[Skiznibbler]

I'm seriously confused as to what R represents in chemical structures. Does it always represent a hydrocarbon or alkyl group or can it denote any functional group? 3 functional groups that are confusing me are the ketone group, the aldehyde group and the carbonyl group. I can't see the difference between the 3.

Starting with the carbonyl group

First of all can the "A" and the "B" represent any element or functional group? Secondly lets say A and B denote hydrogen then we have an aldehyde (formaldehyde).
-
now is this a carbonyl group or an aldehyde group or both? What complicates me even more is that this compound also meets the criteria of a ketone

if I substitute the 2 R's of the ketone with hydrogen then once again I have formaldehyde. So what is it? Is it a ketone, is it an aldehyde, is it a carbonyl group or is it all 3?

What is the difference between these 3 functional groups? What is the difference between the ketones "R's" and the carbonyl groups "A and B" and what separates an aldehyde from a ketone?

[Yggdrasil]

The Rs generally represent general carbon-containing chains (of length one or greater).  So, in the place of the R you would have a carbon atom connected to some other things.  R groups are almost never used to represent hydrogen atoms in this context.  R groups also won't be used to represent heteroatoms (e.g. nitrogen, oxygen, halogens) in this context.

To help clear up your confusion, the carbonyl group is a broad category that includes both aldehydes and ketones (as well as carboxylic acids, esters, amides, etc.).  In other words, aldehydes and ketones are different functional groups; however, all ketones contain a carbonyl and all aldehydes contain a carbonyl.

[Skiznibbler]

God bless you my friend god bless you!! That was a lifesaver of a reply. That question was driving me mad. So I can safely assume that the R will most likely represent a carbon atom and most likely that carbon atom will be part of hydrocarbon chain?

As for the carbonyl group question that completely answers it. I didn't know that functional groups could fall under the category of other functional groups. Thanks alot. 

[Yggdrasil]

In most cases, I wouldn't refer to a carbonyl as a functional group.  A carbonyl is basically a fancy name for a C=O double bond.  All carbonyls can be more accurately classified as a more precise-defined functional group (e.g. ketone, aldehyde, carboxylic acid, etc.).

[Skiznibbler]

Thanks again. Everywhere they say carbonyl "group" so its lodged in my head now but it makes more sense thinking of it as just a word for the C=O bond.

Another quick side question. Is alcohol a functional group or is it just a class of molecules that are basically straight hydrocarbon chains with a hydroxyl group attached to it? Are alcohols by definition alkyl groups with an OH bond? I'm guessing alkenes and alkenyl groups attached with a hydroxyl group attached (i.e. phenol) don't count.

[azmanam]

alcohol is just a class.  C-O-H = alcohol.

the alkENe with an alcohOL attached is referred to as an ENOL.  It is a combination of those 2 functional groups.  Phenol is a special case where the O-H group is directly attached to an aromatic (referred to sometimes as an aryl ring).  But they're all still alcohols.

compounds with more than one functional group are quite common and often have combination names like enol, enol ether, enamine, beta-keto acid, amino acid, etc.

[Yggdrasil]

I would call alcohols a functional group.  However, not all OH groups signify an alcohol.  OH groups appear in other common functional groups such as enols, carboxylic acids, and phenols.  Basically, the combination of two or more functional groups or classes often creates a new functional group (the phenol group being a prime example, since its the combination of a phenyl group with an alcohol).  Phenols and alcohols are classified as different functional groups because the OH groups have significantly different properties in both cases (e.g. significantly different pKa values).

[Skiznibbler]

s#*$ I was hoping that alcohols were only alkanes with a hydroxyl group so that I could easily identify alcohols and there wouldn't be too many but your saying that ANY molecule with a hydroxyl group is an alcohol? That means theres millions of different alcohols then doesn't it?

So your saying that all these compounds that contain a hydroxyl group may be classed as other compounds like enols and ketones but they are also alcohols? Does that mean phenol is an alcohol and an enol?

What about ethers? I read that an ether is 2 alkyl groups connected by an oxygen atom. When they say ALKYL groups do they really mean alkyl groups as in hydrocarbon chains withOUT heteroatoms or can any functional groups be attached to either side of the ethers O?

I only just got into organic chemistry after reading alot of regular physical chemistry and I love it. Its good fun learning about functional groups but this alcohol thing pisses me off. I had the idea in my head that alcohols where an ALKYL group attached to the hydroxyl group and that apart from the O they had no heteroatoms so they were a specific easily defined class of compounds but now I'm finding out that an alcohol is ANY compound with a hydroxyl group so that broadens the category so literally millions of compounds are alcohols. It makes the term kind of obscure in my opinion.

[azmanam]

Usually when we refer to functional groups, we're only talking about the very localized atoms around the bonding pattern in question.  It rarely matters what else is in the molecule - that particular bonding pattern still retains its functional group name.

I count at least 5 functional groups in this molecule - and it's fairly small as far as organic molecules are concerned.

http://en.wikipedia.org/wiki/Penicillin

Even on that page, you see an R group.  It can be just about anything.  Click through some of the semi-synthetic derivatives for examples, like this one

http://en.wikipedia.org/wiki/Flucloxacillin

alkyl groups as in hydrocarbon chains withOUT heteroatoms or can any functional groups be attached

Any functional group can be attached to an extent...

I only just got into organic chemistry after reading alot of regular physical chemistry and I love it. Its good fun learning about functional groups

The fun's only beginning

Happy chemistry
-AA

[Yggdrasil]

No, you are correct.  A compound is an alcohol if it has a hydroxyl attached to an alkyl carbon.  An OH connected to a carbonyl carbon is not an alcohol.  An OH connected to an aromatic ring carbon is not an alcohol.  An OH attached to a nitrogen is not an alcohol.

One thing that's different about organic chemistry and physical chemistry is that you shouldn't expect rigid and rigorous definitions in organic chemistry, especially in such things as identifying functional groups.  The real point of identifying functional groups is really to identify reactive parts of a molecule and to be able to infer the reactivity of the molecule from its chemical structure.  As the structures that azmanam posted show, real chemical compounds often contain multiple functional groups juxtaposed side by side and it is often hard (and pointless) to develop a rigorous set of rules to classify the functional groups of a complex molecule.  Instead, it is a chemists' job to use his/her knowledge of chemistry to determine how the different part of the molecule will interact in order to figure out the reactivity and properties of the molecule.  Learning the properties of simple functional groups is only the first step in this process.

[azmanam]

@ yggdrasil

I agree totally.  That's the sentiment I was trying to get across, but you said it much better than I did

[Skiznibbler]

Thanks yet again ygg. Your the only person on this site I've actually learned from. Alot of people explain all sorts of things I didn't ask and in the end don't even answer my question. Although I'm always happy to learn new things I usually have to answer the initial question again.

Thats a huge relief. So I can view alcohols and these other groups like I did when I first learned about them. Hydrocarbons with a hydroxyl group attached to it. I was thinking it was a bit strange these classes of molecules having heteroatoms because everywhere I looked the initial definitions of them were things like this

In chemistry, an alcohol is any organic compound in which a hydroxyl group (-OH) is bound to a carbon atom of an alkyl or substituted alkyl group.

In ketones, the carbonyl group has two hydrocarbon groups attached.