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Aldol condensations and Grignard reagents

1. "Crossed" or "mixed" aldol condensations are practical for synthesis, if one of the aldehydes (or ketones) has no alpha-hydrogen atoms. Explain.

2. In the aldol condensation using the conditions of this experiment, why might it be essential that the benzaldehyde contain no benzoic acid?

3. What hydrocarbon would you expect to obtain by the action of water on each of the Grignard reagents listed below?
a. Butylmagnesium bromide
b. sec-Butylmagnesium bromide
c. iso-Butylmagnesium bromide
d. tert-Butylmagnesium bromide

4. What product would each of the Grignard reagents in the previous question yield when treated with D2O?

Solution Preview

"Crossed" or "mixed" aldol condensations are practical for synthesis, if one of the aldehydes (or ketones) has no alpha-hydrogen atoms. Explain.

Normally, a C-H bond is usually stable, nonpolar, and certainly not acid. However, the preence of a carbonyl group results in an acidic alpha hydrogen. Because these alpha hydrogens are acidic, an aldehyde or ketone with an alpha hydrogen can react with a base forming an enolate ion.

For example,

O O
|| ||
CH3CH + OH- -----> -CH2CH

These enolate ions can act as a nucleophile, attacking the carbonyl carbon of another ketone or aldehyde. Then, the negatively charged O (which used to be the carbonyl oxygen) picks off a hydrogen from water, forming an alcohol. These reactions are called aldol condensations. Aldol stands for "aldehyde and alcohol." In other words, the products of this self condensation is a beta-hydroxyaldehyde. They would look like this if acetaldehyde underwent such a reaction.

OH O
| ||
CH3CHCH2CH

Now, an aldehyde with no alpha hydrogens cannot form an enolate ion and therefore cannot form aldol dimers in an aldol condensation. However, if this kind of ...

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