[Benzene21]

I need the mechanism for this reaction:

[Jorriss]

I need the mechanism for this reaction:

Have you tried anything?

Keep in the mind what the acidity of the hydrogen on the alpha carbon is and the strength of the given nucleophile/base.

This mechanism is called The Claisen Condensation, if you get stuck again you can look it up.

[Benzene21]

Yeah, I've tried. I'm not familiar with reactions of esters. I thought the base was going to attack the carbonyl, but then a friend told me something about the alpha carbon, and now I'm not sure, that's why I'm asking. I read my textbook but there's was nothing about the base/nucleophile attacking the alpha carbon. All the examples had the base/nucleophile attacking the carbonyl. This is what I've got:

The alpha carbon losses a proton and forms a carbanion. The carbanion then attacks another molecule at the carbonyl. There's an oxygen with a negative charge, but it forms the double bond with the carbon again as the ether leaves, right?

Why does the base/nucleophile attack the alpha carbon instead of the carbonyl? I've seen examples and each one has the base/nucleophile attacking the carbonyl.

[nj_bartel]

The base does potentially react with the carbonyl carbon in a substitution reaction.  This is why you use a base (OCH3-) that is the same as what is displaced in the subsitution reaction (OCH3-).

[Jorriss]

Yeah, I've tried. I'm not familiar with reactions of esters. I thought the base was going to attack the carbonyl, but then a friend told me something about the alpha carbon, and now I'm not sure, that's why I'm asking. I read my textbook but there's was nothing about the base/nucleophile attacking the alpha carbon. All the examples had the base/nucleophile attacking the carbonyl. This is what I've got:

The alpha carbon losses a proton and forms a carbanion. The carbanion then attacks another molecule at the carbonyl. There's an oxygen with a negative charge, but it forms the double bond with the carbon again as the ether leaves, right?

Why does the base/nucleophile attack the alpha carbon instead of the carbonyl? I've seen examples and each one has the base/nucleophile attacking the carbonyl.

NJ's, the above poster, got it right. By using methoxide you are using the same nucleophile/base that would potentially be kicked off when the carbonyl reforms. So it likely does attack the carbonyl, it just leads to nothing. So occasionally instead it acts as a base and peels off a hydrogen which does lead to a new product.

If you used a different nucleophile, the mechanism would be different.

[Rishi]

The Claisen condensation has one carbony group attacking the active methylene group (carbon adjacent to the carbonyl carbon) and thus forming the product. The methoxide is a catalyst that initiates the reaction by extracting the proton and forming a carbanion.

Could put the mechanism in the schematic form if only I knew how to put it here.