Aldehydes and Ketones: Structure, Reactivity, and Nucleophilic Addition Reactions

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Aldehydes and Ketones

Introduction to Carbonyl Compounds

Aldehydes and ketones are organic compounds that both contain the carbonyl group (C=O). The carbonyl group is a key functional group in organic chemistry, influencing the reactivity and properties of these molecules. Aldehydes have the carbonyl group at the end of a carbon chain, while ketones have it within the chain.

Aldehyde: R-CHO (carbonyl at terminal position)
Ketone: R-CO-R' (carbonyl at non-terminal position)
Both are important in biology (e.g., sugars, hormones) and industry (e.g., solvents, preservatives).
Examples: Formaldehyde (preservative), Acetone (solvent)

Common Aldehydes and Ketones

Many biologically and industrially relevant molecules are aldehydes or ketones:

Vanillin (vanilla flavor, aldehyde)
Cinnamaldehyde (cinnamon flavor, aldehyde)
(R)-Carvone (spearmint flavor, ketone)
Benzaldehyde (almond flavor, aldehyde)
Progesterone and Testosterone (steroid hormones, ketones)

Nomenclature of Aldehydes and Ketones

Systematic Naming Steps

Naming aldehydes and ketones follows IUPAC rules similar to those for alkanes and alcohols:

Identify and name the parent chain (must include the carbonyl carbon).
Identify and name substituents (side groups).
Assign a locant (number) to each substituent, giving the carbonyl carbon the lowest possible number.
Assemble the name alphabetically.

Note: The aldehyde group is often abbreviated as –CHO in structures.

Suffixes and Numbering

For aldehydes, replace the “-e” ending with “-al” (e.g., butanal).
For ketones, replace the “-e” ending with “-one” (e.g., butanone).
The carbonyl carbon in aldehydes is always assigned position 1.
For ketones, the position of the carbonyl is indicated by a number (e.g., 2-butanone).

Special Cases

If the aldehyde group is attached to a ring, use the suffix carbaldehyde (e.g., cyclohexanecarbaldehyde).
IUPAC recognizes some common names as parent names: formaldehyde, acetaldehyde, benzaldehyde.

Preparation of Aldehydes and Ketones

Methods of Synthesis

Aldehydes and ketones can be prepared by various oxidation and synthetic methods:

Oxidation of primary alcohols yields aldehydes (e.g., PCC oxidation).
Oxidation of secondary alcohols yields ketones.
Other methods include ozonolysis of alkenes, hydration of alkynes, and Friedel-Crafts acylation.

Carbonyl Group Reactivity

Electronic Structure and Polarity

The carbonyl group is highly reactive due to its polarization:

Both C and O are sp2 hybridized; the bond angle is approximately 120°.
The C=O bond is polarized toward the more electronegative oxygen, creating a partial positive charge on carbon and a partial negative charge on oxygen.
This polarization makes the carbonyl carbon electrophilic and susceptible to nucleophilic attack.

Nucleophilic Addition Mechanism

Nucleophilic addition is the key reaction pattern for aldehydes and ketones:

A nucleophile attacks the electrophilic carbonyl carbon, forming a tetrahedral intermediate.
There are two main variations based on reaction conditions:

Condition	Nucleophile	Mechanism Features
Basic	Strong nucleophile (e.g., OH-)	Irreversible addition, poor leaving group
Acidic	Weak nucleophile (e.g., H2O, ROH)	Carbonyl is protonated to increase reactivity

Drawing Mechanisms

Under acidic conditions, mechanisms must avoid strong bases (e.g., OH-).
Under basic conditions, mechanisms must avoid strong acids (e.g., H3O+).

Reactivity: Aldehydes vs. Ketones

Comparative Reactivity

Aldehydes are generally more reactive toward nucleophiles than ketones.
Steric effects: Aldehydes are less hindered (only one alkyl group).
Electronic effects: Aldehydes have a larger partial positive charge on the carbonyl carbon.

Nucleophilic Addition Reactions

Types of Nucleophiles

Aldehydes and ketones react with various nucleophiles:

Oxygen nucleophiles: H2O, ROH (alcohols), diols
Nitrogen nucleophiles: RNH2 (amines)
Hydrogen nucleophiles: Hydride (H-)
Carbon nucleophiles: Grignard reagents (RMgX), cyanide (CN-)

Oxygen Nucleophiles: Hydrates and Acetals

Hydrate Formation

In the presence of water, aldehydes and ketones can form hydrates:

General reaction:
Equilibrium does not favor hydrate formation except for simple aldehydes.
Reaction rate is slow unless catalyzed by acid or base.

