Question

What is the structural formula of carbohydrates?

Answer 1

I wish there was one...

Explanation:

First, we have to define a bit more what Carbohydrates are.
Strictly speaking, the term means Hydrates of Carbon.

Trouble is, there are so many different ones, with a varying amount of Carbon atoms. The smallest amount of `C` -atoms is 3: the Trioses.

Carbohydrates, also known as Saccharides, are divided into four groups: Mono- Di-, Oligo- and PolySaccharides. The smaller ones (primarily the Mono- and Disaccharides) are commonly referred to as Sugars.

But the Oligo- and Polysaccharides are simply chains of the sugar monomers: any between 3 and 10 is generally referred to as Oligo- , anything from 10 upwards to MANY monomers (thousands and thousands, often branched) as PolySaccharides. Best known amongst the latter are Starch (aka Amylum ) and Cellulose...

BTW: The word Saccharide is derived from the Greek word for Sugar anyway, so what's in a name...

As we are dealing with hydrated Carbon, it follows that the generic, empirical formula for them is `C_x(H_2O)_y`. This certainly holds true for most Monosaccharides, though even there you can find exceptions: For instance, Ribose certainly conforms to this rule (`C_5H_10O_5`), but the central sugar in DNA doesn't: Deoxy -Ribose. (`C_5H_10O _color(red)(4)`)

As the di- oligo- and polysaccharides are simply polymers of the monosaccharides (and sometimes disaccharides), we'll concentrate on these...

The smallest ones are Trioses, and there are three:

There are two glyceraldehydes, because the carbon atom in the middle has four different adjacent groups. This means that it is Chiral, resulting in two mirror images of the same molecule.
Both have an aldehyde group at the top, and therefore are called Aldoses, whereas the third one is a Ketone, and therefore called a Ketose.

In each of these three we can add a `H-C-OH` -group, resulting in Tetroses, or 4-carbon sugars.:

It goes without saying that for each of these an L-variant (mirror image) exists. Ketoses usually, if not always, have a name ending in -ulose...

We can repeat this process a number of times, until we reach the Heptoses( 7 carbon atoms). There are no sugar monomers with 8 or more C-atoms, as they are too unstable...

In water, these molecules often reconfigure into ring structures, but that would be beyond the scope of this answer....