Introduction to Ambident Enolate Anions
Because the negative charge of an enolate anion is delocalized over the alpha-carbon and the oxygen, as displayed previous, electrophiles might bond to either atom. Ambident are the reactants that having two or more reactive sites, so this word is correctly applied to enolate anions. Modestly electrophilic reactants like alkyl halides are not adequately reactive to combine with neutral enol tautomers, but the increased nucleophilicity of enolate anion conjugate base allows such type of reactions to occur. Since alkylations are generally irreversible, their results should reflect the inherent (kinetic) reactivity of the distinct nucleophilic sites.
If an alkyl halide goes through an SN2 reaction at the carbon atom of an enolate anion the result is an alkylated aldehyde or ketone. Alternatively, if the SN2 reaction takes place at oxygen the result is an ether derivative of the enol tautomer; such type of compounds are stable in the nonexistence of acid and may be isolated and characterized. These alkylations that are displayed above are irreversible under the circumstances generally employed for SN2 reactions; therefore the result composition should give a measure of the comparative rates of substitution at carbon versus oxygen. It has been establish that this competition is sensitive to a number of factors, that including negative charge density, solvation, product stability and cation coordination. For alkylation reactions of enolate anions to be helpful, these intermediates might be generated in high concentration in the nonexistence of other strong bases and nucleophiles. The aqueous base circumstances employed for the aldol condensation are not appropriate because the enolate anions of simple carbonyl compounds are created in very low concentration, and alkoxide or hydroxide bases induce competing SN2 and E2 reactions of alkyl halides. So, It is essential, to get complete conversion of aldehyde or ketone reactants to their enolate conjugate bases by treatment with a very strong base (pKa > 25) in a non-hydroxylic solvent before any alkyl halides are added to the reaction system. A number of bases having pKa's greater than 30 were explained previous, and some others that have been employed for enolate anion formation are: NaH (sodium hydride, pKa > 45), NaNH2 (sodium amide, pKa = 34), and (C6H5)3CNa (trityl sodium, pKa = 32). Ether solvents such as tetrahydrofuran (THF) are generally employed for enolate anion formation. With exception of the sodium hydride and sodium amide, several of these bases are soluble in THF. Certain other strong bases, like alkyl lithium and Grignard reagents, cannot be employed to make use of the enolate anions because they quickly and irreversibly add to carbonyl groups. Though, these very strong bases are helpful in making soluble amide bases. In the preparation of the lithium diisopropylamide (LDA), for instance, the only other result is the gaseous alkane butane.
Due to its solubility in THF, LDA is a extensively used base for enolate anion formation. In this application one corresponding of diisopropylamine is formed along with the lithium enolate, but this generally does not interfere with the enolate reactions and is simply removed from the results by washing with aqueous acid. Even though the reaction of carbonyl compounds with sodium hydride is slow and heterogeneous, sodium enolates are created with the loss of hydrogen, and no other organic compounds are formed. The following equation provides instances of electrophilic substitution at both carbon and oxygen for the enolate anion derived from cyclohexanone.
A full analysis of the issues that direct substitution of enolate anions to carbon or oxygen is out of the scope of this text. Though, an outline of some important features that effect the two reactions displayed above is illustrative.
Reactant
Important Factors
CH3-I
The negative charge density is greatest at the oxygen atom (greater electronegativity), and coordination with the sodium cation is stronger there. Because methyl iodide is only a modest electrophile, the SN2 transition state resembles the products more than the reactants. Since the C-alkylation product is thermodynamically more stable than the O-alkylated enol ether, this is reflected in the transition state energies.
(CH3)3Si-Cl
Trimethylsilyl chloride is a stronger electrophile than methyl iodide (note the electronegativity difference between silicon and chlorine). Relative to the methylation reaction, the SN2 transition state will resemble the reactants more than the products. Consequently, reaction at the site of greatest negative charge (oxygen) will be favored. Also, the high Si-O bond energy (over 25 kcal/mole greater than Si-C) thermodynamically favors the silyl enol ether product.
