Organic Chemistry- Aldehydes and Ketones II

Question 1

What is an alpha-carbon?

Answer 1

It is the carbon adajacent to the carbonyl carbon

Question 2

What are alpha-hydrogens?

Answer 2

The hydrogens connected to the alpha-carbon

Question 3

Through induction, what happnes to the alpha-carbon and alpha-hydrogen?

Answer 3

Oxygen pulls some of the electron density out of these C-H bonds, weakening them

Question 4

The weaker of the C-H bonds, makes what easy?

Answer 4

To deprotonate the alpha-carbon of an aldehyde or ketone

Question 5

The acidity of alpha-hydrogens is augmented by what? And increase what?

Answer 5

Resonance stabilization of the conjugate base

This increasethe stability of this enolate intermediate

Question 6

Through _____, the negative charge can be distributed to the more electronegative oxygen atom.

Answer 6

This resonance

Question 7

What stabilizes the carbanion?

Answer 7

The electron-withdrawing oxygen atom

Question 8

What is a carbanion?

Answer 8

A molecule with a negatively charged carbon atom.

Question 9

In basic solutions, what will easily deprotonate?

Answer 9

Alpha-hydrogens

Question 10

The _____ of ketones is less _____ then those of aldehydes. Why?

Answer 10

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

Question 11

In reaction, _____ are slighty more reactive to nucleophiles then ____. WHy?

Answer 11

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

Question 12

Due to the acidity of the alpha-hydrogen, aldehydes and ketones exist in solution as what?

Answer 12

A mixture of two isomers: The familar keto form, and the enol form.

Question 13

How does enol get its name?

Answer 13

The presence of a carbon-carbon double bond and an alcohol

Question 14

What is the difference between the two isomers keto and enol?

Answer 14

They differ in the placement of a proton and the double bond

Question 15

What are the two isomers keto and enol called?

Answer 15

Tautomers

Question 16

Where does the equilibirum between the tautomers lie?

Answer 16

Far to the keto side, so there will be many more keto isomers in solution

Question 17

What is enolization?

Answer 17

The process of interconverting form the keto to the enol tautomer

Question 18

What is the general term for enolization?

Answer 18

Tautomerization

Question 19

What is a alpha-racemization?

Answer 19

Any aldehyde or ketone with a chiral alpha-carbon will rapidly become a racemic mixture as the keto and enol forms interconvert

Question 20

What are important intermediates in many reaction of aldehyde and ketones?

Answer 20

Enols

Question 21

The enolate carbanion results from what?

Answer 21

The deprotonation of the alpha-carbon by a strong base

Question 22

What are common strong bases used to form enol?

Answer 22

The hydroxide ion
Lithium diisopropyl amide (LDA)
Potassium hydride (KH)

Question 23

What is often used to form enolate carbanions? Why?

Answer 23

A 1,3-dicarbonyl which is particularly acidic because there are two carbonyls to delocalize negative charge

Question 24

What is another type of keto-enol tautomerization reaction?

Answer 24

Michael addition

Question 25

What occurs in the michael addition reaction?

Answer 25

The carbanion attacks an alpha,beta-unsaturated carbonyl compound

Question 26

What is the michael addition?

Answer 26

A molecule with a multiple bond between the alpha- and beta-carbon next to a carbonyl

Question 27

Why does the michael addition reaction occur?

Answer 27

Due to the resonance stabilization of the intermediates

Question 28

What can form if a ketone has two different alkyl groups?

Answer 28

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

Question 29

The equilibrium between the formation of the two forms of enolate is dependent on what?

Answer 29

Kinetic and thermodynamic control of the reaction

Question 30

The kinetically controlled enolate formation is formed more ____ and less ____.

Answer 30

More Rapidly but less stable

Question 31

What does the product of the kinetically controlled enolate look like?

Answer 31

The double bond to the less substituted alpha-carbon

Question 32

How is the kinetically controlled enolate formed?

Answer 32

By the removal of the alpha-hydrogen from the less substiuted alpha-carbon because it offers less steric hinderance

Question 33

The thermodynamically controlled produce is formed more _____ but is more ______.

Answer 33

More slowly but is more stable

Question 34

What does the product of the thermodynamically controlled enolate look like?

Answer 34

The double bond being formed with the more substituted alpha-carbon

Question 35

How is the thermodynamically controlled enolate formed?

Answer 35

By the removal of the alpha-hydrogen from the more substituted alpha-carbon

Question 36

When are kinetically controlled enolate products favored?

Answer 36

In a reaction that is rapid, irreversible, at lower temperatures and with a strong, sterically hindered base.

Question 37

When is the thermodynamically controlled enolate product favored?

Answer 37

With higher temperatures, slow, reversible reactions, which have smaller and weaker bases

Question 38

What happens if a reversible reaction uses kinetically controlled enolate products?

Answer 38

The kinetic product can revert to the origianl reactant and react again to form the thermodynamic product

Question 39

What are enamines?

Answer 39

Are tautomers of imines

Question 40

What is an imine?

Answer 40

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

Question 41

What is tautomerization again?

Answer 41

Movement of a hydrogen and a double bond.

Question 42

How can imines be converted to enamines?

Answer 42

Through tautomerization

Question 43

What is another vital reaction in the MCAT?

Answer 43

The aldol condensation

Question 44

What is Aldol Condensation?

Answer 44

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.

Question 45

When acetaldehyde (ethonal) is treated with ______, an ____ is produced?

Answer 45

Catalytic amount of base | An enolte is produced

Question 46

Why is the enolate more nucleophilic than the enol?

Answer 46

Because it is negatively charged

Question 47

In the aldol condensation reaction, the nucleophlic enolate ion does what?

Answer 47

The enolate ion can react with the electrophilic carboyl group of another acetaldehyde molecule

Question 48

What is key i a aldol condensation reaction?

Answer 48

Both species are in the same flask

Question 49

What is the product when acetaldehyde (ethonal) goes through an aldol condensation reacction?

Answer 49

3-hydroxybutanol which is an example of an aldol

Question 50

What is an aldol?

Answer 50

A molecule that contains both aldehyde and alcohol functional groups

Question 51

When ketones are used are they still called aldol reactions?

Answer 51

Yes

Question 52

With a strong base and high temperatures, what then occurs in the aldol condenstation reaction?

Answer 52

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

Question 53

Aldol condensations are most useful only if what are used? Why?

Answer 53

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

Question 54

Multiple products wont form, if what? Why?

Answer 54

If one of the molecules has no alpha-hydrogens because the alpha-carbons are quaternary

Question 55

Why is aldol condensation called a condensation reaction?

Answer 55

Because Two molecules are jointed with the loss of a small molecule

Question 56

Can an aldol condensation reaction also be called a dehydration reaction? Why?

Answer 56

Yes because the small molecule that is lost is water

Question 57

What is the reverse of an aldol condensation reacction called?

Answer 57

Retro-aldo reaction

Question 58

How do you push the reaction in a retro-aldol direction?

Answer 58

Aqueous base is added and heat is applied

Question 59

What is the retro-aldol reaction useful for?

Answer 59

Breaking bonds between the alpha dn beta-carbons of a carbonyl

Question 60

The retro-aldol reaction is facilitated if what?

Answer 60

The intermediate can be stabilized in the enolate form, just as in the forward reaction