54
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 ...
- 1,993
26
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 ...
- 24.1k
20
votes
Accepted
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 ...
- 22.6k
16
votes
Accepted
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 ...
- 18.8k
15
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 ...
- 42.1k
13
votes
Accepted
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/...
- 42.3k
12
votes
Accepted
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 ...
- 68.7k
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 ...
- 5,164
12
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 ...
- 2,284
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}...
- 68.7k
10
votes
Accepted
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 ...
- 304
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 ...
- 17.5k
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 ...
- 640
8
votes
Accepted
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-...
- 3,666
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: ...
- 37.8k
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, $11$% ...
- 30k
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 ...
- 71.9k
7
votes
Accepted
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 ...
- 68.7k
7
votes
Accepted
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 ...
- 68.7k
7
votes
Accepted
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 ...
- 68.7k
7
votes
Accepted
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 ...
- 17.5k
7
votes
Accepted
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 ...
- 42.1k
7
votes
Accepted
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 ...
- 42.1k
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 ...
- 17.5k
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 ...
- 22.6k
6
votes
Why is maltose a reducing sugar but not sucrose, even though they're both disaccharides?
Note that this is not about digesting/cleaving disaccharides into monosaccharides first.
Opening has to occur on the stage of the intact disaccharide.
What is requirement for an opening that ...
- 44.2k
6
votes
Accepted
A non-life process making glucose from carbon dioxide and water using electricity or light?
Popular research these days for CO2 fixation involves the following enzymes with associated conversion rate:
RuBisCO - 10 molecules/sec
ECR(Enoxyl-CoA carboxylase/reductase) - 80
molecules/sec
The ...
- 176
6
votes
Accepted
How do you recognize a carbohydrate molecule?
You've asked a great and challenging question, especially for people just learning this material. The other answers are useful and provide good background. They also illustrate how the great ...
- 242
6
votes
Are glucose and galactose cis-trans isomers of each other?
Glucose and Galactose are best described as epimers of one another.
The three structures below all have the same molecular formula and the same connectivity, but differ in the absolute configuration ...
- 17.2k
6
votes
Accepted
Why is methyl α-D-glucopyranoside preferentially formed from glucopyranose in acidic methanol?
In general, if there is overlap between a filled orbital and an unfilled orbital, a stabilising interaction will take place between the two. A simple example is the resonance present in esters, where ...
- 71.9k
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