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Course: MCAT > Unit 6
Lesson 13: Carbohydrates- Carbohydrate questions
- The structure of monosaccharides
- Carbohydrates - naming and classification
- Carbohydrates - absolute configuration, epimers, common names
- Carbohydrates - Cyclic structures and anomers
- Carbohydrates- di and polysaccharides
- Keto-enol tautomerization (by Jay)
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Carbohydrates - Cyclic structures and anomers
Explore how chains of carbon atoms form carbohydrates, discover the magic of intramolecular reactions, and learn about the formation of stable rings. Uncover the secrets of pyranoses and furanoses, and get to grips with the Haworth diagram and chair confirmation. Created by Ryan Scott Patton.
Want to join the conversation?
- Aren't we enzymatically programmed to digest L- sugars. Arent mostbiological sugars L?(9 votes)
- Most amino acids are "L" while most carbohydrates are "D" for humans.(77 votes)
- Is the alpha/beta distinction based on whether it is axial or equatorial, or whether it is cis/trans to the last carbon.(11 votes)
- One of two ring forms can emerge during cyclization of a sugar molecule: α or β. Because these two molecules differ at the anomeric carbon, they are termed anomers of one another. In glucose, the α-anomer has the –OH group of C-1 trans to the –CH2OH substituent (axial and down), whereas the β-anomer has the –OH group of C-1 cis to the –CH2OH substituent (equatorial and up).(5 votes)
- can you elaborate the anomeric carbon(5 votes)
- The anomeric carbon is the carbon derived from the carbonyl carbon (the ketone or aldehyde functional group) of the open-chain form of the carbohydrate molecule and is a stereocenter. An important feature is the direction of the OH group attached to the anomeric carbon, indicating that it is either alpha or beta(12 votes)
- Are we responsible for the chair configurations for the mcat?(5 votes)
- yeah - under 5D of the outline it would fit into "cyclic structure and conformations of hexoses"(4 votes)
- At0:55the instructor says carbinol. Does he mean carbonyl? I am not sure I have heard of carbinol in my organic chemistry classes.(4 votes)
- Carbinol carbons are C atoms bonded to an -OH group.(2 votes)
- why is the 6th carbon (CH2OH) axial and not equatorial? Bulky substituents are more energetically favored in the equatorial position. Also, since there is a single bond between c5 & c6 the bond can easily rotate to the more favorable equatorial position.(4 votes)
- Could you please explain the difference between carbinol and carbonyl?(3 votes)
- At2:00, what causes the alcohol to deprotonate?(2 votes)
- A basic solution or if the sugars are placed in water.(3 votes)
- After about 8 minutes, when he talks about C1 and C6 as cis for Beta, does he mean they are both up, or are both equatorial?(2 votes)
- Cis talks about whether they are both up up or down. It would be cis up, but the one that happens to by up is up on the equatorial position for the anomeric carbon. Trans would be down, and in the axial position for the anomeric carbon.(3 votes)
- When talking about the C6 group, why isn't this group right-down since the H attached to the 5C is on the left? Wouldn't the H be up, so the C6 group would have to be down?(2 votes)
- In the D-configuration of sugars, which is the configuration seen in the human body, the CH2OH points in the upwards direction. In the L-configuration of sugars, the CH2OH would point down.(2 votes)
Video transcript
- [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.