Chapter 16 – Aldehydes and Ketones

Chapter 16 – Ketones & Aldehydes

  • Course: CH 108 – Introduction to General, Organic, and Biological Chemistry

  • Instructor: Dr. Steven Petrovic

  • Semester: Fall 2024

The Carbonyl Group (Chapter 16.1)

  • Defining Characteristics:

    • Functional groups characterized by the carbonyl group (C=O).

    • Aldehydes: At least one hydrogen atom bonded to the carbonyl carbon.

    • Ketones: Alkyl groups are bound to both sides of the carbonyl carbon.

Carbonyl Structure & Bonding (Chapter 16.1)

  • Shape:

    • Carbonyl carbon is trigonal planar with bond angles of 120°.

  • Polarity:

    • The carbonyl group is polar due to oxygen's higher electronegativity.

    • Oxygen is electron-rich while carbon is electron-poor.

Chemical Nomenclature (Chapter 16.2)

Naming Aldehydes

  • IUPAC Naming Process:

    1. Identify the longest chain containing the CHO group.

    2. Change the “-e” ending of the parent alkane to “-al”.

    3. Number the chain to position the CHO group at C1 and omit the number.

    4. Apply all other nomenclature rules.

Common Names for Aldehydes

  • Suffix “-aldehyde” is added to the base name. Examples include:

    • Formaldehyde

    • Acetaldehyde

    • Benzaldehyde

Naming Ketones

  • IUPAC Naming Process:

    1. Identify the longest chain containing the carbonyl group.

    2. Change the “-e” ending of the parent alkane to “-one”.

    3. Number the chain to provide the carbonyl carbon with the lowest number.

    4. Apply all other nomenclature rules.

Common Names for Ketones

  • Constructed by naming both alkyl groups in alphabetical order and adding “-ketone”.

  • Examples include:

    • Acetone

    • Acetophenone

    • Benzophenone

Physical Properties (Chapter 16.3)

  • Polarity: Aldehydes and ketones have a polar carbonyl group, making them polar molecules.

  • Intermolecular Forces:

    • They have stronger intermolecular forces than hydrocarbons but weaker than alcohols due to the absence of OH bonds.

  • Boiling Points:

    • Higher boiling points than similar hydrocarbons and lower than similar alcohols.

  • Solubility:

    • Soluble in organic solvents.

    • Molecules with 6 carbons or fewer are soluble in water, while those with 7 or more are not.

Reactions of Aldehydes and Ketones (Chapter 16.5)

General Reaction Types

  1. Aldehydes can be oxidized to carboxylic acids.

  2. Both aldehydes and ketones can undergo addition reactions.

Oxidation Process

  • Oxidation: Converts the aldehyde C-H bond to a carboxylic acid C-OH bond.

  • Ketones: Cannot be oxidized due to the absence of C-H bonds in the carbonyl group.

Selective Oxidation

  • Aldehydes can be selectively oxidized using the Tollens reagent.

Reduction of Aldehydes and Ketones (Chapter 16.6)

  • Definition: Reduction results in a decrease in C-O bonds and/or an increase in C-H bonds.

  • Aldehyde Reduction: Produces a 1° alcohol.

  • Ketone Reduction: Produces a 2° alcohol.

  • Common reduction reagent: H2 gas in the presence of Pd metal.

Biological Reduction

  • Biological systems use NADH instead of H2 and Pd metal. NADH is oxidized to NAD+, serving as a biological oxidizing agent.

Example: Chemistry of Vision (Chapter 16.7)

  • Components: Human eye contains rod and cone cells.

  • Aldehyde Role: 11-cis-retinal undergoes isomerization when exposed to light, converting to trans-retinal, sending a nerve impulse to the brain.

  • Regeneration: All-trans-retinal is reduced by NADH to become all-trans-retinol (Vitamin A), which is then isomerized back into 11-cis-retinol.

Acetal Formation (Chapter 16.8)

  • Aldehydes react with alcohols to form hemiacetals and acetals in the presence of H2SO4.

  • Hemiacetal Formation: One molecule of alcohol reacts with an aldehyde.

  • Acetal Formation: A second alcohol molecule reacts with the hemiacetal.

Distinction Between Acetals and Ethers

  • Acetals: Contain one carbon bonded to two OR groups.

  • Ethers: Contain one oxygen atom bonded to two carbons.

Cyclic Hemiacetals

  • Stable compounds formed from intramolecular reactions of compounds containing both an OH group and an aldehyde.

  • Glucose primarily exists as a cyclic hemiacetal.

  • Conversion: Cyclic hemiacetals can be converted to cyclic acetals by reacting with another alcohol.

  • Hydrolysis Reaction: Acetals can revert to aldehydes and alcohols through hydrolysis.