Unit 20: Enols and Enolates: Alpha-Carbon Chemistry
The carbon next to a carbonyl leads a double life: acidic enough to deprotonate, and nucleophilic enough to build a whole new carbon-carbon bond.
8 topics · 30 questions
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Keto-Enol Tautomerism
- How it worksHow does a proton move from the alpha carbon to the oxygen while a π bond shifts from carbon-oxygen to carbon-carbon, and back?
- Worked exampleFind the enol tautomer of acetone and identify which atoms actually moved to form it.
- Common mistakeWhy aren't tautomers the same thing as resonance structures, even though both show two versions of one molecule?
- Going furtherWhat happens to the keto-enol equilibrium position when the enol form is unusually stabilized, as in a 1,3-diketone?
Your turn1 of 2
A carbonyl compound's enol and keto tautomers are related by an equilibrium constant K = [enol]/[keto] = 3.13. What percent of the compound exists as the enol tautomer at equilibrium?Enolate Formation and Alpha-Carbon Acidity
- How it worksHow does removing an alpha proton generate a carbanion that resonance spreads out onto the carbonyl oxygen as an enolate?
- Worked exampleFind the pKa range of an alpha proton on a simple ketone and explain why it sits so far below a typical alkane C-H.
- Common mistakeWhy isn't an enolate just a carbanion with its negative charge stuck permanently on the carbon?
- Going furtherWhat happens to alpha-proton acidity when a second carbonyl group flanks the same alpha carbon, as in a 1,3-diketone?
Your turn1 of 2
What is the approximate pKa range of an alpha proton on a simple ketone, and why does it sit so far below a typical alkane C-H (pKa around 50)?Alpha-Halogenation of Ketones
- How it worksHow does the enol form of a ketone attack bromine with its electron-rich double bond to install a halogen at the alpha carbon?
- Worked exampleFind the product when acetone reacts with bromine under acidic conditions, halogenating its alpha carbon.
- Going furtherWhat happens to a methyl ketone when it is treated with excess halogen and base in a haloform reaction?
Your turn1 of 2
Acetone reacts with bromine under acidic conditions, halogenating its alpha carbon. What is the product?The Aldol Addition and Condensation
- How it worksHow does an enolate's carbon attack the carbonyl carbon of a second aldehyde molecule to form a beta-hydroxy carbonyl?
- How it worksHow does that beta-hydroxy aldol product lose water under heating to form a conjugated enone?
- Worked exampleFind the aldol addition product when acetaldehyde is treated with dilute sodium hydroxide.
- Common mistakeWhy doesn't every aldol addition automatically dehydrate into the condensation product without extra heat or acid?
- Going furtherWhat happens to the aldol product's structure when the starting carbonyl is a ketone instead of an aldehyde?
Your turn1 of 2
Acetaldehyde is treated with dilute sodium hydroxide. What is the aldol addition product?Crossed and Directed Aldol Reactions
- How it worksHow does using a carbonyl with no alpha hydrogens, like benzaldehyde, force a crossed aldol to give just one clean product?
- Worked exampleFind the crossed aldol condensation product when benzaldehyde reacts with acetone under basic conditions.
- Common mistakeWhy can't two ordinary ketones, each with their own alpha hydrogens, be combined in a crossed aldol for one clean product?
Your turn1 of 2
Benzaldehyde reacts with acetone under basic conditions in a crossed aldol condensation. What is the product?The Claisen Condensation
- How it worksHow does the tetrahedral intermediate from an enolate attacking an ester collapse by ejecting an alkoxide leaving group?
- Worked exampleFind the beta-ketoester product formed when ethyl acetate undergoes a Claisen condensation with sodium ethoxide.
- Common mistakeWhy doesn't a Claisen condensation stop at the same beta-hydroxy stage an aldol addition does?
- Ties togetherHow does the Claisen condensation reuse the exact enolate-attacks-carbonyl step from the aldol, just aimed at an ester?
Your turn1 of 2
Ethyl acetate undergoes a Claisen condensation with sodium ethoxide. What is the beta-ketoester product?The Michael Addition
- How it worksHow does conjugation let a nucleophile attack the beta carbon of an enone, with the negative charge landing on oxygen through the π system?
- Worked exampleFind the Michael addition product when the enolate of diethyl malonate attacks methyl vinyl ketone.
- Going furtherWhat happens to a synthesis when a Michael addition is followed by an intramolecular aldol condensation in a Robinson annulation?
Your turn1 of 2
The enolate of diethyl malonate attacks methyl vinyl ketone in a Michael addition. What is the product?Malonic Ester and Acetoacetic Ester Synthesis
- How it worksHow does alkylating diethyl malonate's acidic alpha carbon, then hydrolyzing and heating to lose CO2, install a new alkyl chain?
- Worked exampleFind the final carboxylic acid product when diethyl malonate is alkylated with 1-bromopropane, then hydrolyzed and decarboxylated.
- Common mistakeWhy does a beta-ketoester or malonic ester lose carbon dioxide so easily on heating when an ordinary carboxylic acid does not?
- Ties togetherHow does malonic ester synthesis let a chemist build the exact substituted acetic acid a plain alkylation could never make cleanly?
Your turn1 of 2
Diethyl malonate is alkylated with 1-bromopropane, then hydrolyzed and decarboxylated. What is the final carboxylic acid product?