Report
Experiment #9 – Identification of Aldehydes and Ketones
Introduction Aldehydes and ketones share the carbonyl functional group which features carbon doubly bonded to oxygen. In the case of ketones there are two carbon atoms bonded to the carbonyl carbon and no hydrogens. In the case of aldehydes there is at least one hydrogen bonded to the carbonyl carbon; the other attachment may be to a carbon or a hydrogen. In all cases the carbon(s) that are attached to the carbonyl group may be aliphatic (not part of an aromatic ring) or aromatic (part of an aromatic ring). Since they share the carbonyl group, aldehydes and ketones share much of their chemistry, but they are different enough to be considered different classes of compounds. This situation is similar to that of alcohols and phenols which both share the -OH group. Aldehydes and ketones both undergo a reaction type known as nucleophilic addition. Under less acidic conditions, in this type of reaction a nucleophile (a species that can donate a pair of electrons, in other words a Lewis base) donates a pair of electrons toward the carbonyl carbon forming a single bond to it. At the same time the double bond between the carbonyl carbon and oxygen becomes a single bond as one bonding pair of electrons in the double bond moves to become an unshared pair on the oxygen. The oxygen now has one bond to it and it holds three pairs of unshared electrons, so it has a negative charge. Consequently, the oxygen picks up a proton from somewhere (possibly one that was attached to the nucleophilic atom that attacked the carbonyl carbon) and becomes an -OH group. Under more acidic conditions the results are pretty much the same, but the sequence in which they happen is more or less reversed. In this case, a proton (from an acid) attaches itself to one of the unshared pairs of electrons on the oxygen. The carbonyl group now has a +1 charge and is very inviting to even a weak nucleophile. [You can’t have a strong nucleophile in an
Introduction Aldehydes and ketones share the carbonyl functional group which features carbon doubly bonded to oxygen. In the case of ketones there are two carbon atoms bonded to the carbonyl carbon and no hydrogens. In the case of aldehydes there is at least one hydrogen bonded to the carbonyl carbon; the other attachment may be to a carbon or a hydrogen. In all cases the carbon(s) that are attached to the carbonyl group may be aliphatic (not part of an aromatic ring) or aromatic (part of an aromatic ring). Since they share the carbonyl group, aldehydes and ketones share much of their chemistry, but they are different enough to be considered different classes of compounds. This situation is similar to that of alcohols and phenols which both share the -OH group. Aldehydes and ketones both undergo a reaction type known as nucleophilic addition. Under less acidic conditions, in this type of reaction a nucleophile (a species that can donate a pair of electrons, in other words a Lewis base) donates a pair of electrons toward the carbonyl carbon forming a single bond to it. At the same time the double bond between the carbonyl carbon and oxygen becomes a single bond as one bonding pair of electrons in the double bond moves to become an unshared pair on the oxygen. The oxygen now has one bond to it and it holds three pairs of unshared electrons, so it has a negative charge. Consequently, the oxygen picks up a proton from somewhere (possibly one that was attached to the nucleophilic atom that attacked the carbonyl carbon) and becomes an -OH group. Under more acidic conditions the results are pretty much the same, but the sequence in which they happen is more or less reversed. In this case, a proton (from an acid) attaches itself to one of the unshared pairs of electrons on the oxygen. The carbonyl group now has a +1 charge and is very inviting to even a weak nucleophile. [You can’t have a strong nucleophile in an