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55 votes

Why does wood burn but not sugar?

Combustion is a gas phase reaction. The heat of the flame vapourises the substrate and it's the vapour that reacts with the air. That's why heat is needed to get combustion started. Anyhow, wood ...
John Rennie's user avatar
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26 votes
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Does Br2/H2O oxidize all aldehydes including carbohydrates?

Aldehydes, including aldoses, are oxidized to their respective carboxylic acids in the presence of $\ce{Br2}$ in $\ce{H2O}$. The reason this reaction is often discussed with carbohydrates is that it ...
ringo's user avatar
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20 votes
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How long would it take for sucrose to undergo hydrolysis in boiling water?

This is a nice well-defined question, and luckily there is excellent data for which we can provide a quantitative answer. Richard Wolfenden's research group has sought for many years to characterize ...
Curt F.'s user avatar
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16 votes
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Why does the fructose monomer in sucrose appear different from isolated fructose?

Ah, a (fairly) common conundrum that assails us Chemistry students when we start Biochem. ;-) At first glance, the fructose molecule in your first picture, and that in the second picture appear to ...
paracetamol's user avatar
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14 votes

Can sugars dissolve in liquid ammonia?

Can sugars dissolve in liquid ammonia? Yes, according to Ref.1, liquid ammonia is used to extract sugars in sugar-beet chips: 5.88 kilograms of sugar-beet chips having a moisture content of 5.4 ...
Mathew Mahindaratne's user avatar
13 votes

Why does wood burn but not sugar?

With hydrocarbons a certain amount of oxygen (n) and a certain amount of heat energy (Q) are required for complete combustion. In complete combustion the byproducts are carbon dioxide and water in ...
Agriculturist's user avatar
13 votes
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Why might a prepared 1% solution of glucose take 2 hours to give maximum, stable reading on a glucometer?

Crystalline glucose is the alpha anomer, but the enzyme in the glucometer is specific for the beta form. They interconvert slowly in solution. See https://www.jofem.org/index.php/jofem/article/view/...
Karsten's user avatar
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12 votes
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Gases produced by pyrolysis of cellulose

During pyrolysis, organic compounds are thermally decomposed in the absence of oxygen. The pyrolysis products are classified into categories based on their physical state of existence: char (solid), ...
ringo's user avatar
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12 votes
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Ambiguous nature of aldehydic group in glucose

The distal hydroxy groups are in a perfect distance to form pyranose (six-membered oxane) or furanose (five-membered oxolane) rings. Any equilibrium reaction that leaves the carbonyl group liberated ...
Jan's user avatar
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12 votes

How do you recognize a carbohydrate molecule?

The name "carbohydrate", which literally means“carbon hydrate,” arises from their chemical composition, which is roughly $\ce{(C.H2O)_n}$, where $n\ge 3$. The basic units of carbohydrates are ...
xavier_fakerat's user avatar
11 votes
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Why is the reduction by sugars more efficient in basic solutions than in acidic ones?

This is based in the underlying redox rection. If we take e.g. mannose and attempt to oxidise that, the (unbalanced) half-reaction we need is the following: $$\ce{C6H12O6 -> C6H10O6 + 2e-}\tag{Ox1}...
Jan's user avatar
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10 votes
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Why is it important that glucose’s third OH group points to the left?

This is the concept called stereochemistry. Each carbon with 4 bonds has an approximately tetrahedral geometry. We assign a configuration to the situation by numbering the substituents in terms of ...
Lighthart's user avatar
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10 votes
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Does cooling a potato change the nature of its carbohydrates?

Does cooling a potato change the nature of its carbohydrates? Yes, retrogradation is a reaction that takes place when the amylose and amylopectin chains in cooked, gelatinized starch realign ...
Rob's user avatar
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9 votes

Periodic cleavage of carbohydrates

Yes, there are specific rules regarding the products of periodic acid cleavage of 1,2-diols (Malaprade reaction). As to the mechanism, it is illustrated here. It is similar to the mechanism for KMnO4 ...
user55119's user avatar
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8 votes

Before the CIP system was created, how did chemists ensure that the D/L system matched the R/S assignments?

