Chapter 24 - Aldehydes and Ketones

Why are aldehydes generally more reactive than ketones?

Aldehydes have less steric hindrance and less electron donation from alkyl groups.

What is the general mechanism of nucleophilic addition to aldehydes and ketones?

Nucleophile attacks the carbonyl carbon → C=O π electrons move to oxygen → Alkoxide is protonated.

What happens when water reacts with an aldehyde or ketone?

A geminal diol (hydrate) forms, containing two OH groups on the same carbon.

What is a cyanohydrin, and how is it formed?

A compound with OH and CN on the same carbon, formed by cyanide addition to an aldehyde or ketone followed by protonation.

What is the difference between a hemiacetal and an acetal?

Hemiacetal: One OH and one OR group on the same carbon.

Acetal: Two OR groups on the same carbon.

How are acetals formed and broken down?

Formation: Carbonyl + alcohol + acid catalyst, with water removal.

Hydrolysis: Aqueous acid (H₃O⁺).

Why are acetals useful in organic synthesis?

They protect carbonyl groups from reacting with reagents such as Grignard reagents.

What reagents are used to protect a ketone as a cyclic acetal?

Ethylene glycol (HOCH₂CH₂OH) and an acid catalyst.

What is the Wittig reaction?

A reaction that converts an aldehyde or ketone into an alkene using a phosphorus ylide.

How is a Wittig ylide prepared?

Alkyl halide + PPh₃ → Phosphonium salt → Strong base (such as n-BuLi) → Ylide.

How do you determine the starting materials for a Wittig reaction from an alkene product?

Disconnect the C=C bond. One carbon comes from the carbonyl compound, and the other comes from the ylide.

What is the difference between imines and enamines?

Imine: Contains C=N.

Enamine: Contains C=C adjacent to nitrogen.

What type of amine forms an imine versus an enamine?

Primary amine → Imine.

Secondary amine → Enamine (when alpha hydrogens are present).

What is the general mechanism of imine formation?

Amine attacks carbonyl → Carbinolamine forms → Proton transfers → Water eliminated → Iminium ion deprotonated → Imine.

Why is mildly acidic pH (approximately 4.5) preferred for imine formation?

Acid promotes the reaction, but too much acid protonates the amine and reduces its nucleophilicity.

What is the functional group priority order from Chapter 24?

Aldehyde > Ketone > Alcohol > Alkene > Alkyne > Alkane

How do you name an aldehyde using IUPAC nomenclature?

Find the longest carbon chain containing the aldehyde.

Number the aldehyde carbon as C1.

Replace the alkane ending -e with -al.

Identify and number substituents.

Ex: CH₃CH₂CH₂CHO → Butanal

How do you name a ketone using IUPAC nomenclature?

Find the longest carbon chain containing the carbonyl.

Number from the end nearest the carbonyl.

Replace the alkane ending -e with -one.

Include the carbonyl position and substituents.

Ex: CH₃COCH₂CH₃ → Butan-2-one (2-butanone)

How do you name an aldehyde attached directly to a ring?

Use the ring name followed by -carbaldehyde.

Ex: Cyclopentanecarbaldehyde

What are the common names of important aldehydes and ketones?

HCHO → Formaldehyde

CH₃CHO → Acetaldehyde

C₆H₅CHO → Benzaldehyde

CH₃COCH₃ → Acetone