a methyl group with
a formal negative charge so we can just go back up here
and replace all these hydrogens now let's pretend this is a methyl group
here
so this is no longer hydrogen that's just a regular methyl group so the rest
is the same
so here's our CH3 and then if we look
at the transition state we're seeing the same thing happening
whereas now it's simply that that carbon atom
is weakly bound to each of these carbons in the transition state
so an entire methyl group or we should say a methanide group
is shifting from this carbon to satisfy that secondary carbocation
and in doing so leaving a tertiary carbocation which again is more
stable
and that's the driving force behind this rearrangement
always thermodynamics, everything is trying to get to the lowest energy
possible
so if it can shift and become more comfortable it will do so
and then once again we can just add a methyl group
all around to represent that methyl group that shifted
and once again probably Zaitsev will dominate here so
just to reiterate the take-home is that when you have
a secondary carbocation adjacent to
either a tertiary or a quaternary carbon
if that can rearrange it will do by either doing a hydride or methane shift
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