Flashcards in Organic Chemistry- Aldehydes and Ketones II Deck (60)
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1
What is an alpha-carbon?
It is the carbon adajacent to the carbonyl carbon
2
What are alpha-hydrogens?
The hydrogens connected to the alpha-carbon
3
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
4
The weaker of the C-H bonds, makes what easy?
To deprotonate the alpha-carbon of an aldehyde or ketone
5
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
6
Through _____, the negative charge can be distributed to the more electronegative oxygen atom.
This resonance
7
What stabilizes the carbanion?
The electron-withdrawing oxygen atom
8
What is a carbanion?
A molecule with a negatively charged carbon atom.
9
In basic solutions, what will easily deprotonate?
Alpha-hydrogens
10
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
11
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
12
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.
13
How does enol get its name?
The presence of a carbon-carbon double bond and an alcohol
14
What is the difference between the two isomers keto and enol?
They differ in the placement of a proton and the double bond
15
What are the two isomers keto and enol called?
Tautomers
16
Where does the equilibirum between the tautomers lie?
Far to the keto side, so there will be many more keto isomers in solution
17
What is enolization?
The process of interconverting form the keto to the enol tautomer
18
What is the general term for enolization?
Tautomerization
19
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
20
What are important intermediates in many reaction of aldehyde and ketones?
Enols
21
The enolate carbanion results from what?
The deprotonation of the alpha-carbon by a strong base
22
What are common strong bases used to form enol?
The hydroxide ion
Lithium diisopropyl amide (LDA)
Potassium hydride (KH)
23
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
24
What is another type of keto-enol tautomerization reaction?
Michael addition
25
What occurs in the michael addition reaction?
The carbanion attacks an alpha,beta-unsaturated carbonyl compound
26
What is the michael addition?
A molecule with a multiple bond between the alpha- and beta-carbon next to a carbonyl
27
Why does the michael addition reaction occur?
Due to the resonance stabilization of the intermediates
28
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
29
The equilibrium between the formation of the two forms of enolate is dependent on what?
Kinetic and thermodynamic control of the reaction
30
