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Topic: In Deep Discussion: Double Aldol Condensation  (Read 18799 times)

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Offline Momer

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In Deep Discussion: Double Aldol Condensation
« on: May 24, 2009, 12:03:23 PM »
Aldol Condensation
Under the influence of base or dilute acid, two molecules of an aldehyde or a keton may combine to form a β-hydroxy ketone. This reaction is called the aldol condensation. In every case the product results from the addition of one molecule of aldehyde (or ketone) to a second molecule in such a way that the α-carbon of the first becomes attached to the carbonyl carbon of the second. For example:

Until now we have gotten hydroxyl aldehyde or ketone. To reach our compound; we can get further by dehydration the product i.e. , to do double aldol condensation.
The β-hydroxy aldehydes and β-hydroxy ketones obtained from aldol condensation are very easily dehydrated; the major products have the carbon-carbon double bond between the α- and the β-carbon atoms.
As we can see in our experiment we have acetone and benzaldehyde:

Once you look to this product you could surprise because it's different from what we talk above.
Don't worry, we said before we want to do a double aldol condensation; let's talk about the mechanism to learn how such reaction could proceeds:
Our product is very simple, just we have inserted two benzal into one acetone, so we need 2 equivalent weight of benzaldehyde to do that.
Before that what could happens at first:

You see that the hydroxide ion took the α-hydrogen from acetone to yield a water molecule ( the hydroxide part marked by asterisk " * " to indicate that this is the same hydroxide ion) and to yield the enolate our nucleophile, which is more basic than OH-.
This nucleophile is very attracted by the electrophile ( the carbon of the carbonyl group ) which is the step-2.
Then the enolate ion attached to the carbonyl carbon which is now sp3-hyderdized.
In step-3- the oxygen got the proton from our likely hydroxide.

But there is something confusing here !! why the hydroxide ion took the α-hydrogen rather than attack the carbonyl carbon? Which is the likely "addition" reaction !!.

Carbonyl group provides a site at which nucleophilic addition can take place. But, like the carbon-carbon double bond and the benzene ring, the carbonyl can play another role, not as a functional group, but as a substituent. Now we are ready to learn the other part of the story: how the carbonyl group strengthens the acidity of the α-carbon.

But who is the predominant? The addition or the substitution to form the cabanion?
Actually both reactions are possible at once, but, who is the predominant; that depends on the environment.

To solve this problem let's follow up each kind of them.
First, the addition :

Suppose this reaction is possible, but, let's see the other one:

Let's judge:
Both reaction are reversible and both product could be present (the dihydroxyl product could be consider as impurities, which is a big problem).
The solution to this problem arises from this reversibility, the produced enolate is very reactive                      ( strong nucleophile ) and once it's formed it attacks the carbonyl carbon of the banzaldehyde, by this way the concentration of the enolate ions reduced; results in shifting the equilibrium to the right in order to adjust that change according to Henri Louis Le Châtelier  principle.
While the 2,2-Dihydroxyl propane there isn't any reaction could undergoes, except its hydrolysis to come back into acetone to adjust the previous changing in acetone concentration.
There is question: Why doesn't the enolate ion attack the carbonyl carbon of unionized acetone rather than the benzaldehyde?
This is possible and if you notice I wrote in the first figure 2.2 Eq.Wt. of benzaldehyde, that mean the probability to attack the benzaldehyde is more than 75% than aceton, and because of the bulky nature of the benzaldehyde the enolate will find benzaldehyde more easily than acetone. And because the attack of aldehyde is more reactive than ketone ( aldehyde more reactive than ketone).
The following steps of our reaction are:

Notice: you have to add the acetone to the mixture of benzaldehyde and NaOH not the reverse. To prevent the self-aldol condensation of the the acetone, i.e. prevent acetone enolate attacks the neutral acetone carbonyl, while when we add acetone to the benzaldehyde ( which is more electrophile) the enolate have no chance otherwise to attack the carbonyl of benzaldehyde.
Notice also the C-C double bond is conjugated with the carbonyl group, if you face this type of reaction         ( aldol condensation ) you should know the position of the double bond takes place in the most stable state, in other words, in the conjugated form with the carbonyl group.
1-   How could you modify the experiment in order to make benzalacetone?
Answer: by  using 1 Eq.Wt of benzaldehyde.

2-   What ingredients would you use to synthesize benzalacetonphenone?
Answer: by aldol condensation of benzaldehyde and CH3COC6H5.
Organic Chemistry: Morrison & Boyed.

Written By Mohammad Momer Hakky, Administrator of

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