Carbohydrates : Chemical reactions

[mathiasbb]

Hello, i really need your help and some explanation on how should i think about this exercice. 

A disaccharide reduces Fehling's liquor. 

Permethylation followed by hydrolasis gives 2,3, 4, 6 tetramethyl-D-galactopyranose and 2,3,6 trimethyl-D-glucopyranose. 

Deduce from these data the possible formula(s) of this disaccharide and give the corresponding name(s). 

Please i really need your help to be able to understand how to solve this problem. 

Thank you i advance ❤️

[exchemist]

32 minutes ago, mathiasbb said:

Hello, i really need your help and some explanation on how should i think about this exercice. 

A disaccharide reduces Fehling's liquor. 

Permethylation followed by hydrolasis gives 2,3, 4, 6 tetramethyl-D-galactopyranose and 2,3,6 trimethyl-D-glucopyranose. 

Deduce from these data the possible formula(s) of this disaccharide and give the corresponding name(s). 

Please i really need your help to be able to understand how to solve this problem. 

Thank you i advance ❤️

OK this is not my area of chemistry (I hated carbohydrate chemistry at university), but maybe we can crack this problem between us.

First, what does Fehling's solution test for, i.e. what can you say about the disaccharide from the fact it reduces it?  

Second, what does permethylation do to a sugar? 

Third, what would a disaccharide involving galactopyranose and glucopyranose suggest to you?  

Edited February 21, 2022 by exchemist

[mathiasbb]

1 hour ago, exchemist said:

OK this is not my area of chemistry (I hated carbohydrate chemistry at university), but maybe we can crack this problem between us.

First, what does Fehling's solution test for, i.e. what can you say about the disaccharide from the fact it reduces it?  

Second, what does permethylation do to a sugar? 

Third, what would a disaccharide involving galactopyranose and glucopyranose suggest to you?  

Helloo 

Fehling liquor is reduced means that the disaccharide is a reducing sugar, and have at least one free anomeric carbon 

The permathylation is the process where we add a methyl group(CH3) on each and every carbon having an OH (hydroxyl group)

 

A disaccharide having a galactose and a glucose is a lactose and these 2 saccharides are linked in a glycosidic bond which is beta 1- 4

[exchemist]

23 minutes ago, mathiasbb said:

Helloo 

Fehling liquor is reduced means that the disaccharide is a reducing sugar, and have at least one free anomeric carbon 

The permathylation is the process where we add a methyl group(CH3) on each and every carbon having an OH (hydroxyl group)

 

A disaccharide having a galactose and a glucose is a lactose and these 2 saccharides are linked in a glycosidic bond which is beta 1- 4

OK, so what can you say now about this disaccharide?    

[mathiasbb]

So it's simply the lactose disaccharide? 

[exchemist]

1 hour ago, mathiasbb said:

So it's simply the lactose disaccharide? 

Well I that's how it looks to me, yes -  unless someone else can point out an elephant trap of some kind that we have both missed.

But maybe in your answer you need to go through what happens with each procedure, i.e. how Fehling's solution reacts with the free aldehyde, how the -OH groups get methylated, and so forth, in order to show that you have understood the chemistry the question talks about.

Do you think that is reasonable?   

P.S. Why do you think the glucopyranose is only trimethylated? 

 

Edited February 21, 2022 by exchemist

[mathiasbb]

Actually it's been 10 mins and still thinking about your PS question but i don't know. 

Could you maybe explain why? 

And also that's what i wanted help with how can i explain the process step by step to clarify why i chose the lactose to be the initial reactif, could you maybe help me with this 

I will be really thankful for that

[exchemist]

1 hour ago, mathiasbb said:

Actually it's been 10 mins and still thinking about your PS question but i don't know. 

Could you maybe explain why? 

And also that's what i wanted help with how can i explain the process step by step to clarify why i chose the lactose to be the initial reactif, could you maybe help me with this 

I will be really thankful for that

I don't know the answer: I was hoping you might, having had it drawn to your attention! But, looking at the ring structure of lactose, on the glucose ring one of the  -OH groups is part of the hemi-acetal. If you exclude that one, there remain 3 of them on the glucopyranose ring and 4 on the galactopyranose ring.  

HOWEVER, looking in my ancient Finar Part 2 (old-fashioned, even at Oxford in the 1970s!) it looks to me as if the methylated OH that is part of the hemi-acetal is hydrolysed in the following hydrolysis step, leaving you with only 3 methylated hydroxyls on the glucose side. So that would account for it. (That would make sense as acetals are easily hydrolysed to give the aldehyde or ketone.)

I think that's about the best I can do on this. I hope you can construct an answer now that makes some sort of sense. If we've got it wrong, I'd be interested if you can come back here and let me know the right answer in due course.

I must say I found carbohydrate chemistry terribly confusing and tedious, due to the close similarity of all these structures. And the lecturer we had was the most boring man imaginable. So I'm afraid the details are not something that has stayed in my mind 45 years later......... 😁   

Edited February 21, 2022 by exchemist

[mathiasbb]

Hahahahahaha i will update you for sure in case we got it wrong. 

Thank you so much for your time and your explanation ❤️

Have a nice day 

[exchemist]

9 minutes ago, mathiasbb said:

Hahahahahaha i will update you for sure in case we got it wrong. 

Thank you so much for your time and your explanation ❤️

Have a nice day 

It's been fun. Hope we both got something out of it, anyway.