Aldehydes and Ketones: Structure, Properties, and Reactions (GOB Chemistry Study Notes)

Introduction to Organic and Biochemistry: Aldehydes and Ketones

This study guide covers the structure, properties, nomenclature, and key reactions of aldehydes and ketones, focusing on their importance in organic and biochemistry. These notes are suitable for students in a GOB (General, Organic, and Biochemistry) Chemistry course.

The Carbonyl Group

Structure and Definition

• Carbonyl group: A functional group that consists of a carbon atom double-bonded to an oxygen atom (C=O).

• Found in several important classes of organic compounds, including aldehydes and ketones.

• Carbonyl compounds are classified based on the atoms or groups bonded to the carbonyl carbon.

Polarity of the Carbonyl Group

• The carbonyl group is highly polar due to the difference in electronegativity between carbon and oxygen.

• The oxygen atom is more electronegative, pulling electron density away from the carbon, resulting in a partial negative charge on oxygen and a partial positive charge on carbon.

• This polarity makes the carbonyl carbon susceptible to nucleophilic attack.

Carbonyl-Containing Functional Groups

Overview and Classification

• Carbonyl groups are present in several functional groups, including:

• Aldehydes: Carbonyl group bonded to at least one hydrogen atom.

• Ketones: Carbonyl group bonded to two carbon atoms.

• Other groups: Carboxylic acids, esters, amides, and more (not covered in detail here).

Aldehydes and Ketones: Structure and Nomenclature

Definitions

• Aldehyde: An organic compound with a carbonyl group 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'.

Naming Aldehydes

• Common names are often used for simple aldehydes (e.g., formaldehyde, acetaldehyde).

• IUPAC names: Replace the -e ending of the parent alkane with -al.

• The carbonyl carbon is always carbon 1 in aldehydes.

• Examples: Methanal (formaldehyde), Ethanal (acetaldehyde).

Naming Ketones

• Common names: List the alkyl groups attached to the carbonyl carbon in alphabetical order, followed by 'ketone' (e.g., methyl ethyl ketone).

• IUPAC names: Replace the -e ending of the parent alkane with -one. Number the chain so the carbonyl carbon has the lowest possible number.

• Examples: Propanone (acetone), Butanone.

Properties of Aldehydes and Ketones

Physical Properties

• Aldehydes and ketones have higher boiling points than alkanes of similar molecular weight due to dipole-dipole interactions.

• They have lower boiling points than alcohols because they cannot form hydrogen bonds with each other.

• Small aldehydes and ketones are soluble in water due to their ability to form hydrogen bonds with water molecules.

• Solubility decreases as the carbon chain length increases.

Table: Comparison of Boiling Points

Compound	Boiling Point (°C)
Propane (alkane)	-42
Propanal (aldehyde)	49
Propanone (ketone)	56
Propanol (alcohol)	97

Hydrogen Bonding

• Aldehydes and ketones cannot hydrogen bond with themselves, but can form hydrogen bonds with water.

• This explains their moderate solubility in water.

Common Aldehydes and Ketones

Formaldehyde (HCHO)

• Colorless gas with a pungent odor; highly toxic.

• Used in the production of resins, disinfectants, and as a preservative.

Acetaldehyde (CH3CHO)

• Colorless, volatile liquid with a fruity odor.

• Produced naturally in plants and during alcohol metabolism in the liver.

Acetone (CH3COCH3)

• Colorless, volatile liquid; common solvent in laboratories and industry.

• Miscible with water and most organic solvents.

Benzaldehyde (C6H5CHO)

• Colorless liquid with an almond-like odor.

• Used in flavorings and perfumes.

Reactions of Aldehydes and Ketones

Oxidation of Aldehydes

• Aldehydes can be oxidized to carboxylic acids using mild oxidizing agents.

• Ketones are generally resistant to oxidation under similar conditions.

Key Oxidizing Agents

• Tollens' Test: Uses silver(I) ion (Ag+) in ammonia; positive result forms a silver mirror.

• Benedict's Reagent: Contains copper(II) ions; positive result forms a red precipitate of copper(I) oxide.

Reduction of Aldehydes and Ketones

• Aldehydes are reduced to primary alcohols; ketones are reduced to secondary alcohols.

• Common reducing agents: Sodium borohydride (NaBH4), lithium aluminum hydride (LiAlH4).

$\text{Aldehyde} + 2[H] \rightarrow \text{Primary Alcohol}$ $\text{Ketone} + 2[H] \rightarrow \text{Secondary Alcohol}$

Addition Reactions: Hemiacetals, Hemiketals, Acetals, and Ketals

Addition of Alcohols

• Aldehydes and ketones react with alcohols to form hemiacetals (from aldehydes) or hemiketals (from ketones).

• Further reaction with alcohol (in the presence of acid catalyst) forms acetals or ketals.

Hemiacetal/Hemiketal Formation

• Hemiacetals and hemiketals contain both an -OH and an -OR group on the same carbon.

• They are often unstable and revert to aldehyde/ketone and alcohol.

• In sugars, stable cyclic hemiacetals/hemiketals are common.

Acetal/Ketal Formation

• Acetals and ketals have two -OR groups attached to the same carbon.

• Formed by reaction of a hemiacetal/hemiketal with an additional alcohol under acidic conditions.

Acetal Hydrolysis

• Acetals and ketals can be hydrolyzed back to aldehydes/ketones and alcohols in the presence of acid and water.

$\text{Acetal} + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{Aldehyde/Ketone} + 2\text{Alcohols}$

Summary Table: Key Differences Between Aldehydes and Ketones

Property	Aldehyde	Ketone
General Structure	R-CHO	R-CO-R'
Oxidation	Readily oxidized to carboxylic acids	Resistant to oxidation
Reduction	Gives primary alcohol	Gives secondary alcohol
Location of Carbonyl	End of carbon chain	Within carbon chain

Key Terms

• Carbonyl group: C=O functional group.

• Aldehyde: Organic compound with a terminal carbonyl group.

• Ketone: Organic compound with an internal carbonyl group.

• Oxidation: Increase in the number of bonds to oxygen (or loss of hydrogen).

• Reduction: Increase in the number of bonds to hydrogen (or loss of oxygen).

• Hemiacetal/Hemiketal: Intermediate formed by addition of alcohol to aldehyde/ketone.

• Acetal/Ketal: Product formed by addition of a second alcohol to a hemiacetal/hemiketal.

Applications and Examples

• Aldehydes and ketones are found in many natural and synthetic compounds, including sugars, hormones, and solvents.

• They play important roles in metabolism and industrial chemistry.