Questions tagged [carbohydrates]

Carbohydrates are one of three classes of primary metabolites and thus natural products. They are polyhydroxylated hydrocarbons and their derivatives. Colloquially, they are known as sugars. This tag may be applied to all questions that concern carbohydrate species in solution, reaction mechanisms and structures.

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24
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1answer
69k views

Why does fructose reduce Tollen's reagent and Fehling's solution?

Even though fructose is a ketohexose (ketone-containing hexose, a six-carbon monosaccharide), it reduces Tollen's reagent and Fehling's solution. Generally, a ketone does not reduces Tollen's reagent ...
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4answers
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Can paper burn without oxygen or air?

If you put a paper inside an oven vacuum packed and go beyond $250~\mathrm{^\circ C}$, will it burn? There's no oxidizer. If not, what will happen?
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Why do other sugars melt whereas sucrose decomposes?

Sucrose decomposes at 186 °C. It doesn’t melt but when it reaches decomposition temperature, sucrose decomposes into fructose and glucose. After cooling and hardening it is no longer sucrose. Am I ...
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2answers
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Why is maltose a reducing sugar but not sucrose, even though they're both disaccharides?

I came across this on Wikipedia: Many disaccharides, like cellobiose, lactose and maltose, also have a reducing form, as one of the two units may have an open-chain form with an aldehyde group. ...
8
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1answer
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Periodic cleavage of carbohydrates

Carbohydrate like glucose undergoes periodic cleavage by $\ce{HIO4}$ and forms respective products are five moles of formic acid and one mole of formaldehyde. Here is my question , what is mechanism ...
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What's the difference between alpha-glucose and beta-glucose?

What's the difference between $\alpha$-D-glucose and $\beta$-D-glucose? Are they isomers? Or do they differ in their elemental composition?
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2answers
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Does Br2/H2O oxidize all aldehydes including carbohydrates?

In many books, it is mentioned that aldehydes are easily oxidized even by mild oxidizing agents. However, I have never seen bromine water explicitly mentioned. But in the carbohydrates chapter, it is ...
10
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1answer
353 views

Gases produced by pyrolysis of cellulose

I heated cotton in a sealed container (with a small hole) over a natural gas flame. Some gases and smoke were produced. What would they probably be? I can come up with some guesses based on the ...
10
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1answer
3k views

Why does the fructose monomer in sucrose appear different from isolated fructose?

I understood that the representation of fructose is: But during the formation of sucrose: How did fructose, whose representation is given in the first picture, turn out to be different in the ...
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1answer
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Carbon numbering in carbohydrates

Carbons have been numbered differently in two different pictures. What are the rules of carbon numbering?
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1answer
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How to Convert Sucrose to Glucose and Fructose

If you have some sort of sports drink that contains sucrose, how would you take that sucrose and break it down into its components, glucose and fructose? I know that sucrose + water will break it ...
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1answer
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Do L-sugars (enantiomers of natural sugars) have a sweet taste?

I'm wondering about how L-sugars taste. They are obviously not digestible by humans, but what do they taste like? If they are sweet, too, why isn't sucrose with the unnatural chirality used as a zero-...
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2answers
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How does existence of alpha and beta form of glucose prove that it exists as a cyclic structure

My book says that Glucose is found to exist in two different crystalline forms which are named as $\alpha$ and $\beta$. Next it says that This behaviour could not be explained by the open ...
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3answers
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Are glucose and galactose cis-trans isomers of each other?

Glucose and galactose are diastereomers of each other. However, is it correct to say that they are cis-trans isomers of each other? Does it make a difference with regard to terminology if glucose and ...
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1answer
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How are glycosidic linkages formed?

Consider glycosidic linkages in disaccharides. I know a condensation reaction occurs, with the loss of water. But which OH is lost? The O in the linkage belongs to which monosaccharide? Example: ...
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1answer
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Clarification in the mechanism for Molisch's test for glucose

Wikipedia says Molisch's test is a sensitive chemical test, named after Austrian botanist Hans Molisch, for the presence of carbohydrates, based on the dehydration of the carbohydrate by sulfuric ...
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2answers
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Reduction of glucose to hexane with hydroiodic acid

My book (NCERT India) states that n-hexane is formed when glucose is heated in the presence of hydrogen iodide. That is, the following reaction takes place: $$\ce{C6H12O6 + HI ->[{\Delta}] C6H14}$$ ...
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1answer
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Oxidation of fructose with conc. HNO3

I recently studied carbohydrates. I was taught that fructose on oxidation with strong oxidizing agents like concentrated $\ce{HNO3}$ yields glycolic acid and tartaric acid along with trihydroxy ...
14
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2answers
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Why starch (amylose) and cotton (cellulose) are so different?

