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I've always wondered about this...

I can look up the exact chemical structure of (say) glucose, which is a sugar. That's easy enough. But what I cannot figure out is this: if I proffer some random molecule at you, how do you determine whether it can be considered a "sugar" or not? I can't seem to locate anywhere a definition of what it means to be a sugar; I can only find lists of specific molecules which are sugars.

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There is no standard "rule" for identifying sugars because in chemistry there are always exceptions to any rule. Nonetheless, the following guidelines will help you correctly identify a simple sugar (a monosaccharide) in most cases.

Simple sugars have the molecular formula $\ce{C_{n}(H_2O)_{n}}$, where n is at least 3. They also have to be capable of forming an aldehyde or ketone carbonyl group. For example, in glucose the cyclic forms predominate, but there is an isomer with a carbonyl group.

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Whereas compounds in the inositol family fit the molecular formula requirement, but are not considered to be sugars because they are incapable of forming a carbonyl.

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Sugars are a family of compound that includes monosaccharides and (poly-/oligo)-saccharides. The latter are derivide from the former by means of inter-esterification. Monosaccharides include mostly linear-chain molecules with three to seven carbon atoms that consist mostly of $\ce{-CH(OH){-}}$ and one $\ce{-C(O){-}}$ units terminated with hydrogen on chain ends. Some C8- and C9-molecules are also considered to be monosaccharides

Monosaccharides often exhibit linear-ring tautomery, and polysaccharides are usually built from ring forms of monosaccharides.

Traditionally, some molecules including $\ce{-CH2{-}}$ block, like deoxyribose, are also considered monosaccharides, though it may be debatable.

While brutto-formula of sugars usually is close to $\ce{C_{n}(H2O)_{m}}$, it is not s sufficient criterion to find one. For example, dimethylcarbonate has formuls $\ce{CH3-OC(O)O-CH3}$, i.e. $\ce{C3(H2O)3}$, but is not a sugar.

The relevant wiki article http://en.wikipedia.org/wiki/Sugar

some common monosaccharides

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  • $\begingroup$ Actually sucrose is a disaccharide and does not conform to $\ce {C_{n}(H_2O)_{n}}$, it is $\ce {C12H22O11}$. $\endgroup$ – LDC3 Sep 27 '14 at 17:24
  • $\begingroup$ @LDC3 please, reread the answer. a) I'm not talking abount the link, but amount of carbon in a monosacharide and b)no it does not, and the point is that some molecules traditionally assumed to be monosacharides has less oxygen than 'ideal' carbohydrates with same abount of hydrogen. $\endgroup$ – permeakra Sep 27 '14 at 17:28

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