MCAT Aldehydes and Ketones

Question 1

Major resonance v. Minor resonance

Answer 1

Major resonance is when every atom has a full octet.

Minor resonance is when the pi electrons are displaced onto the oxygen making carbon have an incomplete octet.

Question 2

Describe the boiling point and melting point of aldehydes and ketones

Answer 2

Aldehydes and ketones have higher M.P. and B.P. than alkanes due to polarity but lower than alcohols because they only have hydrogen acceptors and not donors.

B.P. decreases with branching.

Question 3

Describe the solubility of aldehydes and ketones

Answer 3

Aldehydes and ketones up to four carbon atoms are highly solube in water.

Question 4

Mild oxidation agents v. Strong oxidation agents

Answer 4

Mild is a two - electron oxidation

Strong - four electron oxidation

Question 5

What happens when we expose a carboxylic acid or carboxylic acid derivative to a mild reducing agent?

Answer 5

We turn it into an aldehyde.

Question 6

Describe nucleophilic addition reactions of aldehydes and ketones in acidic v. basic conditions

Answer 6

Nucleophilic addition reactions is a reaction in which the hydrogen is added to the oxygen and nucleophile added to the carbonyl carbon.

Acidic conditions: acid protonates oxygen first and then nucleophile attacks carbonyl carbon. Hydrogen on nucleophile is removed.

Basic conditions - nucleophile attacks, protonation of oxygen, deprotonation of nucleophile.

Strong nucleophiles we do under basic conditions and weak nucleophiles under acidic conditions.

Question 7

What happens when we hydrate an aldehyde or ketone?

Answer 7

We get an aldehyde hydrate or a ketone hydrate. The result is a geminal diol. Forward (hydration) and reverse (condensation) reactions exist in dynamic equilibrium.

Question 8

Describe the formation of cyanohydrin?

Answer 8

When we react a cyanide ion ( NaCN) with an aldehyde or ketone we get a cyanohydrin in which you get a cyanide and hydroxyl group.
Because nucleophile is strong it’s done under basic conditions.

1. Cyanide attacks first bumping pi electrons onto oxygen.
2. Oxygen gets protonated

Question 9

Describe attack by ammonia or amines?

Answer 9

If attacked by ammonia or a primary amine you get an imine ( schiff base).

If attacked by secondary amine you get a schiff base which can tautomerize into an enamine.

A condensation reaction and happens under acidic conditions.

Question 10

What are the variants of imine formation of aldehydes are ketones?

Answer 10

• Ammonia attacks lead to imine.
• Primary amine attack leads to imine ( Schiff base)
• Hydroxylamine attack leads to oxime.
• Hydrazine attack leads to hydrazone.

Question 11

What happens when we react two molar concentrations of alcohols to aldehydes or ketones?

Answer 11

We form ketals or acetals.

a condensation reaction and happens under acidic conditions.

Hemiacetal intermediate has an alkoxy group (-OR) and hydroxyl group ( -OH_ bonded to the same carbon.

Question 12

What functions does acetals have?

Answer 12

Ther serve as protecting groups. Most commonly with 1,2- diols and 1,3- diols heated with a strong acid (- TsOH).

The protecting group can be removed with a strong acid mixed with water- miscible solvent.

Because aldehydes are less sterically hindered, they react faster than ketones and so can be used to selectively protect aldehydes or ketones.

Question 13

Describe the Tollen’s test

Answer 13

A way to test for aldehydes.

We use the tollens reagent ( Ag( NH3)2^+). When aldehydes are present we get a silver mirror.

Ketones can also produce a silver if they form aldehydes through keto-enol tautomerization.

Question 14

Alpha reactions (alpha substitution reactions)

Answer 14

When the alpha carbon (carbon next to carbonyl carbon) acts as a nucleophile.

Question 15

Describe the formation of enols and enolate ions

Answer 15

Enol is formed when deprotonation of alpha- hydrogen causes a double bond to form between alpha carbon and carbonyl carbon and bumping pi electrons onto oxygen. This forms the enolate ion.

When the enolate ion is protonated, we get an enol.

Question 16

What contributes to the reactivity of the alpha hydrogen?

Answer 16

It’s very acidic due to being next to the polar carbonyl carbon and its inductive effect of withdrawing electron density away from it.

Question 17

What happens when you get a ketone with different R groups each with alpha hydrogens?

Answer 17

Regioisomers form

• When the alpha- hydrogen is removed and double bond forms on side that’s least substituted you get the kinetic product (high energy, less stable). Forms quick.
• When the alpha- hydrogen is removed and the double bond forms on the side that’s most substituted you get the thermodynamic product (low energy, highly stable). Doesn’t form quick

Question 18

alkylation reactions

Answer 18

Reactions in which an enolate ion reacts with an alkyl halide resulting in the alkyl group being added and the halide being released.

Question 19

Stork- enamine synthesis

Answer 19

Reaction in which we alkylate an enamine.

Happens under acidic conditions and through an SN2 mechanism.

Question 20

Alpha- halogenation reaction

Answer 20

When the enol or enolate ion reacts with X2, replacing a hydrogen with a halogen atom.

Can happen with all the other halogens except fluorine.

Under basic conditions the nucleophile attacks first forming an enolate ion to react.

Under acidic conditions the hydrogen is protonated forming an enol which reacts.

Question 21

Aldol condensation

Answer 21

A condensation reaction in which an enol or enolate ion attacks a similar species creating a carbon-carbon bond. The product is called the aldol product.

Question 22

Retro - aldol reaction

Answer 22

The reverse of the aldol condensation reaction in which you get the formation of the two separate compounds from the aldol product.

Question 23

What happens when you apply heat or let the reaction go for a long period of time?

Answer 23

the formation of an alpha, beta unsaturated carbonyl. The product is now referred to as the aldol condensation.

Question 24

How can we stop the aldol condensation product from forming?

Answer 24

Perform the reaction under cold conditions.

Question 25

Crossed aldol condensation reaction

Answer 25

When the two reactants are not the same compound. This can lead to multiple alpha, beta unsaturated aldehyde compounds.

To maximize formation of one product over the other is:

1. The nucleophile is the only compound with alpha hydrogens.
2. We present the electrophile in excess and it’s more electrophilic than the nucleophile is nucleophilic.
3. Reactants are added in a specific order.

Question 26

Intramolecular aldol condensation

Answer 26

When the two aldehyde or ketone products are within the same compound, they perform an intramolecular aldol condensation reaction forming a cyclic compound.

Question 27

1,4- addition v. 1,2- addition

Answer 27

When we add a nucleophile to a diene, we get 1,2- addition product which is the kinetic product.

When we add a nucleophile to a diene we get a 1,4- addition when resonance happens which is the thermodynamic product.

Question 28

Micheal addition

Answer 28

Reaction in which a Micheal donor attacks the Micheal acceptor creating a Micheal adduct. The Micheal acceptor is added to the beta carbon while the hydrogen is added to the alpha carbon.

Question 29

Robinson annulation

Answer 29

When a 1,3- dicarbonyl ( Micheal donor) reacts with an alpha, beta unsaturated ketone resulting in a cyclohexanone ring.

Question 30

Strecker synthesis

Answer 30

A way to make amino acids in vitro.

Uses an aldehyde precursor, with the substituent of the aldehyde being the R group. The carbonyl carbon the aldehyde becomes the alpha carbon.

The reagents used are NH3,HCN
HCI, H20