professor Dave here let's talk about potassium permanganate.
so we're already familiar with potassium permanganate as an oxidizing agent and we've seen what
it can do with with alcohols primary alcohol secondary alcohols and and
aldehydes as well so now let's look at at what potassium permanganate can do to
oxidize alkenes and there's actually two different ways this can go
dependent largely on the temperature so we're gonna look at cold and hot
conditions so first let's take a look at the cold conditions here and actually
what's going to happen so we've got cyclohexene and we've got potassium
permanganate in basic conditions so we've got sodium hydroxide there and
what this is going to achieve is a syn dihydroxylation this is the permanganate
ion and so it's actually very similar to the addition reaction we already know
for sin dihydroxylation using osmium tetroxide right OsO4
we already know how to do that and the reason that's a syn dihydroxylation is
because both the both of the oxygens that are being delivered to the molecule
are coming from the same molecule so it has to kind of lower down and it just
delivers both oxygens so actually the permanganate ion is going to do exactly
the same thing we've got two oxygens that are being delivered to the molecule
so we're gonna get this syn dihydroxylation resulting in this
vicinal diol so just remember if you're making flashcards which i highly
recommend if you're trying to memorize what certain conditions do what kind of
transformations they promote when you see potassium permanganate operating on
an alkene if conditions are cold very important that's going to be a syn
dihydroxylation you're gonna get that vicinal diol exactly like this right
just like that so now with hot conditions it's just a little bit
trickier it enhances the oxidizing ability of potassium permanganate so
we've got potassium permanganate this time in acidic conditions so we've got
acidic conditions just generic acid h3o plus and remember the triangle means
heat we're gonna cook it up a little bit so we've got our same substrates
cyclohexene what's going to happen this gonna add just like we know and so
we've got this intermediate right here and then this is actually going to cause
this carbon-carbon Sigma bond to I've actually rotated this sorry about that
so just pretend this pi bond was there but so this is going to cleave right
here this will go to the manganese atom and then so this sort of cyclization of
sigma electrons is what's going to happen and it's going to break this ring
apart right this this carbon-carbon sigma bond is actually going to cleave
and so we've opened up that ring and once protonation occurs we can see that
we've got two aldehyde functional groups okay so we've popped to that ring open
and we've got the carbons that were participating in the pi bond are now
aldehyde carbonyl carbons but however we know that potassium permanganate is it's
still in solution it's a pretty strong oxidizing agent
we know that potassium permanganate can't take a secondary alcohol a primary
can oxidize a primary or secondary alcohol but also a an aldehyde right so
aldehydes are going to get further oxidized this is a generic symbol for
oxidation in the presence of potassium permanganate so we're gonna get all the
way to the carboxylic acid so just remember in such a situation right these
carbonyl carbons were the the carbons that were participating in the pi bond
and so what happens is we cleaved them and notice that if we cleave these would
each be considered primary carbons and so you're gonna take those primary
carbons and you're gonna take them all the way to the carboxylic acid
functional group level so that's how that one is gonna go but because of that
we need to look at what is going to happen with carbons of other
substitution and so let's say we have these two carbons right this is very
similar but we've added some methyl groups right there okay so what happens
there we're gonna cleave that all the same right the first pi bond goes away
when the permanganate ion coordinates and then the second of the sigma bond
goes away when when the ring opens up but what we're left with is we're gonna
be left with ketones right this is going to be a diketone
a dione, right because here we had an aldehyde we had two aldehydes which
could be further oxidized to get to the carboxylic acid but here we're left with
ketones and potassium permanganate is not going to be able to oxidize these
further it's not going to break carbon-carbon bonds right so we're gonna
be left with a die ketone again same conditions and then lastly just one
slightly different situation let's say we have a terminal alkyne so we have an
alkyne at a terminus or basically we've got two protons at the alright we've got
two hydrogen atoms on the other side so this time when that cleaves instead of
instead of what would be left as a primary carbon or what would be left as
a secondary carbon right when this cleaves this carbon is connected to two
others so we're gonna consider it a secondary carbon briefly here we're left
with a methyl carbon it's that carbon and no other carbons and in such a case
that's going to become CO2 so this carbon here is going to become the
carbonyl carbon in this ketone and then this one is going to be the carbon in
CO2 okay so with the hot conditions the acidic conditions
it's going to be a little bit trickier but what we know is we're going to
cleave the alkene right we're gonna cleave that double bond both the pi bond
and the sigma bond so we're gonna open that up or if it was a linear molecule
we would pull we would pull the molecule into two fragments and then you look at
the two carbons that were participating in the pi bond so if those two carbons
would end up being primary carbons that is going to produce carboxylic acid
functional groups if those two carbons end up being secondary carbons that is
going to produce a ketone right we have a die on here and then if that carbon
ends up being a methyl carbon that is going to produce CO2 so that is a
summary of some of the oxidative abilities of potassium permanganate in
the context of alkenes both in the cold conditions and in the hot conditions