hey guys, professor Dave here. i wanna talk to you about organic
oxidation-reduction
so we're probably familiar with oxidation-reduction reactions
as reactions involving electron-transfer. while that's totally true
in organic chemistry this tends to have a significance more on par of
oxygen content so we're looking at oxidation reactions
as ones in which carbon gains bonds to oxygen
and reduction reactions as ones in which carbon atoms
lose bonds to oxygen. so we can see some different functional groups here
here's a carboxylic acid and an aldehyde and alcohol
and just an alkane. so here this carbon has three bonds to oxygen atoms, there's
1, 2, 3 there
here it just has two, here this carbon has one and then
none any longer so this in this direction
this is reduction in this direction this is oxidation
so there are different compounds that will
nduce different transformations so there are reducing
agents
that will reduce any of these compounds in this direction and there are oxidizing
agents
that will oxidize compounds in this direction so let's take a look at a few of them
and see how they differ in strength and why we might use one over another
so just to take a look at oxidations
we can oxidize primary or secondary
alcohols, a tertiary alcohol cannot be oxidized because that carbon already has
three bonds to other carbons so there are not any bonds that can be replaced
to give that carbon more bonds to oxygen. we can only lose bonds
to hydrogen so this carbon here has one bond to oxygen and has two
hydrogens
so it is going to be possible to gain more oxygen content here
now a primary alcohol can be oxidized
either to the aldehyde or to the carboxylic acid
level of functional group so PCC, pyridinium chlorochromate,
this is a softer oxidizing agent that is only
going to oxidize a primary alcohol
to the aldehyde functional group level so it's gonna stop there with two bonds to
oxygen
something stronger like potassium permanganate or chromic acid
or something else is gonna take a primary alcohol
and oxidize all the way to the carboxylic acid
functional group level so depending on what you're going for
in say synthetic pathway or something that might
guide you in terms of what oxidizing agent you want to use
now a secondary carbon can only be
sorry secondary alcohol can only be oxidized to the ketone
functional group level because we can't oxidize any further, there's only one
additional bond to hydrogen that can be that can be replaced with another bond
to oxygen
we're not gonna break any of these carbon-carbon bonds so for a secondary
alcohol it doesn't matter which we're going to use
we're just gonna get a ketone so it's mainly for these primary alcohols where
we're gonna
have to make a decision. so now let's look at reducing agents
whereas oxidizing agents are going to help us go in this direction we saw that
we can go from primary alcohol
to aldehyde with the PCC or primary alcohol
directly to carboxylic acid with potassium permanganate
now let's see what's going on in the opposite direction. so we have different
reducing agents and again these can have different strengths
so let's take a look at something like sodium borohydride, thats NaBH4
so we have borohydride with the sodium counter ion
so this is BH4 minus, now boron has three valence electron so it's gonna have a
tendency to make three covalent bonds
but but now that has a fourth here this is
the boron is gonna have a formal negative charge so is going to be very interested
in losing a hydride
so if H minus leaves, leaving just BH3
that boron is going to neutralize and that's an energetically favorable
situation
so that's why sodium borohydride is a good source of
hydride or an H minus nucleophile
so hydrides have a tendency to attack carbonyls
thus reducing them so what we're looking at here
is sodium borohydride is capable of producing an aldehyde
to alcohol functional group level however it is
a soft
reducing agent, it does not have the strength to reduce
something like a carboxylic acid so in the presence of
NaBH4 there will be no reaction with a carboxylic acid
but there are stronger or harder reducing agents like say
lithium aluminum hydride so here is an aldehyde that can be reduced
perfectly well with lithium aluminum hydride but lithium aluminum hydride is
also strong enough to reduce
the carboxylic acid all the way to the
primary alcohol functional group level so
let's take a look at why that might be. so here's lithium aluminum hydride and
sodium borohydride
so what happens is that the positively charged counter ion
in solution is gonna go and coordinate to the
oxygen in the carbonyl, now lithium is a smaller ion so it's a better Lewis acid
so it is gonna more strongly interact in
ion-dipole fashion with this oxygen atoms and that is going to generate
something of a partial positivity
on that oxygen because some of the electron density in the lone pairs
is going to be tied up with this partial... it's actually quite strong
interaction is nearing the level of a covalent bond
so a lot of this electron density in response is going to skew even further
towards the oxygen
in an attempt to neutralize so that creates a heightened
partial positivity on that
on that carbon atom so that's part one. part two
is that because aluminum is less
electronegative than boron that means that it is holding onto these hydrides
less tightly so they're even more available for nucleophilic attack
so sodium a bit larger is unable to interact quite as well
with the carbonyl as lithium is. secondly
boron being slightly more electronegative is holding on to those
hydrides just a little bit tighter so those are two
things that act in unison to
make lithium aluminum hydride a stronger reducing agent than sodium borohydride
so basically we have all these different stages of oxidation and reduction
and we are able to jump from one to another
and there are a lot of oxidizing agents and reducing agents and I spoke
about very few of
them, in your class you're probably using a great deal more
but all you need to do is understand the transformation that agent is capable
of producing
and be able to apply that to any substrate.
thanks for watching guys, subscribe to my channel for more tutorials
and as always feel free to email me with questions