Can someone help me identify the functional groups in ascorbic acid?

Question

I just started organic chemistry and am having a hard time identifying functional groups.

Answer 1

Go to wikipedia, for example...now, look at the structure of ascorbic acid or vitamin C:

 

Going strictly by the definition: you have two alcohols (-OH groups) on the chain, you have one cyclic ester (a lactone), and then you have two more hydroxyl -OH's on the ring with a carbon-carbon double bond between them (by definition, an alkene). So from just that standpoint you have:

1 lactone

4 alcohols

1 alkene

 

However, this is where it becomes complicated.

 

You will realize that there is a different combination of functional groups than are mentioned above because of one thing. The two hydroxyl groups on the ring next to each other are vinyl (on the alkene) and therefore in conjugation. Vinyl alcohols are known as enols. (alk"ene" + alcoh"ol").

1 lactone

2 alcohols

2 enols

 

Each one is an enol, however there is no "functional group" that is an en-1,2-diol. That is because an enol is almost always in equilibrium with an isomer which are the more commonly known functional groups, a ketone and an alcohol. (the exceptions are mentioned below and involves a different phenomenom in organic chemistry)

You therefore must have a ketone and an alcohol (each OH can play either role) instead of 2 enols.

The most stable structure (the one that's usually shown as ascorbic acid) is the most stable and therefore the one that is written because the C-C double bond is in resonance with the carbonyl of the ester. (a,b-unsaturation). However, each of those hydroxyls can (and should) be drawn as a ketone which makes the other a regular alcohol (sp3 hybridized--without the conjugation). Then you can actually catalog the proper functional group names that you are accustomed to. 

 

Ascorbic acid should be drawn in equilibrium with two other isomeric structures. (There are three acceptable structures in equilibrium). The only enol that is completely in its enol form is phenol (and even 1,2-dihydroxybenzene or 1,2-catechol) because that structure is aromatic and so stable it doesn't tautomerize). Thus, you must account for the other two structures and then the compound therefore contains, regardless of which enol you tautomerize:

1 lactone

3 alcohols

1 ketone

 

This unusual structure gives ascorbic acid the ability to trap single-electron species (free-radicals) giving it antioxidant properties. The job of Vitamin C is to scavenge radicals in aqueous or hydrophilic environments. Other vitamins (A, E, etc.) play this role in lipid/fatty or hydrophobic environments.

 

Ascorbic acid is quite unique and the question is more complicated than it appears. But if you understand this, you truly have a grasp on an important phenomenon that recurs often in organic chemistry. Keto-enol tautomerization.   

Answer 2

I can't draw the structure here, but if you look it up, you will find the following functional groups:

(a)  there are four (4) alcohol groups (they are the -OH groups)

(b)  there is one (1) ester (it is O=C-O-C in the ring part)

(c)  there is one (1) alkene (it is the C=C in the ring)