Lab 12 An Aldol Reaction-The Synthesis of Tetraphenylcyclopentadienone

Lab 12 An Aldol Reaction-The Synthesis of Tetraphenylcyclopentadienone

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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. RC H + R'MgX O RC H OMgX R' H + H 2 O RC H R' OH aldehyde Grignard reagent salt acidification II o alcohol RCR'' + R'MgX O RCR'' OMgX R' H + H 2 O acidification RCR'' R' OH III o alcohol Grignard reagent salt ketone addition addition 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 II o alcohol; whereas, the ketone produces a III o 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. Lab 12 1
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O :Nu C O - Nu 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 shown in Figure 1. Formation of an Enolate from an Aldehyde or Ketone Aldehydes and ketones that possess alpha hydrogen atoms can form enolates. The Greek alphabet ( α , β, γ , etc.) is used by chemists to identify carbon atoms in relation to the carbon atom of a carbonyl group. An alpha carbon atom is a carbon atom that is bonded directly to the carbon atom of a carbonyl group. A beta carbon atom is the second carbon atom from the carbonyl carbon, a gamma carbon is the third carbon away from the carbonyl, etc. Likewise, hydrogen atoms are named according to the name of the carbon atom to which they are bonded . A hydrogen atom bonded to an alpha carbon is called an alpha hydrogen, etc. Figure 3 shows these relationships for acetaldehyde and 2- pentanone. α
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This note was uploaded on 05/04/2010 for the course CHEM Orgo taught by Professor Su during the Spring '10 term at Rowan.

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Lab 12 An Aldol Reaction-The Synthesis of Tetraphenylcyclopentadienone

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