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Summary Sheet 2: Enols and Enolates

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Summary Sheet 2: Enols and Enolates Summary Sheet 2: Enols and Enolates Name Structure Example pKA* of H Structural Features of the Carbonyl Group: Effects on acidity of alkyl groups Effect on reactivity of alkenes: 2 O O •Carbon, oxygen: sp hybridized Likewise, the presence of a carbonyl group activates alkenes toward The carbonyl is an electron withdrawing system with aldehyde 17 •O–C–C bond angle ~120 ° π nucleophilic attack: low-lying π* orbitals. It stabilizes adjacent negative charge. R H H3C H •C=O bond strongly polarized O – δ toward oxygen. M H3C CH3 M H H H O O •Carbonyl carbon is partially positive H2C CH3 LiNEt2 CH X CH3 δ+ 2 does not form ketone 20 therefore electrophilic! Ethane R R' H C Me Conjugate base NEt2 3 •Lone pairs render oxygen weakly pKa = ~50 H H NEt2 nucleophilic (will react with strong acid) highly unstable O O OMe OMe ester 25 O O O Li O O R OR H C OMe Key Concept: Tautomerism CH3 CH3 3 H t-Bu H t-Bu t-Bu M O LiNEt2 O H X O O O O OH Methyl propionate Conjugate base (enolate) lactone pKa = ~25 ~1025 more acidic just by H H Et2N Et2N O H C O H H (cyclic 2 C R R replacing H with a carbonyl! ( ) H enolate: stabilized! forms readily! ester) n 2 H Why the huge difference in acidity? The lone pair is keto form enol form stabilized by donation into the carbonyl π system. The reactivity of the alkene toward nucleophilic attack is directly related O to the stability of the enolate that forms - O Tautomerism: a form of isomerism where OMe OMe amide 30 a keto converts to an enol through the movement of O O R NR CH3 CH3 O O 2 H2C NMe2 a proton and shifting of bonding electrons O O O O M M NH2 = 1° amide For acetone (R=CH ) the keto:enol ratio is ~6600:1 at 23 > > > > 3 NR2 OR Me CF RO OR NHR = 2 ° amide °C. Main reason is the difference in bond strengths between Enolate Enolate 3 (Carbanion resonance form) (oxy-anion resonance form) NR1R2 = 3 ° amide the two species. O O The enol tautomer is most significant for ketones and Question: How do you explain the relative acidity of the increasing reactivity toward nucleophilic attack following series? lactam NH(R) Me aldehydes. (You may also encounter it with acid chlorides in increasing stability of enolate H2C N O (cyclic O O O ( )n the mechanism of the Hell-Vollhard-Zolinsky reaction). Esters Can predict the course of the reaction by pKa! amide) and amides are less acidic and exist almost exclusively as the > > > H3C CF3 H3C CH3 H3C OR H3C NR2 O O keto form (e.g. >106 : 1 keto: enol for ethyl acetate) carboxylic Note 1 Acetone in D O will slowly incorporate deuterium at the acid R OH H C OH 2 α− most acidic least acidic Key Reaction: Enolate Formation 3 carbon. The enol form is responsible for this behavior. Enolate = deprotonated enol The rate of keto/enol tautomerism is greatly increased by Answer: The more electron-poor the carbonyl, the greater will O O acid (see below right) be its ability to stabilize negative charge. Conversely, the greater the donating ability of a substitutent on the carbonyl, i-Pr i-Pr O Li Li O acid O N * Note 2 the less it will be able to stabilize negative charge. chloride R Cl H3C Cl Five factors that influence the relative Li (LDA) H H H C OEt OEt OEt proportion of keto/enol: The aromatic electrophilic substitution chart is a good proxy 3 (or other O O O O Note 2 1. Aromaticity for the ability of a functional group to donate to a carbonyl: H H strong base) ester enolate anhydride O OH C-bound form * NR , O > OH > OR, NHAc > CH , R > Cl, Br, F, I > C(O)OR Carbonyl compound O-bound form R O R H3C O Me 2 3 CF3, etc. Substitution of the α-carbon by a second carbonyl 25 Important: the Enolate is a NUCLEOPHILE derivative makes the α-proton even more acidic: nitrile R–CN Me-CN Amphiphilic =nucleophilic at both O and C; note: though an ester enolate Not observed Exclusive O O O O O O here we focus on the reactions at C. is shown here, the reaction of Note 1: The carboxylic acid is deprotonated any enolate with an aldehyde 2. Hydrogen bonding stabilizes the enol form. first. Subsequent deprotonation of the α-carbon Two key examples: is generally called an "Aldol". Me Me Me OMe MeO OMe would form a dianion, which has a high H-bond O activation barrier due to charge repulsion. It O M OH O H pKa = 9 pKa = 11 pKa = 13 can be done, but it requires a very strong base. O O O O R H Aldol reaction OEt R OEt Note 2: It is difficult to measure the pKa of these species due to their reactivity. Me Me Me Me O 24 : 76 (at 23 °C) The rate of keto/enol interconversion isgreatly enhanced by acid: O M OH O Claisen Condensation 3. Strongly hydrogen bonding solvents can disrupt Acid makes carbonyl more electrophilic, increasing acidity of α- R OR *source: P. Y. Bruice, "Organic Chemistry" this, however. The above equilibrium is 81:19 protons, facilitating formation of enol: this increases K1 OEt R OEt using water as solvent. Note that some of of these values differ slightly from H 4. Conjugation is stabilizing. O O H O textbook to textbook and instructor to instructor. H X K1 H X However, there is universal agreement that for R R R The goal here is clarity and consistency. I O OH R R R a given structure, pKas increase in the order K would greatly appreciate feedback on any aldehyde < ketone < ester < amide H H H H 2 H errors, omissions, or suggestions for Me Me Keto Enol improvements. Thanks! If you have a vastlyl different set of values I Preferred enol form X: Enol is nucleophilic at α-carbon, acid is would appreciate it if you brought it to my attention. 5. As with alkenes, increasing substitution increases version 1.0, 4/13/10 thermodynamic stability (assuming equal steric factors) excellent electrophile: this increases K2 copyright James Ashenhurst, 2010. For a comprehensive list of pKa values see: suggestions/questions/comments? 1)evans.harvard.edu/pdf/evans_pKa_table.pdf Net result: Addition of acid speeds proton [email protected] 2) www.chem.wisc.edu/areas/reich/pkatable/ exchange between the keto and enol forms. masterorganicchemistry.wordpress.com.
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