Organic Chemistry- Aldehydes and Ketones II Flashcards
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.
This resonance
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?
Alpha-hydrogens
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?
Tautomers
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?
Tautomerization
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?
Enols
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?
Michael addition
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
The kinetically controlled enolate formation is formed more ____ and less ____.
More Rapidly but less stable
What does the product of the kinetically controlled enolate look like?
The double bond to the less substituted alpha-carbon
How is the kinetically controlled enolate formed?
By the removal of the alpha-hydrogen from the less substiuted alpha-carbon because it offers less steric hinderance
The thermodynamically controlled produce is formed more _____ but is more ______.
More slowly but is more stable
What does the product of the thermodynamically controlled enolate look like?
The double bond being formed with the more substituted alpha-carbon
How is the thermodynamically controlled enolate formed?
By the removal of the alpha-hydrogen from the more substituted alpha-carbon
When are kinetically controlled enolate products favored?
In a reaction that is rapid, irreversible, at lower temperatures and with a strong, sterically hindered base.
When is the thermodynamically controlled enolate product favored?
With higher temperatures, slow, reversible reactions, which have smaller and weaker bases
What happens if a reversible reaction uses kinetically controlled enolate products?
The kinetic product can revert to the origianl reactant and react again to form the thermodynamic product
What are enamines?
Are tautomers of imines
What is an imine?
Is a compound that contains a C=N bond. This nitrogen in the imine may or may not be bonded to an alkyl group or other subtituent
What is tautomerization again?
Movement of a hydrogen and a double bond.
How can imines be converted to enamines?
Through tautomerization
What is another vital reaction in the MCAT?
The aldol condensation
What is Aldol Condensation?
This reaction follows the same general mechanism of nucleophilic addition to a carbonyl, however, an aldehyde or ketone acts both as an electrophile (in its keto forms) and a nucleophile (in its enolate form), and the end result is the formation of a carbon-carbon bond.
When acetaldehyde (ethonal) is treated with ______, an ____ is produced?
Catalytic amount of base
An enolte is produced
Why is the enolate more nucleophilic than the enol?
Because it is negatively charged
In the aldol condensation reaction, the nucleophlic enolate ion does what?
The enolate ion can react with the electrophilic carboyl group of another acetaldehyde molecule
What is key i a aldol condensation reaction?
Both species are in the same flask
What is the product when acetaldehyde (ethonal) goes through an aldol condensation reacction?
3-hydroxybutanol which is an example of an aldol
What is an aldol?
A molecule that contains both aldehyde and alcohol functional groups
When ketones are used are they still called aldol reactions?
Yes
With a strong base and high temperatures, what then occurs in the aldol condenstation reaction?
Dehydration occurs by an E1 or E2 mechanism: We kick of a water molecule and form a double bond, producing an alpha, beta-unsaturated carbonyl
Aldol condensations are most useful only if what are used? Why?
One type of aldehyde or ketone
If there are multiple aldehydes or ketones, we cannot easily control which will act as the nucleophile and which will act as the electrophile, and a mixture of products will result
Multiple products wont form, if what? Why?
If one of the molecules has no alpha-hydrogens because the alpha-carbons are quaternary
Why is aldol condensation called a condensation reaction?
Because Two molecules are jointed with the loss of a small molecule
Can an aldol condensation reaction also be called a dehydration reaction? Why?
Yes because the small molecule that is lost is water
What is the reverse of an aldol condensation reacction called?
Retro-aldo reaction
How do you push the reaction in a retro-aldol direction?
Aqueous base is added and heat is applied
What is the retro-aldol reaction useful for?
Breaking bonds between the alpha dn beta-carbons of a carbonyl
The retro-aldol reaction is facilitated if what?
The intermediate can be stabilized in the enolate form, just as in the forward reaction
