If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:11:12

Carbohydrates - Cyclic structures and anomers

Video transcript

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 1 to 2 to 1 ratio of carbon hydrogen and oxygen and of course I'll keep using glucose as an example now I've also used the term poly hydroxylated 2 to refer to the numerous hydroxyl groups that are in these carbohydrates and really I bring all of this verbiage back up to hopefully spark your ability to see that carbohydrates have all the makings of an internal or or intramolecular I guess reaction between the carbonyl carbon right here and one of the hydroxyl groups because essentially what we have is is carbonyl and alcohol chemical reactions just kind of waiting to happen and what happens when an alcohol nucleophile attacks an aldehyde or a ketone well you know if there's an excess of alcohol we end up with a product that is either an acetal or a key towel but what happens if there's only one nucleophilic attack by an alcohol if we just have one alcohol and that's going to 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 hemi key towel and really that terminology is just kind of a review of asset allen key towel chemical reactions that would fall under I guess if you're looking at an organic chemistry book aldehyde or ketone reactions probably in the carbonyl section so let's show how this process is happening in the context of our glucose over here first I'm going to highlight the particular hydroxyl oxygen that's going to act as the nucleophile so we'll make that pink and after being deprotonated so after losing after losing this proton this oxygen is going to have an extra set of electrons right here and those electrons are going to target that carbonyl carbon so I'll draw the 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 bottom double bond is being hugged by this oxygen so the oxygen has a partially negative charge and the 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 and so after the oxygens electrons attack this carbonyl carbon what's going to happen is these the electrons from this double bond are going to kind of kick back up to the to the oxygen up here and eventually they're going to attract another proton and will form another hydroxyl group out of some of the electrons from that bond now you might be asking and it's a perfectly valid question why it's this particular oxygen the one that I've highlighted that's acting as the nucleophile and you're going to see as soon as I get the product drawn that we've formed a six-member ring so it really has a lot to do with product stability and if you remember the basis for the formation of the ring and the first place was the increased stability over the straight carbon chain so it makes sense that we're going to form the most stable ring that we can now when we end up with a six membered carbohydrate ring such as the case with glucose here we call a product at Piron OHS pyrrha knows the OHS again as the suffix for sugar and then in the P R apart for to indicate that this ring is is at sugar with six carbons and then if the carbohydrate ring is a five carbon ring we call it a furanose which is a bit easier for me to remember because furanose and five both start with the letter F so that's kind of the the memory jogger for me and maybe a good example for that would be ribose with its five carbon chain but I'll kind of stop there because almost every ring forming carbohydrate that I can think of with with biological implications at least forms either a five or a six membered ring so pyrrha Knossos and and furiosa's so just by convention you can see that I've placed the oh in this corner up here and that places the formerly 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 the highlighted oxygen and it's bonded to another hydroxyl group as well so I'll keep it distinguished in color and we also distinguishes name now as the anomeric carbon so 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 and we call this diagram a ha worth diagram so Hall worth diagram and the hall worth diagram doesn't show us that actual configuration of the Ring because in reality you know six membered rings are going to show up in a more stable chair shape but it is beneficial and telling us which substituents are above or below the ring so to keep this convention straight in my mind I remember the phrase down right up left II so downright uplifting kind of a play on I guess the phrase that's downright uplifting but downright uplifting because as i fill in the substituents those on the right side of the fischer diagram will point down and those on the on the left side of the fischer diagram are going to point up so we can actually see that that one's up and we'll make sure that this number is off right this one's up as well and it maybe will 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 1 2 3 4 5 6 so our 3 carbon and the Hollow Earth diagram is pointed up and our 3 carbon on the on the Fischer diagram has its substituent on the left so down right up left and as we get to the last carbon which kind of forms this tail down here I remember that if it's a D sugar that group is going to point up in the hay with her in the hall - excuse me projection so this is a D sugar and you can see in the Hall worth projection that this last carbon points up as well and really this is going to be the case for a lot of sugar chemistry that you deal with because again we're enzymatically programmed to digest D sugars so we often end up with this last group pointing up now the last thing I want to show you is the chair conformation so the chair conformation and that's because this is this is the kind of diagram that's going to give us a sense for the actual configuration of D glucose but it really does just follow the follow suit with the ha worth projection as far as the substituents being above or below the ring so let me just kind of keep filling in the substituents here and I'll number them off again just so you can kind of see that there's some consistency here so 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 excuse me 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 the S configuration so that last hydroxyl group can actually be in two different positions on one hand the hydroxyl group would be sis to the last carbon in the in the equatorial position so it'd be sis to this last carbon over here and it's in the equatorial position and we call this the beta anomer then on the on the other hand I yes it could be trans to that last carbon group which would place it in that axial position down here so I guess it could be done here in the axial position and we call it the alpha anomer when that when the hydroxyl group is in the axial and I kind of remember that a little bit easier alpha for the the axial position of that substituent and I guess I've also heard that fishies 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 with some amount of acid or base and the amount of acid in base in water is actually kind of capable of doing that because that's what facilitated this ring closing process in the first place and in water the ring can actually open and close spontaneously and when it opens up the the c1 and c2 bond right here can actually rotate and when it closes again you can form the either the alpha or the beta product so this thing is constantly opening and opening and closing to form the two different products and we call that process where it opens and rotates and closes again mutarotation so this thing is muta rotating in the water at all times so mutarotation and the outcome is that we end up with both configurations that beta and the Alpha and the equilibrium concentration so for glucose that's going to be about 36% alpha and about 64% in beta and the reason that the Alpha configuration is less favored in equilibrium for glucose is because the trans positioning of the hydroxyl group creates some steric hindrance but this is pretty individualized for different sugars so I guess the most general rule I suppose that you can apply to all cyclic sugars would be to say that the beta anamur again anamur that the beta anamur is the one with the anomeric oxygen in the system with respect to the last carbon