Aldehydes, Ketones, and Chiral Molecules

Carbonyl Group: Structure and Properties

The carbonyl group (C=O) is a functional group found in both aldehydes and ketones. It consists of a carbon atom double-bonded to an oxygen atom.

• Bond Angle: The carbonyl carbon is sp2 hybridized, resulting in a bond angle of approximately 120°.

• Polarity: The C=O bond is highly polar due to the difference in electronegativity between carbon and oxygen.

Example: In acetone (CH3COCH3), the central carbon is part of the carbonyl group.

Recognizing Aldehydes and Ketones

• Aldehydes: Contain a carbonyl group bonded to at least one hydrogen atom. General formula: RCHO.

• Ketones: Contain a carbonyl group bonded to two carbon atoms. General formula: RCOR'.

Example: Formaldehyde (HCHO) is an aldehyde; acetone (CH3COCH3) is a ketone.

Naming Aldehydes and Ketones

• IUPAC Naming:

  • Aldehydes: Replace the -e of the parent alkane with -al (e.g., ethanal).

  • Ketones: Replace the -e of the parent alkane with -one (e.g., propanone).

  • Cyclic ketones: Name the ring and add 'one' (e.g., cyclohexanone).

• Common Names:

  • Formaldehyde (methanal), acetaldehyde (ethanal), acetone (propanone).

• Multiple Functional Groups: Use appropriate locants and prefixes to indicate positions and types of groups.

Physical Properties

• Boiling Points: Higher than alkanes due to dipole-dipole interactions, but lower than alcohols (no hydrogen bonding).

• Solubility: Lower members are soluble in water due to hydrogen bonding with water; solubility decreases with increasing carbon chain length.

Oxidation of Aldehydes

• Aldehydes can be oxidized to carboxylic acids.

• Ketones generally do not undergo oxidation under mild conditions.

Equation:

Tollen's Test and Benedict's Test

• Tollen's Test: Used to distinguish aldehydes from ketones. Aldehydes reduce Ag+ to metallic silver.

• Benedict's Test: Aldehydes reduce Cu2+ to Cu2O (red precipitate).

Hemiacetals and Acetals

• Hemiacetal: Formed by the addition of an alcohol to an aldehyde or ketone.

• Acetal: Formed by the addition of a second alcohol to a hemiacetal.

• Cyclic Hemiacetals/Acetals: Common in sugars (e.g., glucose forms a cyclic hemiacetal).

Equation for Hemiacetal Formation:

Equation for Acetal Formation:

Isomerism: Constitutional vs. Stereoisomers

• Constitutional (Structural) Isomers: Same molecular formula, different connectivity.

• Stereoisomers: Same connectivity, different spatial arrangement.

Chirality and Enantiomers

• Chiral Center: A carbon atom bonded to four different groups.

• Achiral: Molecules that are superposable on their mirror images.

• Enantiomers: Non-superposable mirror images.

Example: Lactic acid has a chiral center and exists as two enantiomers.

Fischer Projections and D/L Nomenclature

• Fischer Projection: A two-dimensional representation of a three-dimensional molecule, commonly used for sugars and amino acids.

• D- and L- Isomers: Based on the position of the -OH group on the chiral carbon farthest from the carbonyl group in Fischer projections.

Carboxylic Acids and Esters

Carboxylic Acid Group and Naming

• Carboxylic Acid Group: Contains a carbonyl and hydroxyl group on the same carbon (–COOH).

• Common Names: Formic acid (methanoic acid), acetic acid (ethanoic acid).

• IUPAC Naming: Replace the -e of the parent alkane with -oic acid (e.g., ethanoic acid).

• Aromatic Carboxylic Acids: Benzoic acid is a common example.

• Multiple Functional Groups: Use locants and prefixes as needed.

Preparation of Carboxylic Acids

• Oxidation of primary alcohols or aldehydes yields carboxylic acids.

