Flashcards in Organic Chemistry- Aldehydes and Ketones II Deck (60)
What is an alpha-carbon?
It is the carbon adajacent to the carbonyl carbon
What are alpha-hydrogens?
The hydrogens connected to the alpha-carbon
Through induction, what happnes to the alpha-carbon and alpha-hydrogen?
Oxygen pulls some of the electron density out of these C-H bonds, weakening them
The weaker of the C-H bonds, makes what easy?
To deprotonate the alpha-carbon of an aldehyde or ketone
The acidity of alpha-hydrogens is augmented by what? And increase what?
Resonance stabilization of the conjugate base
This increasethe stability of this enolate intermediate
Through _____, the negative charge can be distributed to the more electronegative oxygen atom.
What stabilizes the carbanion?
The electron-withdrawing oxygen atom
What is a carbanion?
A molecule with a negatively charged carbon atom.
In basic solutions, what will easily deprotonate?
The _____ of ketones is less _____ then those of aldehydes. Why?
Alpha-hydrogens of ketons tend to be slightly less acidic than those of aldehydes.
Due to the electron-donating properties of the additional alkyl group in a ketone
In reaction, _____ are slighty more reactive to nucleophiles then ____. WHy?
Aldehydes are slightly more reactive to nucleophiles than ketones.
This is due to steric hinderance in the ketone, which arises from the additional alkyl group ketones contain. When the nucleophile approaches the ketone or aldehyde in order to react, the additional alkyl groups on the ketone are in the way, more so than the single hydrgen of the aldehyde. This makes for a higher-energy, more crowded intermediate step. Remember, higher-energy intermediates mean that the reaction is less likely to proceed
Due to the acidity of the alpha-hydrogen, aldehydes and ketones exist in solution as what?
A mixture of two isomers: The familar keto form, and the enol form.
How does enol get its name?
The presence of a carbon-carbon double bond and an alcohol
What is the difference between the two isomers keto and enol?
They differ in the placement of a proton and the double bond
What are the two isomers keto and enol called?
Where does the equilibirum between the tautomers lie?
Far to the keto side, so there will be many more keto isomers in solution
What is enolization?
The process of interconverting form the keto to the enol tautomer
What is the general term for enolization?
What is a alpha-racemization?
Any aldehyde or ketone with a chiral alpha-carbon will rapidly become a racemic mixture as the keto and enol forms interconvert
What are important intermediates in many reaction of aldehyde and ketones?
The enolate carbanion results from what?
The deprotonation of the alpha-carbon by a strong base
What are common strong bases used to form enol?
The hydroxide ion
Lithium diisopropyl amide (LDA)
Potassium hydride (KH)
What is often used to form enolate carbanions? Why?
A 1,3-dicarbonyl which is particularly acidic because there are two carbonyls to delocalize negative charge
What is another type of keto-enol tautomerization reaction?
What occurs in the michael addition reaction?
The carbanion attacks an alpha,beta-unsaturated carbonyl compound
What is the michael addition?
A molecule with a multiple bond between the alpha- and beta-carbon next to a carbonyl
Why does the michael addition reaction occur?
Due to the resonance stabilization of the intermediates
What can form if a ketone has two different alkyl groups?
Two forms of the enolate can form, with the carbon-carbon double bond between the carbonyl carbon and either the more or less substituted carbon
The equilibrium between the formation of the two forms of enolate is dependent on what?
Kinetic and thermodynamic control of the reaction