Oxygen positively-charged species

We must also consider the situation for certain important radionuclides from a variety of different groups in the Periodic Table which are able to exist in the environment as negatively charged oxygenated species as well as various positively charged species. Important illustrations of this type are TcOf, I-and IO3. 

The reaction of aldehydes and ketones with an acid catalyst gives the corresponding oxygen-stabilized cation. Protonation of carbonyl compounds also gives carbocations (Scheme 2.11). Here, note that carbocations are positively charged species. They are much more likely to be formed in acidic than in basic media in fact, carbocations are never seen under conditions that are strongly basic. 

Notice that the mechanisms for imine, enamine, hydrate, and acetal formation are similar. The nucleophile in each reaction has a lone pair on its attacking atom. After the nucleophile has added to the carbonyl carbon, water is eliminated from a protonated tetrahedral intermediate, forming a positively charged species. In imine and hydrate formation, a neutral product is achieved by loss of a proton from a nitrogen and an oxygen, respectively. (In hydrate formation, the neutral product is the original aldehyde or ketone.) In enamine formation, a neutral product is achieved by the loss of a proton from an a-carbon. In acetal formation, a neutral compound is achieved by the addition of a second equivalent of alcohol. 

Notice, too, that the carbonyl oxygen of the carboxylic acid is protonated in the first step and not the hydroxyl oxygen. The species formed by protonation of the carbonyl oxygen is more stable because it is stabilized by electron delocalization. The positive charge is shared equally by both oxygens. 

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