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Aldol condensations are important in organic synthesis, because they provide a good way to form carbon–carbon bonds. Sophos ap100c. In its usual form, it involves the nucleophilic addition of a ketone enolate to an aldehyde to form a β-hydroxy ketone, or “aldol” (aldehyde + alcohol), a structural unit found in many naturally occurring molecules and pharmaceuticals.
The name aldol condensation is also commonly used, especially in biochemistry, to refer to just the first (addition) stage of the process—the aldol reaction itself—as catalyzed by aldolases. However, the aldol reaction is not formally a condensation reaction because it does not involve the loss of a small molecule.
The reaction between an aldehyde/ketone and an aromatic carbonyl compound lacking an alpha-hydrogen (cross aldol condensation) is called the Claisen-Schmidt condensation. This reaction is named after two of its pioneering investigators Rainer Ludwig Claisen and J. G. Schmidt, who independently published on this topic in 1880 and 1881. An example is the synthesis of dibenzylideneacetone. Quantitative yields in Claisen-Schmidt reactions have been reported in the absence of solvent using sodium hydroxide as the base and plus benzaldehydes.
Mechanism
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The first part of this reaction is an aldol reaction, the second part a dehydration—an elimination reaction (Involves removal of a water molecule or an alcohol molecule). Dehydration may be accompanied by decarboxylation when an activated carboxyl group is present. The aldol addition product can be dehydrated via two mechanisms; a strong base like potassium t-butoxide, potassium hydroxide or sodium hydride in an enolate mechanism, or in an acid-catalyzed enol mechanism. We will focus on the base-catalyzed mechanism, which is more widely used.
Enolate mechanism
If the catalyst is a moderate base such as hydroxide ion or an alkoxide, the aldol reaction occurs via nucleophilic attack by the resonance-stabilized enolate on the carbonyl group of another molecule. The product is the alkoxide salt of the aldol product. The aldol itself is then formed, and it may then undergo dehydration to give the unsaturated carbonyl compound. The scheme shows a simple mechanism for the base-catalyzed aldol reaction of an aldehyde with itself.
Base-catalyzed aldol reaction (shown using −OCH3 as base)
Base-catalyzed dehydration
Although only a catalytic amount of base is required in some cases, the more usual procedure is to use a stoichiometric amount of a strong base such as LDA. In this case, enolate formation is irreversible, and the aldol product is not formed until the alkoxide of the aldol product is protonated in a separate acid-base workup step. Mixtures of stereoisomers (E & Z) are obtained from some reactions, though the E product is generally favored. Overall the general reaction involves a dehydration of an aldol product to form an alkene:
What is an alpha, beta-unsaturated ketone?
1 Answer
It looks something like this:
The #alpha# -carbon is the one adjacent to (on the right of) the carbonyl carbon. The #beta# -carbon is the very bottom-right carbon. Hence, #mathbf(alpha,beta)# -unsaturated ketones have a double bond across the#mathbf('C'_alpha-'C'_beta)# connection.
You would see these in second-semester organic chemistry courses, and they often react on the #beta# -carbon due to the electron-withdrawing behavior of oxygen in these contexts (the #beta# -carbon becomes electropositive).
An interesting example called the Robinson Annulation is shown below, where an #alpha,beta# -unsaturated ketone reacts with another ketone in base with some heat applied.
If you have heard of it before, then you should know that the steps are as follows:
- Formation of an enolate using a base such as sodium hydroxide to steal a proton at the
#alpha# -carbon. - Nucleophilic attack of the
#alpha,beta# -unsaturated ketone at the#mathbf(beta)# -carbon, which is the common target for a nucleophile that is of the strength of a typical enolate. - The enolate takes a proton from the water that just formed from the hydroxide instead of attacking the carbonyl carbon because it cannot make any ring (a four-membered ring is too unstable).
- The formation of another enolate occurs---this time, five carbons away from the leftmost carbonyl carbon, so a ring can be made (thus giving the sixth bond in a six-membered ring).
- A ring is made via an intramolecular enolate nucleophilic attack.
- This step is to show how the bond is connected. It is not actually realistically stable. :P
- The water that just formed from the hydroxide that was used in step 4 donates a proton to the oxide intermediate.
- Yet another enolate formation occurs. This time, facilitated by heat.
- There is heat applied, so that, in addition to how the atoms are aligned, are such that the hydroxide can be eliminated from the product.
We have formed our product! :D As you can see, this is a ring formation reaction. It makes not one (like the acetylene synthesis), but TWO #'C'-'C'# bonds! NICE!
Alpha Carbon Definition
SIDENOTE: Steps 1-3 are called the Michael reaction. Steps 4-7 are an intramolecular aldol addition. Steps 8-9 form another#alpha,beta# -unsaturated ketone, from a #beta# -hydroxyketone. Steps 4-9 are collectively called an intramolecular aldol condensation.
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