Qualified and Experienced Organic Chemistry Online Tutors at www.tutorsglobe.com
Tutors at the www.tutorsglobe.com take pledge to provide full satisfaction and assurance in Ambident Enolate Anions homework help via online tutoring. Students are getting Chemistry homework help services across the globe with 100% satisfaction by online tutors. We value all our service-users. We provide email based Ambident Enolate Anions homework help - assignment help. You can join us to ask queries 24x7 with live, experienced and qualified Chemistry online tutors specialized in Ambident Enolate Anions. Through Online Tutoring, you would be able to complete your homework or assignments at your home.
Latest technology based Organic Chemistry Online Tutoring Assistance
Tutors at the www.tutorsglobe.com are committed to provide the best quality online tutoring assistance for homework help and assignment help services. They use their experience, as they have solved thousands of the Ambident Enolate Anions assignments, which may help you to solve your complex Ambident Enolate Anions homework. You can find solutions for all the topics come under the Ambident Enolate Anions. The dedicated tutors provide eminence work on your Organic Chemistry homework help and devoted to provide K-12 level Chemistry to college level chemistry help before the deadline mentioned by the student. Ambident Enolate Anions homework help is available here for the students of school, college and university. TutorsGlobe assure for the best quality compliance to your homework. Compromise with quality is not in our dictionary. If we feel that we are not able to provide the homework help as per the deadline or given instruction by the student, we refund the money of the student without any delay.
Class C Amplifiers tutorial all along with the key concepts of Modes of class c amplifier, Output characteristics of Class C power amplifier, Class C power amplifier circuit diagram
Theory and lecture notes of Nash Equilibrium all along with the key concepts of nash equilibrium, Battle of the Sexes, Mixed Strategies, market structure. Tutorsglobe offers homework help, assignment help and tutor’s assistance on nash equilibrium.
a flow chart is pictorial representation of an algorithm. it gives an easy and clear understanding of an algorithm. the understanding of an algorithm is made easy by flow charts, as compared to textual representation of an algorithm.
there are two kinds of dc armature windings that are the lap and wave windings. for development of dc armature windings, a number of pitches related to the types of dc armature windings are back pitch, front pitch and winding pitch.
the optical fibre elements are generally individually coated with plastic layers and consisted in a protective tube appropriate for the environment in which the cable will be deployed.
www.tutorsglobe.com offers Collection and Classification of Overheads homework help, assignment help, case study, writing homework help, online tutoring assistance by accounting tutors.
We encompass an optimum panel of tutors offering Human Anatomy Assignment Help, which surely obtain you top grades.
Angular Momentum tutorial all along with the key concepts of Common Theory and Detailed Origin, Ls coupling, JJ Coupling, Term symbols, Nuclear coupling, Review of Quantum Numbers
solutions and phase equilibrium tutorial all along with the key concepts of pure substances and solutions, differentiating between pure substances and solutions, phase equilibrium and definition of phase
Interference by division of Amplitude tutorial all along with the key concepts of Stroke's analysis of phase change, Interference in thin films, Interference by a wedge-shaped film and Newton's Ring
biology tutorial tutorial all along with the key concepts of the order mononegavirales, viriods, satellites, viral replication, absorption/attachment, uncoating, transcription, synthesis of viral components, assemblage/morphogenesis and mode of transmission viruses and diagnosis of viral infections
alkenes tutorial all along with the key concepts of classification of alkenes, structure of monoenes and dienes, physical properties, spectral properties, preparation of alkenes, dehydrohalogenation of alkyl halides, dehydration of alcohols, wittig reaction
Need authentic World Regional Geography Assignment Help to gain maximum scores? Hire best professional tutors and score well!
Benzopyridines tutorial all along with the key concepts of Quinoline, Skraup Synthesis, Friedlander Synthesis, Methylquinolines, Aminoquinolines, Conrad-Limpach-Knorr Synthesis, Isoquinoline, Bischler-Napieralski Synthesis and Pictet-Spengler Synthesis
Aldehydes and Ketones tutorial all along with the key concepts of Structures of Aldehyde and Ketones, IUPAC Nomenclature of Aldehydes and Ketones, Bonding and reactivity, Properties of aldehydes and ketones
1957474
Questions Asked
3689
Tutors
1487811
Questions Answered
Start Excelling in your courses, Ask an Expert and get answers for your homework and assignments!!