- [Voiceover] All righty, so we've been speaking so far
about carbohydrates as chains of carbon atoms
and these are chains of carbon atoms that feature
an aldehyde or a ketone functional group
and that falls into this general kind of one to two
to one ratio of carbon, hydrogen and oxygen
and of course I'll keep using glucose as an example.
Now I've also used the term polyhydroxylated
to refer to the numerous hydroxyl groups
that are in these carbohydrates
and really I bring all of these verbiage back up
to hopefully spark your ability to see that
carbohydrates have all the makings of an internal
or intramolecular, I guess reaction between
the carbinol carbon right here
and one of the hydroxyl groups
because essentially what we have
is carbinol and alcohol chemical reaction
just waiting to happen.
What happens when an alcohol nucleophile attacks an aldehyde
or a ketone?
Well if there's an excess of alcohol,
we end up with a product that is either
an acetal or a ketal
but what happens if there's only one nucleophilic
attacked by an alcohol.
If we just have one alcohol
and that's gonna be the case in the ring closing
intramolecular reaction we have going on here.
Well in that case we end up with a hemiacetal
or a hemiketal, and really that terminology
is just a review of acetal and ketal chemical reactions
that would fall under I guess if you're looking
in an organic chemistry book aldehyde
or ketone reactions probably in the carbonyl section.
Let's show how this process is happening
in the context of our glucose over here.
First I'm gonna highlight the particular hydroxyl oxygen
that's gonna act as the nucleophile,
so we'll make that pink.
After being deprotonated,
so after losing this proton,
this oxygen is gonna have an extra set
of electrons right here and those electrons
are gonna target that carbonyl carbon.
I'll draw the carbonyl carbon in green
and remember that the carbonyl carbon
has a partial positive charge on it,
it has a partial positive charge
because a lot of the electron density
in this double bond is being hugged by this oxygen.
The oxygen has a partially negative charge
and that carbonyl carbon is partially positive
and that makes it a perfect target
for the nucleophile that's been created
in the deprotonation process of this oxygen.
After the oxygen's electrons attack this carbonyl carbon,
what's gonna happen is the electrons from the settle bond
are gonna kick back up
to the oxygen up here
and eventually they're gonna attract another proton
and will form another hydroxyl group
out of some of the electrons from that bond.
You might be asking and it's a perfectly valid question
why is this particular oxygen
the one that I've highlighted
that's acting as the nucleophile.
You're gonna see as soon as I get the product drawn
that we formed a six member ring.
It really has a lot to do with product stability
and if you remember the basis for the formation of the ring
in the first place was the increase stability
over the straight carbon chain.
It makes sense that we're gonna form the most stable ring
that we can.
When we end up with the six membered carbohydrate ring
such as the case with glucose here,
we call the product a pyranose,
the ose again as the suffix for sugar
and the pyr part to indicate
that this ring is a sugar with six carbons
and then if the carbohydrate ring is a five carbon ring,
we call a furanose
which is a bit easier for me to remember
because furanose and five both start with a letter F.
That's kind of the memory jogger for me
and maybe a good example for that would be ribose
with its five carbon chain
but I'll kinda stop there
because almost every ring forming carbohydrate
that I can think of with biological implications
at least forms, either a five or a six membered rings,
so pyranoses and furanoses.
Just by convention, you can see
that I've placed the O in this corner up here
and that places the formally carbonyl carbon
down here, right below it
and it's actually no longer the carbonyl carbon
but it's still significant
because it's the only carbon here
that is bonded to two oxygen atoms,
the highlighted oxygen and it's bonded
to another hydroxyl here as well.
I'll keep a distinguishing color
and we also distinguish it's name now
as the anomeric carbon.
That's the anomeric carbon
and then we can go ahead and fill in
the rest of the substituents in the diagram,
so a hydroxyl group and another and another.
We call this diagram a Haworth diagram.
The Haworth diagram doesn't show
as the actual configuration of the ring
because in reality six membered rings
are gonna show up in a more stable chair shape
but it is beneficial in telling us which substituents
are above or below the ring.
To keep this convention straight in my mind,
I remember the phrase downright up lefting.
Kind of a play on I guess the phrase
that's downright uplifting
but downright up lefting
because as I fill in the substituents,
those on the right side of the Fischer diagram
will point down and those on the left side
of the Fischer diagram are gonna point up.
We can actually see that that one's up
and we'll make sure that this number's off right.
This one's up as well and maybe we'll name this
or maybe we'll start numbering with one, two,
three, four, five, six
and we can do that over here.
This would be one, two, three,
four, five, six.
Our three carbon in the Haworth diagram is pointed up
and our three carbon on the
Fischer diagram has its substituent on the left,
so downright, up, left.
As we get to the last carbon group
which forms this tail down here.
I remember that if it's a d sugar,
that group is gonna point up in the Haworth projection.
This is a d sugar
and you can see in the Haworth projection
that this last carbon points up as well
and really this is gonna be the case
for a lot of sugar chemistry that you deal with
because again we're into matically programmed
to digest d sugars so we often end up
with this last group pointing up.
Now the last thing I wanna show you
is the chair confirmation
and that's because this is the kind of diagram
that's gonna give us a sense for that actual configuration
of a d glucose but it really does just follow through
with the Haworth projection
as far as the substituents being above
or below the ring.
Let me just keep filling in the substituents here
and I'll number them off.
Again just so you can see
that there's some consistency here.
We've got one, two, three, four, five, six
and again this three carbon right here
is the only one with the hydroxyl group pointing up
and I guess I better change the color
of our one carbon to keep that consistent as well.
Now I didn't indicate the position of the anomeric
carbons hydroxyl group yet
because I think it makes more sense to show it
in this diagram.
Remember that the original nucleophilic attack
by the oxygen way back over here.
That could have created two different products,
one with an r configuration about the anomeric carbon
and the other with a s configuration.
That last hydroxyl group can actually be
in two different positions.
On one hand the hydroxyl group
would be cis to the last carbon
in the equatorial position.
It would cis to this last carbon over here
and it's in the equatorial position
and we call this the beta anomer
then on the other hand I guess it could be trans
to that last carbon group which would place it
in that axial position down here.
I guess it could be down here in the axial position
and we call it the alpha anomer,
when the hydroxyl group is in the axial
and I can remember that a little bit easier.
Alpha for the axial position of that substituent.
I guess I've also heard that fishes are down in the sea
and birds are up in the air
so if that helps you keep them straight,
you might be able to use that also.
Now you've got to remember that what caused this ring
to close in the first place was some amount of acid or base
and the amount of acid and base and water
is actually capable of doing that
because that's what facilitated this ring closing process
in the first place.
In water, the ring can actually open
and close spontaneously.
When it opens up,
the c1 and c2 bond right here
can actually rotate
and when it closes again,
you can form either the alpha or the beta product.
This thing is constantly opening and closing
to form the two different products
and we call that process where it opens
and rotates and closes again, mutarotation.
This thing is mutarotating in the water at all times.
Mutarotation and the outcome is that we end up
with both configurations, the beta and the alpha
and the equilibrium concentration.
For glucose, that's gonna be about 36% alpha
and about 64% beta.
The reason that the alpha configuration is less favored
in equilibrium for glucose
is because the transpositioning of the hydroxyl group
creates some steric hindrance
but this is pretty individualized for different sugars.
I guess the most general rule I suppose
that you could apply to all cyclic sugars
would be to say that the beta anomer, again anomer,
that the beta anomer is the one
with the anomeric oxygen
in the cis position with respect to the last carbon.