Both amylose and cellulose have the same "monomer" structure (glucose), so what makes them look/form so differently?
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Why do gummy bears explode when added to hot potassium chlorate?

This link shows that a gummy bear explodes when in contact with heated potassium chlorate, $\ce{KClO3}$. But what in a gummy bear creates this reaction? Also, do other foods (fruit, icing sugar...) ...
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2answers
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Why is H bonded to C5 drawn below the plane in Haworth formula for D-glucose?

I have always seen the hydrogen atom bonded to C5 drawn below the plane as in the following figure. Shouldn't it be drawn above the plane since it is drawn on left side in Fischer projection?
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1answer
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Monosaccharides configurations (alpha beta, D L) identification

I am really finding it difficult to identify the correct configuration(s) of a given monosaccharide, in fact my friends and I have been trying to solve this, much to our annoyance. In one of our ...
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1answer
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Why is the reduction by sugars more efficient in basic solutions than in acidic ones?

Why is it said that reduction by sugars is much more efficient in basic media than in acidic media as in the following quote taken from Satyanaryana’s Biochemistry, 4th edition page 16? In the ...
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1answer
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Which conformer of beta D-glucose is more stable?

I drew the two chair conformers of $ \beta $-D-glucose: Which one is more stable? I think it's the one with all of the OH groups in the equatorial position because of less steric hindrance, but I ...
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1answer
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Stability of furanose vs. pyranose form of glucose?

I am trying to imagine the cyclization of the Fischer projection of glucose as it forms an $\unicode[Times]{x3B1}$-D-glucofuranose and an $\unicode[Times]{x3B1}$-D-glucopyranose (right now I am trying ...
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2answers
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If glucose can react with phenyl hydrazine to form osazone why can't it react with 2,4-DNP to give positive test?

If glucose can react with form phenyl hydrazine to form osazone why can't it react with 2,4-DNP? Yes I know about cyclic form of glucose and its equilibrium with straight chain and all … I read ...
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1answer
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What are the steps to creating synthetic glucose?

I need to know how to go about creating synthetic glucose with generally available materials. As I am aware of both photosynthesis and glucose in nature, I will not need lessons in how plants produce ...
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0answers
606 views

The proper formula of sucrose: why both forms are correct and how one is transformed into another? [duplicate]

During preparations for my exam I've stumbled upon a tormenting question how it happens that the formula of sucrose is written in two different forms: the vertical and the horizontal ones: The ...
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1answer
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What are the differences between D and L glucopyranose?

I have just studied the concept of cyclotron in carbohydrates. My reference book used Glucose as an example and began by saying we can find D-glucose or L-glucose: Depending on what enantiomer we ...
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1answer
427 views

Why pyranosides do not mutarotate?

For mutarotation, at least one hemiacetal group must be present in the sugar. Therefore, non-reducing sugars do not show mutarotation. Why? Consider the following reaction: $$\ce{\beta-D{-glucose}...
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1answer
383 views

Can glucose burn in chlorine?

Chlorine is an oxidizing agent, so could it replace oxygen? If so, what would be the products, and how much energy would such reactions release? Do any other carbohydrates burn in chlorine? I cannot ...
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0answers
392 views

What is a reasonable mechanism for the following reaction?

Hello all! I'm stuck on this one question and am not sure how to finish the mechanism. What I did was attack the alkyne first with the N farthest to the left. I'm not sure what do to after that though....
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How to convert glucose to fructose

How to convert glucose to fructose? Moreover, can there be direct conversion(not necessarily in a single step), or we have to use indirect means, such as from sucrose, or other higher carbohydrates, ...
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1answer
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Haworth's research of glucose

Firstly haworth used CH3I/Ag2O with glucose which replaced 5 H by CH3 forming a acetal which locked the anomer and stopped the equilibirium of ring chain tautomerism. Under dilute acidic conditions ...
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1answer
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How is levulinic acid formed from sucrose? [closed]

I came across the following question: Sucrose on treatment with conc. $\ce {HCl}$ produces? The answer is given as levulinic acid. So how exactly does sucrose produce levulinic acid?
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1answer
651 views

Fischer's proof of the structure of glucose

To differentiate between Mannose and Glucose , Fischer developed a series of reactions to make the aldehyde group to alcohol and terminal alcohol to aldehyde. What was this series of reactions ? Even ...