What is a sugar?

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.




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](http://en.wikipedia.org/wiki/Sugar#Chemistry). 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.


![enter image description here](https://i.stack.imgur.com/mcvGW.gif)


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.


![enter image description here](https://i.stack.imgur.com/f7psz.jpg)


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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


* <http://en.wikipedia.org/wiki/Deoxyribose>
* <http://en.wikipedia.org/wiki/Glucose>
* <http://en.wikipedia.org/wiki/Sucrose>


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