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Name:  Donnie
Status: other
Grade: other
Location:VA
Country: N/A
Date: 11/2/2005


Question:
Can a molecule be in the Hydroxyl, Carbonyl, and Carboxyl group at the same time?


Replies:
The common terminology is a little different than the way you are using these words, Donnie.

Conventionally, there is no "group" of molecules. But we'll understand if you invent a "family" of molecules. Same thing, different word, because the word "group" already means something important to us. A "group" usually means a bonded group of atoms that are a common subset of many different whole molecules. A group is a familiar "sub-molecule", if you will. The atoms [-OH] make a "hydroxyl group", all by itself, regardless what molecule it is part of. Likewise [>C=O] is a carbonyl group, and [-COO-] is a carboxyl group.

Defining our "families" of molecules: If one starts with the various small-to-medium-sized alkanes and alkenes, and then adds one of these groups, or perhaps two of the same group, I can see calling that a family. The hydroxyl family is commonly called "Alcohols". Di-hydroxyls are "di-ols", or maybe "di-alcohols" or "glycols". The carbonyl family is commonly called "ketones" if the carbonyl is in the middle of a carbon chain, or "aldehydes" if it's on the end making a [-CO-H]. Having a carboxyl group at the end of a carbon chain means it is [-COOH], so these are the carboxylic acids. If the carboxyl group is in the middle of that chain, these are the "esters". In this case the non-double-bonded oxygen is in the line of the chain, and we choose to consider this a break in the carbon-chain, so the molecule is named by the two separate carbon-chains it now has on either side of the [-O-] in [-COO-]. But we still like to use the family name "ester".

C2H5-COO-H is a carboxylic acid called propionic acid. Named after propane because there is an unbroken chain of 3 carbons. C2H5-COO-C2H5 is an ester called ethyl propionate. The C2H5 on the right is the ethyl group which replaced the H in propionic acid.

You could easily have a molecule with two or more of these groups on it, but it's difficult to chose a family to put it in. We like our family concepts because:

a) the molecule is simple and it's structure is described by the family definition.

b) the physical, solubility, and reaction properties of molecules in a family resemble each other.

If you put on groups from two families:

a) there are a lot more combinations, so it is not as simple to use it to describe the structure.

b) the properties are a chaotic hybrid of the properties of the two families.

For some properties one group matters most, for others, the other group matters most.

Probably a new family is needed. Or several families.

I think cooking oils, other natural oils, "Fatty acids", and many waxes make a very broad family which includes any two or even all three of these groups on a single molecule. The "ether" group [-O-] gets in there too.

Obviously you are thinking for yourself, which is good. I hope this feels like it helps your thinking along, rather than interrupting it.

Jim Swenson


Hydroxyl groups refer to C--OH bonds, carbonyl groups refer to >C=O bonds, and carboxyl groups refer to --C|=O bonds. These can all be present organic compounds of sufficient complexity to satisfy the

OH

four bonds required for each carbon.

Vince Calder



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