A carbonyl group consists of an oxygen atom bound to a carbon atom by a double covalent bond [C=O]. This is an example of a functional group commonly found loacated in different places in compounds. Carbonyl compounds are seperated into two different classes depending on the position of the carbonyl group. The carbonyl group in Class 1 compounds is a leaving group which leaves the molecule in chemical reactions. Examples of class 1 carbonyl compounds are amides, esters, carboxyllic acids and anhydrides. Class 2 compounds have carbonyl compounds that do not leave the molecule when reacting, examples of this type of carbonyl compound are ketones and aldehydes .
The [C=O] bond is electronegative due to the oxygen atom having two lone pairs of electrons, making the group polar as the carbon is partially positive, and the oxygen is partially negative. The electronegative properties of the group make it prone to nucleophillic attack; the positively charged carbon is attacked by negative nucleophiles in nucleophillic addition reactions. The negatively charged oxygen can attack positively charged hydrogen ions, meaning that the group has acidic properties.