Chapter 19: Aldehydes and Ketones

Question 1

Naming an aldehyde

Answer 1

1. Identify and name the parent chain that includes the aldehyde group
2. Identify and name substituents
3. Assign locant to each substituent giving the aldehyde the lowest number
4. Assemble substituents alphabetically
5. Assign configurations to any chiral centers
6. Replace “e” with suffix −al

Question 2

Answer 2

Cyclohexanecarbaldehyde

Question 3

Answer 3

Formaldehyde

Question 4

Answer 4

Acetaldehyde

Question 5

Answer 5

Benzaldehyde

Question 6

Naming ketones

Answer 6

1. Identify and name the parent chain that includes the ketone carbonyl
2. Identify and name substituents
3. Assign locant to each substituent giving the ketone the lowest number
4. Assemble substituents alphabetically
5. Assign configurations to any chiral centers
6. Replace “e” with suffix −one

Question 7

Naming simple ketones

Answer 7

Simple ketones can be named alkyl alkyl ketones

Question 8

Answer 8

Acetone

Question 9

Answer 9

Acetophenone

Question 10

Answer 10

Benzophenone

Question 11

Preparing aldehydes from primary alcohols

Answer 11

Reagents

PCC & CH₂Cl₂ (dichloromethane)

or

Des-Martin periodinane (DMP) oxidation
DMP and CH₂Cl₂ (dichloromethane)

or

Swern oxidation
1. DMSO & (COCl)₂ (oxalyl chloride)
2. Et₃N (triethylamine)

Question 12

Preparing ketones from secondary alcohols

Answer 12

Reagents

Na₂Cr₂O₇ (sodium dichromate) & H₂SO₄/H₂O

or

xs CrO₃ (chromium trioxide) & H₃O⁺/acetone

or

PCC & CH₂Cl₂ (dichloromethane)

or

Des-Martin periodinane (DMP) oxidation
DMP & CH₂Cl₂ (dichloromethane)

or

Swern oxidation
1. DMSO & (COCl)₂ (oxalyl chloride)
2. Et₃N (triethylamine)

Question 13

Preparing aldehydes or ketones from an alkene

Answer 13

Depending on the substitution of the alkene either can be created

Reagents

1. O₃
2. DMS

Question 14

Forming an aldehyde from a terminal alkyne

Answer 14

Hydroboration oxidation

Reagents

1. R₂BH (alkyl borane)
   Disiamylborane or 9-BBN
2. H₂O₂ & NaOH

Question 15

Forming an acetyl ketone from an alkyne

Answer 15

Acid-catalyzed hydration

Reagents

H₂SO₄, H₂O & HgSO₄ (mercuric sulfate)

Question 16

Aldehyde vs. ketone reactivity

Answer 16

Question 17

Formation of hydrates

Answer 17

Reagents

[H⁺] & ROH

Alcohols can attack carbonyls making acetals

Ketal- specifically from a ketone

• For most ketones, the ketal is NOT favored at equilibrium

Acetal- typically from an aldehyde

• For simple aldehydes the acetal is favored at equilibrium

Question 18

Cyclic acetal formation

Answer 18

Reagents

[H⁺], ethylene glycol

Typically, a non-nucleophilic acid is used like H₂SO₄

Can be used as a protecting group as it is reversible

Question 19

Hemiacetal formation

Answer 19

An acetal with one −OR group and one hydroxyl group

Difficult to isolate however cyclic hemiacetals can be isolated

Question 20

Imine formation

Answer 20

Reagents

[H⁺] & RNH₂

Mechanism

Under acidic conditions aldehydes/ketones react with primary amines to form imines

Question 21

Formation of an enamines

Answer 21

Reagents

[H⁺] & R₂NH

Mechanism

Under acidic conditions aldehydes/ketones react with secondary amines to form enamines

Question 22

Wolff-Kishner Reduction

Answer 22

Reagents

1. [H⁺] & H₂N−NH₂ (hydrazine)
2. KOH, H₂O & heat

Mechansim

Reduce a carbonyl to alkane

Question 23

Hydrolysis of acetals

Answer 23

Reagents

[H⁺] & H₂O

Mechanism

Acetals are hydrolyzed with aqueous acid to yield a ketone (or aldehyde) and two equivalents of alcohol

Simply the reverse of acetal formation

Question 24

Hydrolysis of imines and enamines

Answer 24

Reagents

[H⁺] & H₂O

Mechanism

Acetals are hydrolyzed with aqueous acid to yield a ketone (or aldehyde) and a primary or secondary amine

Question 25

Thioacetal formation

Answer 25

_Reagents_ **[H⁺] & RSH** _Mechanism_ Nucleophilic attack similar to alcohols

Question 26

Cyclic thioacetal formation

Answer 26

_Reagents_ **[H⁺], 1,2-ethanedithiol**

Question 27

Reduction of thioacetals

Answer 27

_Reagents_ **Raney Ni**

Question 28

Hydrogen nucleophiles

Answer 28

_Reagents_ 1. **LiAlH₄** 2. **H₂O** or **NaBH₄, MeOH** _Mechanism_ Can act as reducing agents by delivering *hydrides* Need *basic* conditions because hydrides are strong nucleophiles

Question 29

Gringard reagents

Answer 29

_Reagents_ 1. **RMgX** 2. **H₃O⁺** _Mechanism_ A **Grignard reagent** is formed by the reaction between an alkyl halide and magnesium characterized by a C−Mg bond; acts as a nucleophile and attacks a *carbonyl carbon* CANNOT be used in the presence of a mildy acidic proton as it will just deprotonate the substrate; NOT compatible with **carboxylic acids** _Regeoselectivity_ Attaches the *R* group to the carbonyl *carbon*; subsequent workup protonates the carbonyl *oxygen* creating a hydroxyl group _Stereospecificity_ Nuclephilic attack can occur form either side; forms a mixture of enantiomers when a chiral center is present

Question 30

Cyanohydrin formation

Answer 30

_Reagents_ **KCN & HCN** or **KCN & HCl** _Mechanism_ Produces a mixture of enantiomers if a new chiral center is formed

Question 31

Wittig reaction

Answer 31

_Reagents_ Whittig reagent **R=PPh₃** (phosphorous ylide) _Mechanism_ Converts a ketone or aldehyde into an alkene by forming a new C=C Inserts the *R* group on the ylide at the location where the carbonyl oxygen was When the Whittig reagent is generated from a **simple** alkyl halide the (*Z*)-alkene is generally the major product When the Whittig reagent contains an **electron-withdrawing group** the (*E*)-alkene is generally the major product

Question 32

Whittig reagent preparation

Answer 32

_Reagens_ 1. **PPh₃** (triphenylphosphine) 2. **Strong base** *n*-BuLi. NaH, NaNH₂, or PhLi _Mechanism_ Retrosynthetic analysis reveals two possibly ways to make product. Which is better? Look at which Wittig reagent is easier to make, **methyl halide** is better

Question 33

Baeyer-Villiger oxidation

Answer 33

_Reagents_ **RCO₃H** (peroxy acid) _Mechanism_ Inserts an oxygen between a carbonyl carbon and an alkyl group * Converting an **aldehyde** to a carboxylic acid * Converts a **ketone** to an ester For an aldehyde or unsymmetrical ketone the rate of migration is **H > 3° > 2°, Ph > 1° > methyl**