how the isovaleraldehyde easily give aldol condensation under basic condition
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Aldol condensations are important in organic synthesis, because they provide a good way to form carbon–carbon bonds.[1] For example, the Robinson annulation reaction sequence features an aldol condensation;[1] the Wieland–Miescher ketone product is an important starting material for many organic syntheses.[2] Aldol condensations are also commonly discussed in university level organic chemistry classes as a good bond-forming reaction that demonstrates important reaction mechanisms.[3][4][5] 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.[6][7][8]
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 or ketone having an alpha-hydrogen with an aromatic carbonyl compound lacking an alpha hydrogen 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.[9][10][11][page needed] 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.[12] Because the enolizeable nucleophilic carbonyl compound and the electrophilic carbonyl compound are two different chemicals, the Claisen–Schmidt reaction is an example of a crossed aldol process.
Mechanism
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,[13][14] or in an acid-catalyzed enol mechanism. Depending on the nature of the desired product, the aldol condensation may be carried out under two broad types of conditions: kinetic control or thermodynamic control.[15]
:
animation, base catalyzed
animation, acid catalyzed
Condensation types
It is important to distinguish the aldol condensation from other addition reactions of carbonyl compounds.
When the base is an amine and the active hydrogen compound is sufficiently activated the reaction is called a Knoevenagel condensation.
In a Perkin reaction the aldehyde is aromatic and the enolate generated from an anhydride.
A Claisen condensation involves two ester compounds.
A Dieckmann condensation involves two ester groups in the same molecule and yields a cyclic molecule
A Henry reaction involves an aldehyde and an aliphatic nitro compound.
A Robinson annulation involves a α,β-unsaturated ketone and a carbonyl group, which first engage in a Michael reaction prior to the aldol condensation.
In the Guerbet reaction, an aldehyde, formed in situ from an alcohol, self-condenses to the dimerized alcohol.
In the Japp–Maitland condensation water is removed not by an elimination reaction but by a nucleophilic displacement
Aldox process
In industry the Aldox process developed by Royal Dutch Shell and Exxon, converts propylene and syngas directly to 2-ethylhexanol via hydroformylation to butyraldehyde, aldol condensation to 2-ethylhexenal and finally hydrogenation.[16]
In one study crotonaldehyde is directly converted to 2-ethylhexanal in a palladium / Amberlyst / supercritical carbon dioxide system:[17]
Scope
Ethyl 2-methylacetoacetate and campholenic aldehyde react in an Aldol condensation.[18] The
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 or ketone having an alpha-hydrogen with an aromatic carbonyl compound lacking an alpha hydrogen 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.[9][10][11][page needed] 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.[12] Because the enolizeable nucleophilic carbonyl compound and the electrophilic carbonyl compound are two different chemicals, the Claisen–Schmidt reaction is an example of a crossed aldol process.
Mechanism
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,[13][14] or in an acid-catalyzed enol mechanism. Depending on the nature of the desired product, the aldol condensation may be carried out under two broad types of conditions: kinetic control or thermodynamic control.[15]
:
animation, base catalyzed
animation, acid catalyzed
Condensation types
It is important to distinguish the aldol condensation from other addition reactions of carbonyl compounds.
When the base is an amine and the active hydrogen compound is sufficiently activated the reaction is called a Knoevenagel condensation.
In a Perkin reaction the aldehyde is aromatic and the enolate generated from an anhydride.
A Claisen condensation involves two ester compounds.
A Dieckmann condensation involves two ester groups in the same molecule and yields a cyclic molecule
A Henry reaction involves an aldehyde and an aliphatic nitro compound.
A Robinson annulation involves a α,β-unsaturated ketone and a carbonyl group, which first engage in a Michael reaction prior to the aldol condensation.
In the Guerbet reaction, an aldehyde, formed in situ from an alcohol, self-condenses to the dimerized alcohol.
In the Japp–Maitland condensation water is removed not by an elimination reaction but by a nucleophilic displacement
Aldox process
In industry the Aldox process developed by Royal Dutch Shell and Exxon, converts propylene and syngas directly to 2-ethylhexanol via hydroformylation to butyraldehyde, aldol condensation to 2-ethylhexenal and finally hydrogenation.[16]
In one study crotonaldehyde is directly converted to 2-ethylhexanal in a palladium / Amberlyst / supercritical carbon dioxide system:[17]
Scope
Ethyl 2-methylacetoacetate and campholenic aldehyde react in an Aldol condensation.[18] The
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