Addition Reactions of Carbonyl Compounds
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- 63679
The reactivity of carbonyl compounds is dictated primarily by the polarity of the C=O bond. The carbonyl carbon contains a partial positive charge (d+) and the carbonyl oxygen contains a partial negative charge (d-). Aldehydes are typically more reactive than ketones due to the following factors.
- Aldehydes are less sterically hindered than ketones (a hydrogen atom is smaller than any other organic group).
- The carbonyl carbon in aldehydes generally has more partial positive charge than in ketones due to the electron-donating nature of alkyl groups. Aldehydes only have one e- donor group while ketones have two.
Nucleophilic Addition
The most common reaction of carbonyl compounds is addition of a nucleophile to the carbonyl carbon. In the first mechanism, the nucleophile attacks the carbonyl carbon (d+) with concurrent conversion of the C=O to a single bond. This is the predominant mechanism for strong nucleophiles. Two possible reaction pathways can occur from this point. For aldehydes and ketones, the negatively charged oxygen in this intermediate tends to abstract a proton from the solvent to become an alcohol (as shown below). For carboxylic acids and derivatives, elimination of the leaving group occurs (with simultaneous conversion of the C-O back into C=O).
A second mechanism predominates in strongly acidic environments. In this mechanism, the carbonyl oxygen is protonated, generating a positively charged compound. This intermediate can be drawn as two resonance structures, where the positive charge is localized on either the oxygen or the carbonyl carbon. In the second step, the carbonyl carbon (which carries a partial positive charge) is attacked by a nucleophile. Since the intermediate is already positively charged, the attacking nucleophile does not have to be particularly strong for this reaction to proceed.
Addition of Water and Alcohols
The lone pair of electrons on the oxygen atom in water and alcohols can act as a nucleophile and attack the carbonyl carbon atom. If water is added, the product is referred to as a gem-diol or simply as a hydrate. If one equivalent of alcohol is added, the resulting product is either a hemiacetal or a hemiketal. If a second equivalent of alcohol is added, an acetal or ketal is formed.
Product | Reaction | # -OH | #OR |
---|---|---|---|
hydrate | 2 | 0 | |
hemiacetal | 1 | 1 | |
hemiketal | 1 | 1 | |
acetal | 0 | 2 | |
ketal | 0 | 2 |