Chemists actually still use D- and L- prefixes today, especially for carbohydrates where using it saves a lot of characters: Compare D-ribose to (2R,3R,4R)-2,3,4,5-tetrahydroxypentanal. The ...
Jan's user avatar
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8 votes

Why is it important that glucose’s third OH group points to the left?

Lighthart gave the theoretical explanation of what enantiomers (‘right foot — left foot’) and diastereomers (‘right foot and right shoe — right foot and left shoe’) can be thought of. Here is the ...
Jan's user avatar
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8 votes

How do you recognize a carbohydrate molecule?

A carbohydrate is a biological molecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water); in other words, with the empirical ...
Mesentery's user avatar
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8 votes
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D/L and +/− labels of glucose

For a given configuration there is a corresponding and fixed rotatory effect. However, the actual rotatory effect - that seen by a polarimeter - can either be + or −, independently of the D- or L-...
Alchimista's user avatar
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8 votes

Why is fermentation of cellulose to produce biofuel and nutrients so difficult?

The ultimate reason is that cellulose is designed to be hard to digest The specific chemical reason is well covered in Jan's answer, but there is an explanation that is simpler and more fundamental: ...
matt_black's user avatar
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8 votes

What is the carbon content, by weight, of vegetable oil?

Vegetable oils are mainly made of glycerol esters of oleic acid, linoleic acid and palmitic acid. For example, the fatty acids extracted from olive oil are a mixture of $74$% oleic acid, $11$% ...
Maurice's user avatar
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7 votes

difference between glyceraldehyde-3-phosphate and 3-phosphoglycerate

The difference between the two molecules is highlighted in red. The functional group in 3-phosphoglycerate is a carboxylic acid. That in glyceraldehyde-3-phosphate is an aldehyde. Sugars have the ...
orthocresol's user avatar
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7 votes
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Why is maltose a reducing sugar but not sucrose, even though they're both disaccharides?

The difference in stability between maltose and sucrose boils down to the different structural elements their aldehyde/ketone groups have been turned into during the formation of the disaccharide. In ...
Jan's user avatar
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7 votes
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Why pyranosides do not mutarotate?

In principle, the acetalisation reaction is indeed reversible. Typical conditions to reverse it would be to use $\ce{TsOH}$ or $\ce{PPTS}$ and water. However, both full acetalisation and liberation ...
Jan's user avatar
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7 votes
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Why is fermentation of cellulose to produce biofuel and nutrients so difficult?

Your question is a little bit all over the place, but I believe I can answer it anyway. First, though, allow me to point out that your sum formula for cellulose is wrong. While glucose is indeed a ...
Jan's user avatar
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7 votes
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Identifying the D/L form of any aldose or ketose in cyclic form

Your structure is a ketose because it is a hemiketal of a ketone. It is a furanose because it is a 5-membered (furan-oid) ring. Fructose, which your structure is not, is usually drawn as a furanose as ...
user55119's user avatar
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7 votes
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How does existence of alpha and beta form of glucose prove that it exists as a cyclic structure

According to Wikipedia: Glucose is usually present in solid form as a monohydrate with a closed pyran ring (dextrose hydrate). In aqueous solution, on the other hand, it is an open-chain to a small ...
Mathew Mahindaratne's user avatar
7 votes
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Reaction of glucose acetal with acetic anhydride

I think there's nothing to discussed about this further as all comments directing to the correct answer: $x = 4, \ y = 6,$ and $ z = 5.$ I think it's better show in the structures: Glucose is ...
Mathew Mahindaratne's user avatar
6 votes

If glucose can react with phenyl hydrazine to form osazone why can't it react with 2,4-DNP to give positive test?

Although glucose 1a exists principally in the hemiacetal pyranose forms (α and β), nonetheless it reacts with one equivalent of phenylhydrazine to form the hydrazone 1b via the aldehydic ...
user55119's user avatar
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6 votes

Reduction of glucose to hexane with hydroiodic acid

Hydroiodic acid is a reducing agent. As Wikipedia says: Although harsh by modern standards, HI was commonly employed as a reducing agent early on in the history of organic chemistry. Chemists in ...
Curt F.'s user avatar
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