Carbonyl Compounds: Aldehydes and Ketones I – Structure, Properties, Synthesis, and Reactions

Carbonyl Compounds: Aldehydes and Ketones

Introduction

Carbonyl compounds are a fundamental class of organic molecules characterized by the presence of a carbonyl group (C=O). This group imparts unique chemical and physical properties to the compounds, influencing their reactivity and applications in organic synthesis. Aldehydes and ketones are two major categories of carbonyl compounds, each with distinct structural features and reactivity profiles.

Structure and Bonding

General Structure

• Aldehyde: Contains a carbonyl group bonded to at least one hydrogen atom. General formula: R-CHO.

• Ketone: Contains a carbonyl group bonded to two hydrocarbon groups (alkyl or aryl). General formula: R-CO-R'.

Bonding Characteristics

• The carbonyl carbon is sp2 hybridized, resulting in a trigonal planar geometry (bond angle ≈ 120°).

• The unhybridized p orbital of carbon overlaps with a p orbital of oxygen to form a π (pi) bond.

• The C=O bond is shorter, stronger, and more polarized than the C=C bond in alkenes.

• Bond Lengths: Ketone C=O bond ≈ 1.23 Å; Alkene C=C bond ≈ 1.34 Å.

Polarization: The difference in electronegativity between carbon and oxygen leads to a partial positive charge on carbon and a partial negative charge on oxygen, making the carbonyl group highly reactive toward nucleophiles.

Nomenclature of Aldehydes and Ketones

General Rules

• Aldehydes: Named by replacing the terminal -e of the parent alkane with -al (alkanal).

• Ketones: Named by replacing the terminal -e of the parent alkane with -one (alkanone).

• The parent chain must contain the carbonyl group.

• Number the chain from the end nearest the carbonyl group.

• List substituents alphabetically with their position numbers.

• When an aldehyde is a substituent on a ring, it is referred to as a carbaldehyde group.

Nomenclature Examples

• Aldehyde Priority: The –CHO group has higher priority than most other functional groups (except acids and esters).

• Examples:

  • 4-bromo-3-methylheptanal

  • 3-hydroxybutanal

  • 2-pentenal or pent-2-enal

• Cyclic Aldehydes: If the carbonyl carbon is attached to a ring, the compound is named as a cycloalkanecarbaldehyde (e.g., cyclohexanecarbaldehyde, benzene carbaldehyde/benzaldehyde).

• Ketone Priority: The ketone group has higher priority than ethers and alkanes, but lower than acids, esters, and aldehydes.

• Examples:

  • 3-hexanone

  • 2,4-dimethyl-3-pentanone

  • 2-methylcyclopentanone

Physical Properties and Effects of Polarity

Polarity and Intermolecular Forces

• The C=O bond is highly polar due to the electronegativity difference between carbon and oxygen.

• This polarity leads to strong dipole-dipole interactions, resulting in higher boiling points for aldehydes and ketones compared to ethers and alkanes of similar molecular weight.

• Although aldehydes and ketones cannot form hydrogen bonds with themselves, they can form hydrogen bonds with water and alcohols, increasing their solubility in aqueous media.

Comparative Physical Properties

• Boiling Points: Alcohols > Aldehydes/Ketones > Ethers > Alkanes

• Water Solubility: Alcohols > Aldehydes/Ketones > Ethers > Alkanes

Formula	Common Name	Boiling Point (°C)
CH3CH2CH3	Propane	-42
CH3OCH3	Methoxyethane	49
CH3CH2CHO	Propanal	49
CH3COCH3	Acetone	56
CH3CH2CH2OH	1-Propanol	97

Preparation of Aldehydes and Ketones

Ozonolysis of Alkenes

• Reactants: Alkene and ozone (O3).

• Products: Aldehyde and/or ketone.

• Conditions: Zinc and acid (Zn/H+).

Example Reaction:

$RCH=CHR' + O_3 \xrightarrow{Zn/H^+} RCHO + R'CHO$

Friedel-Crafts Acylation

• Reactants: Benzene and acid chloride (RCOCl).

• Product: Aromatic ketone.

• Conditions: Lewis acid catalyst (AlCl3).

Example Reaction:

$C_6H_6 + CH_3COCl \xrightarrow{AlCl_3} C_6H_5COCH_3 + HCl$

Reduction of Carboxylic Acid Derivatives

• Reactants: Acid chloride (RCOCl).

• Product: Aldehyde (RCHO).

• Conditions: H2-Pd/BaSO4 (Rosenmund Reduction).

