8.S: Aldehydes and Ketones (Summary)

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Concepts & Vocabulary

19.0 Chapter Objectives and Preview of Carbonyl Chemistry

Aldehydes are carbonyl compounds with an R group and a hydrogen attached to the carbonyl carbon.
Ketones are carbonyl compounds with two R groups attached to the carbonyl carbon.

19.1 Naming Aldehydes and Ketones

Aldehydes are named following IUPAC rules with the standard ending -al.
Ketones are named following IUPAC rules with the standard ending -one.
Aldehydes have priority over ketones when both appear in the same molecule.

19.2 Preparing Aldehydes and Ketones

Primary alcohols can be oxidized to aldehydes with PCC (pyridinium chlorochromate) or DMP (Dess-Martin Periodinane).
Terminal alkynes can be hydrated to form aldehydes through hydroboration-oxidation reactions. (preview)
Secondary alcohols can be oxidized to ketones with chromic acid/Jones reagent.
Alkynes can be hydrated to form ketones. (preview)
Aryl ketones can be formed by Friedel-Crafts acylation of aromatic rings. (CHM 223)
Nitriles can be reacted with Grignard reagents to form ketones.
Alkenes can be cleaved with ozone to form aldehydes or ketones. (CHM 223)

19.3 Oxidation of Aldehydes and Ketones

Aldehydes can be oxidized more easily than ketones due to the aldehyde hydrogen.
Jones reagent is a mixture of CrO₃ and aqueous acid.
Aldehydes can be oxidized to carboxylic acids with chromium trioxide/Jones reagent.
Ketones can be oxidized to esters by peroxycarboxylic acids in a reaction known as Baeyer-Villiger oxidation. (CHM 223)

19.4 Nucleophilic Addition Reactions of Aldehydes and Ketones

Carbonyl bonds are polarized with a partial positive charge on the carbon making it an electrophile and a target for attack by nucleophiles.
Neither aldehydes nor ketones have a good leaving group causing both to undergo nucleophilic addition reactions.
Aldehydes are more reactive than ketones to nucleophilic addition reactions.

19.5 Nucleophilic Addition Reactions of Water: Hydration

Gem-diols are molecules that have two hydroxide groups attached to the same carbon.
Aldehydes and ketones can react with water under either acidic or basic conditions to form gem-diols.

19.6 Nucleophilic Addition Reactions of HCN: Cyanohydrin Formation

Cyanohydrins are molecules with a cyanide and a hydroxide attached to the same carbon.
Cyanohydrins can be formed from aldehydes or ketones by reacting with cyanide ion and a weak acid.

19.7 Nucleophilic Addition Reactions of Hydride and Grignard Reagents: Alcohol Formation

Metal hydrides can reduce aldehydes and ketones to alcohols.
Organometallic reagents include carbon bonds to metals which react similarly to carbanions.
Grignard reagents (R-Mg-X) and organolithium compounds add to aldehydes and ketones to form alcohols.

19.8 Nucleophilic Addition of Amines: Imine and Enamine

Imines are characterized by a carbon-nitrogen double bond.
Reaction of aldehydes and ketones with primary amines can form imines.
Reaction of aldehydes and ketones with secondary amines can form enamines.

19.9 Nucleophilic Addition of Hydrazine - The Wolff-Kishner Reaction

Hydrazine will react with aldehydes and ketones to form hydrazones (similar to imine formation). Heating of the hydrazone with base converts it to an alkane.
The combination of hydrazone formation and reduction (of aldehydes and ketones) is called the Wolff-Kishner reaction.

19.10 Nucleophilic Addition of Alcohols: Acetal Formation

Hemiacetals have one hydroxide and one ether attached to a carbon.
Acetals have two ethers attached to a carbon.
Reaction of an aldehyde or ketone with an alcohol under acidic conditions forms a hemi-acetal. Continuation of the same reaction results in an acetal.
Acetals can reform the aldehyde or ketone by reacting with acid.
Acetals are useful as protecting groups due to the reversible nature of acetal formation.

19.11 Nucleophilic Addition of Phosphorun Ylides: The Wittig Reaction

A molecule with positive and negative charges on adjacent atoms is called an ylide.
Wittig reagents are organophosphorus ylides which can be drawn as a double bonded structure called a phosphorane.
The Wittig reaction is used to convert an aldehyde or ketone into an alkene by reacting with an organophosphrous ylide.
Oxaphosphetane intermediates containing a 4 membered ring including one oxygen and one phosphorus atom occur during the Wittig reaction.

19.12 Biological Reductions (reference only)

The Cannizzaro reaction allows an aldehyde to react with another like molecule in strong base to form one oxidized molecule (carboxylic acid) and one reduced molecule (alcohol).
NADH, one of the most important biological reducing agents, uses a similar mechanism to the Cannizzaro reaction while reducing ketones in biological systems to alcohols while being converted to NAD⁺.

19.13 Conjugate Nucleophilic Addition of alpha, beta-Unsaturated Aldehydes and Ketones (CHM 223)

α, β-unsaturated carbonyls have an alkene attached to the carbonyl carbon.
Due to resonance delocalization, the β-carbon of an α, β-unsaturated carbonyl is electrophilic and will react with nucleophiles.
α, β-unsaturated carbonyls can undergo 1,2 or 1,4 addition.
1,4 addition is also called conjugate addition.
Lithium diorganocopper reagents are called Gilman reagents.
Gilman reagents typically react with α, β-unsaturated carbonyls via conjugate addition.

19.14 Spectroscopy of Aldehydes and Ketones

IR of aldehydes and ketones is defined strongly by the carbonyl stretching vibration.
¹H NMR of aldehydes show the aldehyde proton between 10 and 11 ppm as well as the protons on carbon adjacent to the carbonyl at about 2.5 ppm.
¹H NMR of ketones show the protons on carbon adjacent to the carbonyl at about 2.5 ppm.
¹³C NMR of aldehydes and ketones show the carbonyl carbon at about 200 ppm.
Mass spectra of aldehydes and ketones typically yield moderately intense molecular ions, M⁺. They also can undergo some rearrangements that yield common fragmentation patterns.

Skills to Master

Skill 19.1 Explain the relative reactivity of aldehydes vs. ketones to nucleophiles, based on carbonyl bond polarity.
Skill 19.2 Interpret common names of aldehydes and ketones.
Skill 19.3 Describe methods for preparing aldehydes and ketones.
Skill 19.4 Explain general reactivity of aldehydes and ketones through the nucleophilic addition mechanism.
Skill 19.5 Draw mechanisms for nucleophilic addition reactions to aldehydes and ketones including
- Hydration
- Cyanohydrin formation
- Hydride Reduction
- Organometallic reactions
- Addition of amines
- Wolff-Kishner reaction
- Acetal formation
- Wittig reaction