Mechanism

Basic conditions: OH- attacks carbonyl carbon.
Acidic conditions: Carbonyl is protonated, H2O attacks.

Acetal Formation

Aldehydes and ketones react with alcohols (in acid) to form acetals:

Two equivalents of alcohol are required.
Reaction proceeds via a hemiacetal intermediate.
Acetals have a carbon bonded to two alkoxy (RO-) groups.
Hemiacetals have one alkoxy and one hydroxyl group.

Compound	Structure	Key Features
Acetal	R2C(OR)2	Two alkoxy groups
Hemiacetal	R2C(OH)(OR)	One alkoxy, one hydroxyl

Mechanism

Formation of hemiacetal: Protonated carbonyl undergoes nucleophilic addition by alcohol.
Conversion to acetal: Hemiacetal loses water, second alcohol adds.

Equilibrium and Reversibility

Acetal formation is reversible and controlled by water removal/addition.
For simple aldehydes, acetals are favored at equilibrium; for ketones, reactants are favored.
Acetals can be hydrolyzed back to aldehydes/ketones by adding water and acid.

Cyclic Acetals

If a diol is used, both equivalents of alcohol come from the same molecule, forming a cyclic acetal.
Cyclic hemiacetals and acetals can form intramolecularly if a molecule contains both a carbonyl and a hydroxyl group.
Carbohydrates (e.g., glucose) undergo this process to form stable rings.

Nitrogen Nucleophiles: Imines

Addition of Primary Amines

Under acidic conditions, aldehydes/ketones react with primary amines to form imines:

General reaction:
Water is released during the transformation.
Imines are important in biochemistry (e.g., Schiff bases).

Mechanism

Nucleophilic attack by amine on carbonyl carbon.
Proton transfers and loss of water yield the imine.

Effect of pH

Optimal pH for imine formation is 4–5.
Too low: amines are protonated, cannot attack carbonyl.
Too high: not enough acid to catalyze reaction.

Reversibility

Imines can be hydrolyzed back to aldehydes/ketones by adding water and acid.
Mechanism of hydrolysis is the reverse of formation.

Hydride Nucleophiles: Reduction of Aldehydes and Ketones

Hydride Addition and Reducing Agents

Hydride (H-) can reduce aldehydes and ketones to alcohols via nucleophilic addition:

Common reducing agents: Lithium Aluminum Hydride (LiAlH4) and Sodium Borohydride (NaBH4).
LiAlH4 is more powerful and reacts explosively with water; used in dry ether solvents.
NaBH4 is milder and used in alcohol solvents.

Reagent	Solvent	Products
NaBH4	Alcohol	Primary alcohol (from aldehyde), secondary alcohol (from ketone)
LiAlH4	Ether (THF)	Primary alcohol (from aldehyde), secondary alcohol (from ketone)

Mechanism

Hydride attacks carbonyl carbon, forming an alkoxide intermediate.
Protonation yields the alcohol product.
Reaction is irreversible due to poor leaving ability of hydride.

Selectivity

Catalytic hydrogenation (e.g., Pd/C) can also reduce C=O to alcohol.
LiAlH4 and NaBH4 do not reduce C=C bonds.

Carbon Nucleophiles: Grignard Reagents and Cyanide

Grignard Reagents

Grignard reagents (RMgX) are prepared by reacting alkyl halides with magnesium in anhydrous ether:

Grignard reagents act as strong nucleophiles and bases.
They attack carbonyl carbons to form new C–C bonds.
General reaction: (after aqueous workup)
Care must be taken to avoid water/alcohols, which destroy the reagent.

Cyanide Addition: Cyanohydrin Formation

Cyanide ion (CN-) acts as a carbon nucleophile, adding to carbonyls to form cyanohydrins.
General reaction:
Reaction is typically base-catalyzed.

Summary Table: Nucleophilic Addition to Aldehydes and Ketones

Nucleophile	Product	Key Features
H2O	Hydrate	Equilibrium, slow unless catalyzed
ROH	Acetal	Requires acid, reversible
RNH2	Imine	Acid catalyzed, pH sensitive
H- (LiAlH4, NaBH4)	Alcohol	Irreversible, reduction
RMgX	Alcohol (after workup)	New C–C bond formation
CN-	Cyanohydrin	Base catalyzed, reversible

Additional info:

Carbohydrates such as glucose form cyclic hemiacetals and acetals, which are crucial for their ring structures.
Acetals are used in synthesis to temporarily protect carbonyl groups from unwanted reactions.