53
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 ...
- 1,860
25
votes
Accepted
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 ...
- 23.2k
24
votes
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How can 32 g of Gatorade powder contain 33 g of carbohydrates?
Is there a chemical process as some people have suggested that is changing the 32g of powder into 33g of carbs once water is added?
No. Even if there was, it's not relevant in this case: the numbers ...
- 10.3k
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 ...
- 20.2k
17
votes
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Can paper burn without oxygen or air?
No, the paper will not burn without oxygen being present.
Paper is made primarily of cellulose which is a polymer of glucose.
If you heat paper in a vacuum the cellulose simply decomposes to $\ce{...
- 81.6k
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 ...
- 17.9k
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 ...
- 34.6k
13
votes
Can I break starch down into glucose units?
Starch is what plants primarily use as a glucose storage. As such, it is essential that they can break it back down into glucose otherwise it would defeat its own purpose and be removed by evolution ...
- 63.6k
13
votes
Why do other sugars melt whereas sucrose decomposes?
You're mostly right. Sucrose is a disaccharide composed of glucose and fructose.
It is a crystalline material. Heating sucrose results in a complex thermal process that involves both melting and ...
- 81.6k
12
votes
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How do you identify reducing / non-reducing sugar by looking at structure?
Many sugars are drawn in the cyclic, closed form where the carbonyl group has been converted to a hemiacetal.
Once you realize that a hemiacetal can equilibrate with a carbonyl (e.g. it is a ...
- 81.6k
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), ...
- 23.2k
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 ...
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11
votes
Accepted
Why do gummy bears explode when added to hot potassium chlorate?
Potassium chlorate is a source of oxygen. After heating, it decomposes to $\ce{O2}$ and $\ce{KCl}$:
$$\ce{4 KClO3 → KCl + 3 KClO4}$$
$$\ce{KClO4 → KCl + 2O2}$$
The gummy bear is mainly composed of ...
- 837
11
votes
Accepted
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}...
- 63.6k
11
votes
Accepted
Ambigious 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 ...
- 63.6k
11
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 ...
- 2,164
10
votes
Accepted
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 ...
- 6,458
8
votes
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Carbohydrate configurations
There are 32 isomers of C6H12O6, of which you have named 6. The most common one is D-glucose (dextrose). I don't think all of the 16 L-saccharides are found in nature.
α and β refer to the connection ...
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8
votes
Accepted
Why starch (amylose) and cotton (cellulose) are so different?
Your structures don't clearly show the different configurations between α- and β- linked glycopyranosides. Cellulose is a non-branching (poly) β-glycopyranoside. Amylose (a component of starch) is a ...
- 7,892
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 ...
- 63.6k
8
votes
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 ...
- 259
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, $11$% ...
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7
votes
Accepted
How is it that fructose has a different metabolic pathway than glucose but yet glucose is converted to fructose?
The metabolic pathway you are talking about is how fructose is converted into energy and how its concentration in the blood is regulated. It is indeed true that blood fructose level does not affect ...
- 638
7
votes
Why do other sugars melt whereas sucrose decomposes?
What you are doing when you heat a sugar like sucrose is you are dehydrating it. The crystalline structure of sucrose breaks down and the molecules decompose into glucose and fructose and then lose ...
- 563
7
votes
Accepted
Why is H bonded to C5 drawn below the plane in Haworth formula for D-glucose?
The Haworth projection depicts the pyranose (or furanose, as the case may be) as a planar ring. The Fischer projection depicts the open chain carbohydrate with the carbon backbone in a single plane. ...
- 10.4k
7
votes
Accepted
Carbon numbering in carbohydrates
Actually, the numbering is the same, at least for the fructose portion of the molecule (which is the right-hand monosaccharide in your sucrose image - the one on the left is glucose).
The key point ...
- 4,727
7
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 ...
- 63.6k
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 ...
- 65.6k
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