Equation:

Physical Properties

• Boiling Points: High due to hydrogen bonding (often higher than alcohols of similar mass).

• Solubility: Lower members are highly soluble in water; solubility decreases with chain length.

Acidity and Dissociation

• Carboxylic acids are weak acids; they partially dissociate in water.

Equation:

Neutralization and Carboxylate Salts

• React with bases (NaOH, KOH, LiOH) to form carboxylate salts and water.

• Carboxylate salts are generally solid, odorless, and more soluble in water than the parent acid.

Equation:

Esters: Recognition, Formation, and Naming

• Functional Group: –COOR (where R is an alkyl group).

• Formation: Reaction of a carboxylic acid with an alcohol (esterification).

• IUPAC Naming: Name the alkyl group from the alcohol, then the acid part as a carboxylate (e.g., ethyl acetate).

Equation:

Physical Properties of Esters

• Lower boiling points than carboxylic acids and alcohols (no hydrogen bonding between ester molecules).

• Many esters have pleasant, fruity odors.

Hydrolysis of Esters

• Acid Hydrolysis: Ester reacts with water in the presence of acid to yield carboxylic acid and alcohol.

• Base Hydrolysis (Saponification): Ester reacts with base to yield carboxylate salt and alcohol.

Acid Hydrolysis Equation:

Base Hydrolysis Equation:

Identifying Reactants from an Ester

• Given an ester, identify the alcohol (R'OH) and carboxylic acid (RCOOH) used in its formation.

Amines and Amides

Recognizing Amines

• Alkyl Amines: Nitrogen attached to alkyl groups.

• Aromatic Amines: Nitrogen attached to an aromatic ring (e.g., aniline).

• Heterocyclic Amines: Nitrogen is part of a ring structure.

Classification of Amines

• Primary (1°): One alkyl/aryl group attached to nitrogen.

• Secondary (2°): Two alkyl/aryl groups attached to nitrogen.

• Tertiary (3°): Three alkyl/aryl groups attached to nitrogen.

• Quaternary (4°): Four groups attached; nitrogen carries a positive charge.

Naming Amines

• Common Names: Methylamine, ethylamine, dimethylamine, diethylamine, triethylamine.

• IUPAC Naming: Name the longest carbon chain attached to nitrogen, add 'amine' as a suffix.

• For molecules with multiple functional groups, use appropriate locants and prefixes.

Physical Properties of Amines

• Boiling Points: Primary and secondary amines can hydrogen bond, leading to higher boiling points than alkanes but lower than alcohols.

• Solubility: Lower amines are soluble in water; solubility decreases with increasing size.

Reactions of Amines

• With Water: Amines act as bases, accepting a proton from water to form ammonium ions.

Equation:

• With Acids (Neutralization): Amines react with acids to form ammonium salts.

Equation:

Properties of Ammonium Salts

• Generally solid, odorless, and highly soluble in water.

Recognizing Amine Functional Group

• Look for nitrogen atoms bonded to carbon and/or hydrogen atoms.

Preparation and Naming of Amides

• Preparation: Reaction of a carboxylic acid (or derivative) with an amine.

• Common Names: Formamide, acetamide, benzamide.

• IUPAC Naming: Replace the -oic acid of the parent acid with -amide (e.g., ethanamide).

Equation:

Physical Properties of Amides

• High melting points due to hydrogen bonding.

• Lower members are soluble in water.

Hydrolysis of Amides

• Base Hydrolysis: Amide reacts with base to form carboxylate salt and amine.

• Acid Hydrolysis: Amide reacts with acid to form carboxylic acid and ammonium salt.

Base Hydrolysis Equation:

Acid Hydrolysis Equation:

Identifying Reactants from an Amide

• Given an amide, determine the carboxylic acid and amine used in its synthesis.

Summary Table: Functional Groups and Key Properties

Additional info: For best results, students should practice drawing structures and writing reactions to reinforce these concepts.