Aldehydes and Ketones: Synthesis, Properties, and Nucleophilic Addition Reactions

Chapter 16: Aldehydes & Ketones – Nucleophilic Addition to the Carbonyl Group

Introduction to Carbonyl Compounds

Carbonyl compounds are a central class of organic molecules characterized by the presence of a carbon-oxygen double bond (C=O). The two most important types are aldehydes and ketones, which differ in the groups attached to the carbonyl carbon.

• Aldehyde: The carbonyl carbon is bonded to at least one hydrogen atom.

• Ketone: The carbonyl carbon is bonded to two carbon atoms.

• Other carbonyl-containing compounds include esters, amides, and carboxylic acids.

Nomenclature of Aldehydes & Ketones

The systematic naming of aldehydes and ketones follows IUPAC rules:

• Aldehydes: Suffix "-al" (e.g., ethanal, propanal).

• Ketones: Suffix "-one" (e.g., propanone, butanone).

• Number the longest carbon chain containing the carbonyl group, starting at the carbonyl carbon.

• Functional groups such as formyl (methanoyl), acetyl (ethanoyl), and acyl (alkanoyl) are used as prefixes when the carbonyl is not the principal group.

Physical Properties

Aldehydes and ketones have higher boiling points than alkanes of similar molecular weight due to dipole-dipole interactions, but lower than alcohols because they lack hydrogen bonding.

• Example: Butane (bp -0.5°C), Propanal (bp 49°C), 1-Propanol (bp 97.2°C).

Synthesis of Aldehydes

Aldehydes by Oxidation of Primary Alcohols

Primary alcohols can be oxidized to aldehydes using mild oxidizing agents such as PCC (Pyridinium chlorochromate).

• Reaction: $\text{R-CH}_2\text{OH} \xrightarrow{\text{PCC}} \text{R-CHO}$

• PCC prevents further oxidation to carboxylic acids.

Aldehydes by Ozonolysis of Alkenes

Ozonolysis of alkenes is another method to synthesize aldehydes. Ozone cleaves the double bond, and reductive workup yields aldehydes.

• Reaction: $\text{R-CH=CH-R'} \xrightarrow{\text{O}_3, \text{Me}_2\text{S}} \text{R-CHO} + \text{R'-CHO}$

Synthesis of Ketones

Ketones from Alkenes, Arenes, and Secondary Alcohols

Ketones can be synthesized by several methods:

• Ozonolysis of Alkenes: Similar to aldehydes, but yields ketones when both sides of the double bond are alkyl groups.

• Friedel–Crafts Acylation: Aromatic rings react with acyl chlorides in the presence of AlCl3 to form aryl ketones.

• Oxidation of Secondary Alcohols: Secondary alcohols are oxidized to ketones using PCC or chromic acid.

Nucleophilic Addition to the Carbonyl Group

Structure and Reactivity

The carbonyl carbon is sp2 hybridized and trigonal planar, making it susceptible to nucleophilic attack due to its partial positive charge.

• Polarization: The C=O bond is polarized, with the carbon being electrophilic (δ+) and the oxygen nucleophilic (δ−).

• Resonance: Resonance structures further increase the electrophilicity of the carbonyl carbon.

Reversibility of Nucleophilic Additions

Many nucleophilic additions to carbonyl compounds are reversible, and the position of equilibrium depends on the reactants and conditions.

Relative Reactivity: Aldehydes vs. Ketones

Aldehydes are generally more reactive toward nucleophilic addition than ketones due to both steric and electronic factors.

• Steric: Ketones have two bulky groups attached to the carbonyl carbon, making nucleophilic attack more difficult.

• Electronic: Aldehydes have only one electron-donating group, making the carbonyl carbon more electrophilic.

Addition of Amines: Imines, Oximes, Hydrazones, and Enamines

Imines

Aldehydes and ketones react with primary amines to form imines via nucleophilic addition followed by elimination of water.

• Reaction: $\text{R}_2\text{C=O} + \text{RNH}_2 \xrightarrow{\text{H}^+} \text{R}_2\text{C=NR} + \text{H}_2\text{O}$

• All steps are reversible; excess amine drives equilibrium toward imine formation.

Oximes and Hydrazones

Oximes are formed by reaction of aldehydes or ketones with hydroxylamine, while hydrazones are formed with hydrazine.

• Oxime: $\text{R}_2\text{C=O} + \text{NH}_2\text{OH} \rightarrow \text{R}_2\text{C=NOH} + \text{H}_2\text{O}$

• Hydrazone: $\text{R}_2\text{C=O} + \text{NH}_2\text{NH}_2 \rightarrow \text{R}_2\text{C=NNH}_2 + \text{H}_2\text{O}$

Enamines

Secondary amines react with aldehydes or ketones to form enamines, which are useful intermediates in organic synthesis.

• Reaction: $\text{R}_2\text{C=O} + \text{R}_2\text{NH} \xrightarrow{\text{H}^+} \text{R}_2\text{C=NR}_2 \rightarrow \text{Enamine}$

Reactions of Carbonyl Compounds with Nucleophiles

Hydride and Carbanion Addition

Hydride ions (from NaBH4 or LiAlH4) and carbanions (from organometallic reagents) are important nucleophiles for carbonyl addition.

• Reduction: Aldehydes and ketones are reduced to alcohols.

• Organometallic Addition: Grignard and organolithium reagents add to carbonyls to form alcohols.

Oxidation-Reduction Reactions

Oxidation increases the oxygen content or decreases hydrogen content; reduction does the opposite.

• Oxidation: Alcohols → Aldehydes → Carboxylic acids

• Reduction: Carboxylic acids → Aldehydes → Alcohols

Oxidation States in Organic Chemistry

Oxidation state of carbon is calculated based on bonds to more or less electronegative atoms.

• C–H or C–M: -1

• C–C: 0

• C–Z (Z = electronegative): +1

Oxidation of Alcohols

Primary alcohols can be oxidized to aldehydes (PCC) or carboxylic acids (Jones reagent, KMnO4).

Mechanism of Chromate Oxidations

Chromate oxidations proceed via formation of a chromate ester, followed by elimination to yield the oxidized product.

Spectroscopic Evidence for Alcohols

Alcohols show broad O–H stretching in IR (3200–3600 cm−1), and characteristic NMR signals for hydroxyl and α-carbon hydrogens.

Organometallic Compounds

Preparation of Organolithium & Organomagnesium Compounds

Organolithium and Grignard reagents are prepared by reaction of alkyl halides with lithium or magnesium in ether solvents.

• Organolithium: $\text{R-X} + 2\text{Li} \rightarrow \text{R-Li} + \text{LiX}$

• Grignard: $\text{R-X} + \text{Mg} \rightarrow \text{R-MgX}$

Reactions of Grignard Reagents

Grignard reagents react with epoxides and carbonyl compounds to form alcohols.

• Epoxides: $\text{RMgX} + \text{Epoxide} \rightarrow \text{Alcohol}$

• Carbonyls: $\text{RMgX} + \text{Aldehyde/Ketone} \rightarrow \text{Alcohol}$

Summary of Aldehyde and Ketone Addition Reactions

Aldehydes and ketones undergo a variety of nucleophilic addition reactions, including:

• Addition of hydride (reduction to alcohols)

• Addition of cyanide (formation of cyanohydrins)

• Addition of amines (formation of imines, enamines, oximes, hydrazones)

• Addition of organometallic reagents (formation of alcohols)