A carboxyl, also known as a carboxyl or carboxy group, is a very common part of a functional group in an organic compound. It is written as -COOH and occurs on the end or side of a molecule. A functional group is the part of a molecule that gives that molecule its characteristic chemical reactions. A carbonyl, also known as a carbonyl group, is a simple carbon-oxygen double bond in an organic compound and is expressed as C=O.
The main difference between a carboxyl group and a carbonyl group is their chemical structure. A carboxyl group contains a carbonyl within it. The carboxyl group, derived from combining the words "carbonyl" and "hydroxyl," is comprised of a carbonyl group that has its carbon atom in a single bond with a hydroxyl group (-OH).
Difference in Bonding
The bonds in the carboxyl group are at a 120 degree angle. This is due to the C-OH double bond being longer than that of the C=O carbonyl bond. This makes the carboxyl group somewhat asymmetric. The hydrogen atom in the -OH has the ability to enter into a hydrogen bond with the oxygen atom in the carbonyl of a carboxyl group that's close by. The simple carbonyl double bond is a polar bond, making it stretch out in a straight line. This is due to the oxygen being more electronegative than the carbon, as the oxygen's electrons have a slightly higher density than the carbon's electrons.
The most common examples of compounds that have a carbonyl group in them are benzaldehyde, methyl ethyl ketone, acetic acid and ethyl acetate. The carbonyl group is located in the middle of each compound. Examples of carboxylic acids (organic compounds containing a carboxyl group) include formic acid (produced by ants) and acetic acid (found in vinegar).
Aldehydes and Ketones
Aldehydes and ketones both have a carbonyl group in them. The carbonyl group in an aldehyde is on the end of the carbon chain, while ketones have the carbonyl group in the middle of the carbon chain. The C=O double bond is susceptible to chemical reactions, thus paving the way for several common aldehydes and ketones. For example, propanone (acetone) is used in fingernail polish remover, 3-phenyl-2-propenal (cinnamaldehyde) is what causes the odor and flavor of cinnamon and 4-hydroxy-3-methoxy-benzaldehyde (vanillin) is flavor from vanilla beans. The reason for the numbers within the compound's name is the position number of the carbonyl group within the molecule.