Example Reaction:

$RCOCl \xrightarrow{H_2, Pd/BaSO_4} RCHO$

Oxidation of Aldehydes and Ketones

General Oxidation Reactions

• Aldehydes: Can be oxidized to carboxylic acids by various oxidizing agents.

• Ketones: Generally resistant to oxidation under mild conditions.

Aqueous Oxidants

• Chromic acid (H2CrO4), chromate salts (CrO42-), dichromate salts (Cr2O72-), permanganate (MnO4-).

• Observation: Aldehyde turns orange solution green; ketone shows no change.

Compound	Observation with Dichromate
Ketone	No change in orange solution
Aldehyde	Orange solution turns green

Tollen's Test

• Reagent: Diamminesilver(I) ion, [Ag(NH3)2]+.

• Reaction: Aldehyde is oxidized to carboxylic acid; Ag+ is reduced to metallic silver, forming a mirror-like deposit.

$2Ag(NH_3)_2^+ + RCHO + 3OH^- \rightarrow 2Ag + RCOO^- + 4NH_3 + 2H_2O$

Fehling's and Benedict's Tests

• Reagents: Cu2+ ions complexed with tartrate (Fehling's) or citrate (Benedict's).

• Reaction: Aldehyde reduces Cu2+ to Cu2O (brick red precipitate); ketones do not react.

Compound	Observation
Ketone	No change in blue solution
Aldehyde	Dark red precipitate of copper(I) oxide

Note: Aromatic aldehydes are not oxidized by Fehling's or Benedict's reagents.

Relative Reactivity of Aldehydes and Ketones

Resonance and Reactivity

• The carbonyl group has resonance contributors, with the minor form showing a positive charge on carbon and a negative charge on oxygen.

• This makes the carbonyl carbon electrophilic and susceptible to nucleophilic attack.

• Substituent Effects:

  • Steric factor: Bulky groups hinder nucleophilic attack.

  • Electronic factor: Electron-donating groups decrease reactivity; electron-withdrawing groups increase reactivity.

Nucleophilic Addition Reactions

General Mechanism

• Nucleophile attacks the electrophilic carbonyl carbon, forming a new σ bond.

• The π bond breaks, resulting in an alkoxide intermediate.

• Protonation of the alkoxide yields an alcohol derivative.

Types of Catalysis

• Base-catalyzed: Nucleophile is activated under basic conditions.

• Acid-catalyzed: Carbonyl oxygen is protonated, increasing electrophilicity.

Addition of Water (Hydration)

• Reactants: Aldehyde or ketone and H2O.

• Product: Hydrate (geminal diol).

Example: Acetaldehyde + H2O → hydrate (ethane-1,1-diol)

Addition of Alcohol (Hemiacetal and Acetal Formation)

• Hemiacetal Formation: Aldehyde or ketone + 1 molar equivalent of ROH → hemiacetal.

• Acetal Formation: Aldehyde or ketone + 2 molar equivalents of ROH → acetal (geminal diether).

Example: Butanone + CH3OH → 2-methoxy-2-butanol (hemiacetal); further reaction yields acetal.

Addition of Grignard Reagent (RMgX)

• Reactants: Grignard reagent (RMgX) and carbonyl compound.

• Steps: Addition of RMgX, followed by protonation with H3O+.

• Product: Alcohol (primary, secondary, or tertiary depending on the carbonyl compound).

$R_2C=O + R'MgX \rightarrow R_2C(OMgX)R' \xrightarrow{H_3O^+} R_2C(OH)R'$

Addition of HCN (Cyanohydrin Formation)

• Reactants: Aldehyde or ketone and HCN.

• Product: Cyanohydrin (tetrahedral intermediate, then protonated).

• Mechanism: Base-catalyzed generation of CN-, nucleophilic attack on C=O, followed by protonation.

Example: Benzaldehyde + HCN → mandelonitrile (cyanohydrin).

Addition of Sodium Bisulfite (NaHSO3)

• Reactants: Aldehyde or ketone and sodium bisulfite.

• Product: Bisulfite addition compound (water-soluble derivative).

Example: Ethanal + NaHSO3 → ethanal sodium bisulfite.

Summary Table: Key Reactions and Tests

Test/Reagent	Positive for Aldehyde	Positive for Ketone
Dichromate (VI)	Orange to green	No change
Tollen's Test	Silver mirror	No change
Fehling's/Benedict's	Brick red ppt	No change

References

• Carey FA. Organic Chemistry, 9th ed. McGraw Hill, 2014.

• Mur J. Organic Chemistry, 7th ed. Books Cole, 2012.

Additional info: This guide covers the structure, nomenclature, physical properties, synthesis, oxidation, and nucleophilic addition reactions of aldehydes and ketones, suitable for college-level Organic Chemistry students.