The base-catalyzed keto-enol interconversion of 2-nitrocyclohexanone in ionic liquids.
Guido Angelini1, Paolo De Maria, Cinzia Chiappe
1Dipartimento di Scienze del Farmaco, Università G. d'Annunzio Via dei Vestini 31, Chieti, Italy.
The Journal of Organic Chemistry
|July 30, 2009
Summary
The reaction rate of 2-nitrocyclohexanone tautomerization is faster in ionic liquids (ILs) than predicted by permittivity alone. A comprehensive linear solvation energy relationship (LSER) explains this by considering additional solvent properties, revealing no unique IL effect.
Area of Science:
- Physical Chemistry
- Organic Chemistry
- Solvent Effects
Background:
- The tautomerization of 2-nitrocyclohexanone is a key reaction influenced by solvent properties.
- Previous studies indicated solvent permittivity as the primary factor affecting reaction rates in organic solvents.
Purpose of the Study:
- To investigate the solvent effect on the tautomerization rate of 2-nitrocyclohexanone in organic solvents and ionic liquids (ILs).
- To determine if a unique ionic liquid effect exists or if established solvation models apply.
- To analyze the transition state structure using the Brønsted equation.
Main Methods:
- Kinetic studies of 2-nitrocyclohexanone tautomerization in various organic solvents and ILs.
- Application of linear solvation energy relationships (LSER) incorporating solvent parameters like permittivity, polarizability, H-bond acidity, and cohesive pressure.
- Analysis of kinetic data using the Brønsted equation to probe transition state structure.
Main Results:
- Reaction rates in ILs were faster than predicted by permittivity alone.
- A comprehensive LSER model, including polarizability, H-bond acidity, and cohesive pressure, successfully described reaction rates in both organic solvents and ILs.
- No special 'ionic liquid effect' was identified when considering multiple solvent parameters.
- Brønsted beta values approached 1 in ILs, indicating a shift towards an enolate-like transition state structure.
Conclusions:
- The solvent effect on 2-nitrocyclohexanone tautomerization can be comprehensively described by LSER, integrating multiple solvent properties.
- Ionic liquids do not exhibit a unique effect on this reaction beyond their combined physical-chemical properties.
- The transition state for enolization in ILs becomes more enolate-like compared to molecular solvents.
Related Concept Videos
01:14Keto–Enol Tautomerism: Mechanism
The keto and enol forms are known as tautomers and they constantly interconvert (or tautomerize) between the two forms under acid or base catalyzed conditions. Both the reactions involve the same steps—protonation and deprotonation— although in the reverse order.
01:19Nitrosation of Enols
The nitrosation reaction is one of the methods of preparing 1,2-diketones. The enol tautomer of the starting ketone reacts with sodium nitrite in hydrochloric acid, generating the 1,2-diketone after hydrolysis.
01:10α-Alkylation of Ketones via Enolate Ions
Ketones with α protons are deprotonated by strong bases like lithium diisopropylamide (LDA) to form enolate ions. The anion is stabilized by resonance, and its hybrid structure exhibits negative charges on the carbonyl oxygen and the α carbon. This ambident nucleophile can attack an electrophile via two possible sites: the carbonyl oxygen, known as O-attack, or the α carbon, known as C-attack. The nucleophilic attack via the carbanionic site is preferred. This is due to the strong interaction...
01:07Alkylation of β-Ketoester Enolates: Acetoacetic Ester Synthesis
Acetoacetic ester synthesis is a method to obtain ketones from alkyl halides and β-keto esters. The reaction occurs in the presence of an alkoxide base that abstracts the acidic proton of the β-keto esters. The step results in an enolate ion which is doubly stabilized. The enolate then reacts with an alkyl halide via the SN2 process to produce an alkylated ester intermediate with a new C–C bond. The hydrolysis of the intermediate, followed by acidification, results in an alkylated β-keto acid.
01:18Reactivity of Enols
Enols are a class of compounds where a hydroxyl group is attached to a carbon–carbon double bond, which implies that it is a vinyl alcohol. A carbonyl compound with an α hydrogen undergoes keto–enol tautomerism and remains in equilibrium with its tautomer, the enol form. Usually, the keto tautomer is present in a higher concentration than the enol tautomer due to the higher bond energy of C=O compared to C=C. Moreover, the direction of the keto–enol equilibrium is governed by factors like...
01:19Types of Enols and Enolates
Aldehydes and ketones form enols, although only about 1% of the enol is present at the equilibrium for simple monocarbonyl compounds. The enol form is undetectable for acetaldehyde, present as only 1.5 × 10−4 % of acetone, and present as only 1.2% of cyclohexanone. Two kinds of regioisomeric enols are possible for unsymmetrical ketones, and their net composition is 1% at equilibrium. This instability is due to the lower bond energy of C=C than the C=O group. The additional instability of enols...