Aldehydes and Ketones

Aldehydes and Ketones

Concept Map

  • Aldehydes and Ketones with Carbon-Oxygen Double Bonds

    • Aldehydes react to form:

    • Tollens' Test - test positive with 1° Alcohols

    • Ketones react to form:

    • Hemiacetals

    • Benedict's Test - test positive with Carboxylic Acids and 2° Alcohols

    • Acetals

Introduction to Aldehydes and Ketones

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    • Treating skin conditions

    • Assisting in surgeries

    • Performing biopsies and excisions

    • Writing prescriptions

    • Freezing skin lesions

    • Screening for skin cancer

    • Career path: Become a nurse/physician assistant, then specialize in dermatology.

Definition of Aldehydes and Ketones

  • Formalin: A 40% formaldehyde aqueous solution used as a germicide and to preserve biological specimens.

  • Learning Goal: Identify compounds with carbonyl groups as aldehydes and ketones. Write IUPAC and common names for aldehydes and ketones; draw their condensed structural and line-angle formulas.

Carbonyl Group

  • Characteristics:

    • Aldehyde: Bonded to at least one hydrogen atom.

    • Ketone: Bonded to two carbon groups.

  • Structure: Carbonyl group is a carbon-oxygen double bond (C=O), with two groups of atoms attached at angles of 120°.

Polarity of Carbonyl Group

  • Electronegativity: Oxygen atom has two lone pairs and is more electronegative than carbon.

  • Dipole Moment: The carbonyl group exhibits a strong dipole with a partial negative charge (δ−) on oxygen and a partial positive charge (δ+) on carbon.

  • Influence: The polarity affects the physical and chemical properties of aldehydes and ketones.

Naming Aldehydes

  • IUPAC Naming: Replace the 'e' of the corresponding alkane name with 'al'.

  • Position: No number is needed for the aldehyde group as it is always at the end of the chain.

  • Common Names: Aldehydes with 1 to 4 carbons often referred to by common names (e.g., butanal).

  • Example: Benzaldehyde is the aldehyde derived from benzene.

Learning Check: IUPAC Naming for Aldehyde

  • Process to determine the name:

    • Step 1: Identify the longest carbon chain and replace 'e' with 'al'.

    • Step 2: Name and number substituents, considering the carbonyl group as carbon 1.

Naming Ketones

  • Common Naming: Name alkyl groups as substituents and list them alphabetically followed by 'ketone'.

    • Example: Acetone (or propanone) is a common name but is accepted in IUPAC.

  • IUPAC Naming: Replace 'e' in the alkane name with 'one'. Number the chain starting from the end nearest to the carbonyl group for chains with five or more carbons.

  • Cyclic Ketones:

    • Prefix 'cyclo' before the ketone name. Number the ring starting with the carbonyl as 1 to give the lowest numbers to substituents.

Learning Check: IUPAC Naming for Ketone

  • Process to determine the name:

    • Step 1: Identify the longest carbon chain and replace 'e' with 'one'.

    • Step 2: Number the carbon chain nearest to the carbonyl group.

    • Step 3: Name and number any substituents.

Uses and Impacts of Formaldehyde

  • Formalin: Used in industry, reacts to synthesize polymers, insulation materials, carpets, plywood, and kitchen counter plastics.

  • Health Risks: Exposure to fumes can cause irritation to eyes, nose, respiratory tract, and skin rashes, headaches, dizziness, and fatigue.

Acetone

  • Properties: Colorless liquid with a mild odor; used as a solvent in cleaning fluids, paint, nail polish removers, and rubber cement.

  • Body Production: May be produced in conditions like uncontrolled diabetes or fasting when fats are metabolized for energy.

Naturally Occurring Aldehydes

  • Examples:

    • Benzaldehyde: Found in almonds.

    • Vanillin: Found in vanilla beans.

    • Cinnamaldehyde: Found in cinnamon.

Learning Check: Identify as Aldehyde or Ketone

  • Examples:

    • A. CH₃—CH₂—CH₂—CHO - Aldehyde

    • B. CH₃—CH₂—CO—CH₂—CH₂—CH₃ - Ketone

Learning Check: IUPAC Naming for Aldehydes

  • Examples:

    • A. 4-chloropentanal

    • B. 2-ethylbenzaldehyde

Learning Check: IUPAC Naming for Ketones

  • Example:

    • 3-ethyl-1-cyclopentanone

Physical Properties of Aldehydes and Ketones

  • Boiling Points & Solubility: Due to polar carbonyl groups, aldehydes and ketones display varying boiling points depending on their carbon framework.

Polarity Effect on Physical Properties

  • At Room Temperature:

    • Methanal and ethanal are gases. Aldehydes/ketones with 3-10 carbons are liquids.

  • Boiling Point Factors:

    • Higher than alkanes/ethers due to dipole-dipole interactions.

    • Lower than alcohols due to lack of hydrogen bonds.

Boiling Points of Aldehydes and Ketones

  • Table of boiling points and solubility:

    • E.g., Methanal (1 C) = -21°C, soluble; Propanone (3 C) = 56°C, soluble; Pentanal (5 C) = 103°C, slightly soluble.

Solubility of Aldehydes and Ketones

  • Water Solubility:

    • Soluble if 1-4 carbons; lower solubility for 5+ carbons.

    • Solubility is due to hydrogen bonding between the carbonyl oxygen and water hydrogen atoms.

Learning Check: Higher Boiling Point

  • Understand that alcohols form hydrogen bonds, which typically increases boiling points compared to aldehydes and ketones.

Oxidation and Reduction of Aldehydes and Ketones

  • Tollens’ Test: Produces a “silver mirror” reflecting the oxidation of aldehyde reducing silver ions to metallic silver.

Oxidation of Aldehydes

  • Aldehydes readily oxidize to form carboxylic acids; ketones do not oxidize.

Tollens’ Test Characteristics

  • Tests for distinguishing aldehydes from ketones using Tollens’ reagent (AgNO3 with ammonia) which oxidizes aldehydes and reduces Ag+ to form the silver mirror.

Benedict’s Test Characteristics

  • Positive for aldehydes and adjacent hydroxyl groups; heating with the solution (Cu²⁺) forms a brick-red solid Cu₂O. Negatives with simple aldehydes/ketones.

Reductions of Aldehydes and Ketones

  • Can be reduced via hydrogen (H₂) or sodium borohydride (NaBH₄) with a catalyst (nickel, platinum, palladium).

  • Aldehydes reduce to primary alcohols, ketones to secondary alcohols.

Learning Check: Oxidation Product with Tollens’ Reagent

  • Examples:

    • Butanal → Butanoic acid

    • Acetaldehyde → Ethanoic acid

    • Ethyl methyl ketone → No reaction (ketones are not oxidized by Tollens’ reagent).

Learning Check: Addition of Alcohols to Carbonyl Groups

  • Hemiacetal and acetal formation:

    • An alcohol adds to aldehyde or ketone to create a hemiacetal; further addition creates a stable acetal and water.

Hemiacetal and Acetal Structures

  • Hemiacetals consist of an —OH and an —OR group on the same carbon while acetals have two —OR groups on the same carbon.

Formation of Cyclic Hemiacetals

  • Cyclic hemiacetals from carbonyl group and —OH from the same molecule; especially stable in five or six-membered rings (like glucose).

Formation of Maltose

  • Maltose is a disaccharide from two glucose molecules joined by an acetal bond, one retains the cyclic hemiacetal bond.