Chapter 14 – Aldehydes and Ketones: Nucleophilic Additions

Nomenclature of Aldehydes and Ketones

Aldehydes and ketones are organic compounds containing the carbonyl group (C=O). Their nomenclature follows IUPAC rules, with aldehydes named by replacing the terminal '-e' of the parent alkane with '-al', and ketones by replacing '-e' with '-one'.

• Aldehyde: The carbonyl group is at the end of the carbon chain. Example: ethanal (acetaldehyde).

• Ketone: The carbonyl group is within the carbon chain. Example: propanone (acetone).

Physical Properties and Spectroscopy

Aldehydes and ketones have distinct physical properties due to the polar carbonyl group. Their IR spectra show a strong absorption near 1700 cm-1 (C=O stretch). NMR chemical shifts for the carbonyl carbon are typically downfield (high ppm).

• Boiling Points: Higher than alkanes, lower than alcohols.

• Solubility: Small aldehydes and ketones are soluble in water due to hydrogen bonding.

Methods for Preparation of Aldehydes and Ketones

Several methods exist for synthesizing aldehydes and ketones, including oxidation of alcohols and ozonolysis of alkenes.

• Oxidation of Primary Alcohols: Produces aldehydes (mild oxidants like PCC).

• Oxidation of Secondary Alcohols: Produces ketones (e.g., chromic acid).

• Ozonolysis of Alkenes: Cleaves double bonds to form aldehydes and/or ketones.

Reactions of Aldehydes and Ketones

The carbonyl group is highly reactive toward nucleophilic addition. Common reactions include:

• Nucleophilic Addition: Nucleophiles attack the carbonyl carbon, forming addition products.

• Hydration: Formation of geminal diols.

• Acetal Formation: Reaction with alcohols under acidic conditions.

• Imine Formation: Reaction with primary amines.

General equation for nucleophilic addition:

Protecting Groups

Protecting groups are used to temporarily mask reactive functional groups during multi-step syntheses. For carbonyls, acetals are common protecting groups.

• Acetal Formation: Used to protect aldehydes and ketones from unwanted reactions.

• Deprotection: Acetals can be removed under acidic conditions to regenerate the carbonyl.

Key Reagents: DIBAL and LAH

DIBAL-H (Diisobutylaluminum hydride) and LAH (Lithium aluminum hydride) are powerful reducing agents.

• DIBAL-H: Reduces esters to aldehydes under controlled conditions.

• LAH: Reduces carboxylic acids, esters, and aldehydes to alcohols.

Example: Reduction of an ester to an aldehyde using DIBAL-H:

Review and Practice

• Review key concepts, mechanisms, and reactions.

• Practice nomenclature, synthesis, and reaction mechanisms.

• Work assigned exercises to reinforce understanding.

Chapter 15 – Carboxylic Acids

Nomenclature and Structure

Carboxylic acids contain the carboxyl group (-COOH). IUPAC names are based on the parent alkane, replacing '-e' with '-oic acid'. Common names are also used for C1–C6 acids.

• Example: Methanoic acid (formic acid), ethanoic acid (acetic acid).

Physical Properties and Spectroscopy

Carboxylic acids have high boiling points due to hydrogen bonding and dimer formation. Their IR spectra show strong absorption near 1700 cm-1 (C=O) and broad O–H stretch around 2500–3300 cm-1.

• Solubility: Lower carboxylic acids are soluble in water.

• Acidity: Carboxylic acids are more acidic than alcohols and phenols.

Methods for Preparation of Carboxylic Acids

Carboxylic acids can be synthesized by oxidation of primary alcohols and aldehydes, and by hydrolysis of nitriles and esters.

• Oxidation of Primary Alcohols:

• Hydrolysis of Nitriles:

Reactions of Carboxylic Acids

Carboxylic acids undergo several important reactions, including:

• Acid-Base Reactions: Carboxylic acids react with bases to form carboxylate salts.

• Esterification: Reaction with alcohols to form esters (Fischer esterification).

• Decarboxylation: Loss of CO2 from carboxylic acids under heat.

General equation for esterification:

Acidity and Substituent Effects

The acidity of carboxylic acids is influenced by substituents. Electron-withdrawing groups increase acidity, while electron-donating groups decrease it.

Separation and Extraction

Carboxylic acids can be separated from mixtures by extraction as their salts using aqueous base.

• Extraction: Conversion to water-soluble carboxylate salt, then re-acidification to recover the acid.

Review and Practice

• Review key concepts, mechanisms, and reactions.

• Practice nomenclature, synthesis, and reaction mechanisms.

• Work assigned exercises to reinforce understanding.

Additional info:

• Some content inferred from standard organic chemistry curriculum (e.g., typical reactions and mechanisms).

• Key reagents and protecting groups are emphasized for exam preparation (MCAT, DAT).

• Tables and examples added for clarity and completeness.