Aldehydes and Ketones: Structure, Properties, and Reactions

Aldehydes and Ketones

Key Terms and Definitions

• Aldehyde: An organic compound containing a carbonyl group (C=O) bonded to at least one hydrogen atom. General formula: R-CHO.

• Ketone: An organic compound with a carbonyl group bonded to two carbon atoms. General formula: R-CO-R'.

• Carbonyl Group: The functional group consisting of a carbon atom double-bonded to an oxygen atom (C=O).

• Alcohol: An organic compound with a hydroxyl group (-OH) attached to a saturated carbon atom.

• Primary (1o) Alcohol: An alcohol where the carbon bearing the -OH group is attached to only one other carbon.

• Secondary (2o) Alcohol: The -OH group is attached to a carbon bonded to two other carbons.

• Tertiary (3o) Alcohol: The -OH group is attached to a carbon bonded to three other carbons.

• Carboxylic Acid: An organic compound containing a carboxyl group (-COOH).

• Hemiacetal: A compound formed by the addition of an alcohol to an aldehyde or ketone; contains both an -OH and an -OR group on the same carbon.

• Acetal: A compound formed when a hemiacetal reacts with another alcohol; contains two -OR groups on the same carbon.

• Oxidation: A chemical process in which a molecule loses electrons, often increasing the number of bonds to oxygen.

• Reduction: A chemical process in which a molecule gains electrons, often increasing the number of bonds to hydrogen.

• Dehydration: The removal of water (H2O) from a molecule during a chemical reaction.

• Benedict’s Test: A chemical test used to detect the presence of reducing sugars (and aldehydes) by forming a red precipitate.

Section 14.1: Structure and Nomenclature of Aldehydes and Ketones

Identifying and Drawing Aldehydes and Ketones

• Condensed Structural Formula: Shows all atoms in a molecule but omits some or all of the bonds.

• Line-Angle Formula: Each vertex and line ending represents a carbon atom; hydrogens on carbons are usually omitted.

• Aldehyde Example: Ethanal (acetaldehyde): CH3CHO

• Ketone Example: Propanone (acetone): CH3COCH3

Naming Aldehydes and Ketones

• IUPAC Naming:

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

  • Ketones: Replace the -e ending with -one (e.g., propanone, butanone).

• Common Names: Often use prefixes such as form- (1C), acet- (2C), propion- (3C), etc.

Example: Draw and name the following:

• CH3CH2CHO is propanal (IUPAC) or propionaldehyde (common).

• CH3COCH2CH3 is butan-2-one (IUPAC) or methyl ethyl ketone (common).

Section 14.2: Physical Properties of Aldehydes and Ketones

Boiling Points and Solubility

• Boiling Points: Aldehydes and ketones have higher boiling points than alkanes and ethers of similar molecular weight due to dipole-dipole interactions, but lower than alcohols (which can hydrogen bond).

• Solubility: Small aldehydes and ketones (up to about 4 carbons) are soluble in water due to hydrogen bonding with water molecules. Solubility decreases as the hydrocarbon chain length increases.

Example: Acetone (propanone) is miscible with water, while pentanone is only slightly soluble.

Table: Solubility of Aldehydes and Ketones by Carbon Number

Number of Carbons	Solubility in Water
1-4	Soluble
5+	Insoluble or slightly soluble

Additional info: Table 14.1 in the textbook summarizes this trend and should be memorized.

Section 14.3: Reactions of Aldehydes and Ketones

Oxidation and Reduction

• Oxidation of Aldehydes: Aldehydes can be oxidized to carboxylic acids using oxidizing agents such as Tollens' reagent or Benedict's solution.

• Oxidation of Ketones: Ketones are generally resistant to oxidation under mild conditions.

• Reduction: Both aldehydes and ketones can be reduced to alcohols using reducing agents such as NaBH4 or LiAlH4.

Key Equations:

• Oxidation of an aldehyde: $\mathrm{RCHO\ +\ [O]\ \rightarrow\ RCOOH}$

• Reduction of an aldehyde: $\mathrm{RCHO\ +\ 2[H]\ \rightarrow\ RCH_2OH}$

• Reduction of a ketone: $\mathrm{RCOR'\ +\ 2[H]\ \rightarrow\ RCH(OH)R'}$

Tollens' and Benedict's Tests

• Tollens' Test: Used to distinguish aldehydes from ketones. Aldehydes give a positive result (silver mirror), ketones do not react.

• Benedict's Test: Also detects aldehydes (and reducing sugars); a positive result is a red precipitate.

Example: Ethanal gives a positive Tollens' and Benedict's test; propanone does not.

Section 14.4: Hemiacetals and Acetals

Formation and Identification

• Hemiacetal Formation: Aldehydes and ketones react with one equivalent of alcohol to form a hemiacetal.

• Acetal Formation: Hemiacetals react with a second equivalent of alcohol (in the presence of acid) to form an acetal.

• Straight-Chain vs. Cyclic: Hemiacetals and acetals can be straight-chain or cyclic, especially in carbohydrate chemistry.

General Equations:

• Hemiacetal formation: $\mathrm{R_2C=O\ +\ R'OH\ \rightarrow\ R_2C(OH)(OR')}$

• Acetal formation: $\mathrm{R_2C(OH)(OR')\ +\ R'OH\ \xrightarrow{H^+}\ R_2C(OR')_2\ +\ H_2O}$

Example: Glucose forms a cyclic hemiacetal in solution.

Summary Tables and Memorization Points

• Table 14.1: Memorize the number of carbons at which aldehydes and ketones transition from slightly soluble to insoluble in water.

• Parent Prefixes for Aldehydes: Form-, acet-, propion-, butyr-, etc.

• Summary of Reactions: Know the catalysts and reagents for oxidation, reduction, and acetal formation (e.g., NaBH4, LiAlH4, H+).

Additional info: For exam preparation, practice drawing structures, naming compounds, and predicting reaction outcomes as indicated by the textbook problems referenced in each section.