Aldehydes And Ketones Pt2

Question 1

Alpha carbon

Answer 1

The carbon adjacent to the carbonyl carbon

Question 2

Alpha hydrogen

Answer 2

Hydrogen atoms connect to the alpha carbon
Induction from oxygen pulls some electron density out of the CH bonds weakening the bonds and making the hydrogens acidic and able to deprotonate
Resonance stabilization of of the conjugate base augments acidity
Will easily deprotonate in basic solutions

Question 3

Carbanion

Answer 3

A molecule with a negatively charged carbon - the electron withdrawing oxygen provides stabilization while electron donating alkyl groups destabilize it

Question 4

Compare the reactivity and acidity of ketones and aldehydes

Answer 4

Alpha hydrogens in ketones are slightly less acidic because of the electron donating properties of the additional alkyl group
Aldehydes are slightly more reactive to nucleophiles than ketones because of the steric hindrance on the ketone and because the extra alkyl on ketones destabilizes the carbocation

Question 5

Why do we have both keto and enol forms?

Answer 5

The acidity of the alpha hydrogen in ketones and aldehydes causes the formation of 2 isomers in solution

Question 6

Enol

Answer 6

Has a carbon to carbon double bond and an alcohol

Question 7

Tautomers

Answer 7

Differ in the placement of a proton and a double bond

Note: tautomers are not resonance structures

Question 8

Where does the equilibrium lie between keto and enol isomers?

Answer 8

Far towards the keto side

Question 9

How do we get an enolate carbanion

Answer 9

From the deprotonation of the alpha carbon by a strong base such as hydroxide, lithium diisopropyl amide (LDA), potassium hydride (KH)
The nucleophilic carbanion then reacts readily with electrophiles

Question 10

Michael addition

Answer 10

When the carbanion attacks an alpha beta unsaturated carbonyl compound
- resonance stabilization of the intermediates

Question 11

How many enolate forms may a ketone have?

Answer 11

Up to 2 because they can have 2 different alkyl groups

Question 12

Kinetically controlled enolate product

Answer 12

Formed rapidly but is less stable
Has a double bond to the less substituted alpha carbon - product is formed by removing the hydrogen of the less substituted alpha carbon

Question 13

Thermodynamically favoured enolate

Answer 13

Formed more slowly but is more stable
Double bond with the more substituted alpha carbon
Formed by removal of the alpha hydrogen from the more substituted alpha carbon

Question 14

What favours the kinetic products?

Answer 14

Rapid, irreversible, low temperature with a strong sterically hindered base

Question 15

What favours the thermodynamic products?

Answer 15

High temperature, slow, reversible, small weak bases

Question 16

Aldol condensation step 1

Answer 16

Aldehyde or ketone acts as both electrophile (in keto form) and nucleophile (in enol form) - end result is formation of a carbon to carbon bond
Enolate more nucleophilic than enol bc of negative charge
Enolate reacts with electrophilic carbonyl of another acetaldehyde
Product is an aldehyde which contains either ketone or aldehyde and alcohol
Must occur in one vessel

Question 17

Aldol condensation step 2

Answer 17

With a strong base and high temperatures dehydration occurs by an E1 OR E2 mechanism - we kick off a water molecule and form a double bond producing an alpha-beta unsaturated carbonyl

Question 18

Retro aldol reaction

Answer 18

Aqueous base is added and heat is applied to break the bond between the alpha and beta carbons to produce two products (ketone and or aldehyde)