Aldol Condensation

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An Aldol Condensation Reaction: The Synthesis of Tetraphenylcyclopentadienone—An
Example of a Double-Crossed Aldol

Addition Reactions of Carbonyl Groups

The chemical reactivity of aldehydes and ketones is closely associated with the presence of the carbonyl group in their structures. For example, both aldehydes and ketones undergo addition reactions such as the addition of a Grignard reagent to the carbonyl group as shown in Figure 1.

Figure 1. Addition reactions.

The reactions in Figure 1 differ only because the pink H of an aldehyde is replaced by R′′ in the ketone. The addition reaction occurs at the carbonyl group. The carbonyl group is polarized so that the carbon atom bears a partial positive charge and the oxygen atom bears a partial negative charge. The R′ group of the Grignard reagent is negative relative to the positive Mg atom. Thus, the negative R′ group bonds to the positive carbon atom, and the negative oxygen and metallic magnesium form an ionic bond, yielding a salt in each reaction. The addition product is acidified in each case to make a covalent alcohol. The aldehyde produces a IIo alcohol; whereas, the ketone produces a IIIo alcohol owing to the R′′ group.

The two equations for addition reactions in Figure 1 are summarized in Figure 2. A nucleophile (negative species) bonds to the carbonyl carbon (positive), breaking the π bond of the carbonyl group. Figure 2. Addition of a nucleophile to a carbonyl group.

Figure 2 focuses our attention on the salient part of an addition reaction that involves either a ketone or an aldehyde. A nucleophile bonds to the carbonyl carbon. In an aldol addition reaction, the nucleophile is an enolate formed from an aldehyde or ketone by the removal of a hydrogen atom next to the carbonyl group. The enolate (negative nucleophile) then adds to a carbonyl group of another aldehyde or ketone as…...

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