Fiche de révision : Carbonyl Chemistry Masterclass

📋 Course Outline

1. Carbonyl Functional Group
2. Aldehydes and Ketones Nomenclature
3. Preparation Methods
4. Physical Properties
5. Chemical Reactions
6. Reactivity and Mechanisms
7. Carboxylic Acids Structure
8. Carboxylic Acids Reactions
9. Industrial Applications

📖 1. Carbonyl Functional Group

🔑 Key Concepts & Definitions

• Carbonyl group structure: A functional group characterized by a carbon atom double-bonded to an oxygen atom (>C=O). The carbon is sp2 hybridized, forming a trigonal planar geometry with bond angles approximately 120°, as shown in the orbital diagram for the formation of the carbonyl group (see source). (Source)

• Polarity of carbonyl group: The C=O bond is polarized due to oxygen's higher electronegativity, resulting in a partial positive charge on the electrophilic carbon and a partial negative charge on the nucleophilic oxygen. This polarization explains the reactivity of carbonyl compounds in nucleophilic addition reactions. The high dipole moment makes aldehydes and ketones more polar than ethers. (Source)

• Resonance structures of the carbonyl group: The carbonyl group can be represented by two resonance forms—a neutral structure and a dipolar form—highlighting the delocalization of electrons. The dipolar form, with a positive charge on carbon and a negative charge on oxygen, explains the polarity and electrophilicity of the carbonyl carbon. (Source)

• Differences in bonding in aldehydes and ketones: In aldehydes, the carbonyl carbon is bonded to one carbon and one hydrogen atom, whereas in ketones, it is bonded to two carbon atoms. This difference influences their chemical reactivity and physical properties, with aldehydes generally being more reactive due to less steric hindrance and greater electrophilicity. (Source)

📝 Essential Points

• The carbonyl group (>C=O) is fundamental in organic chemistry, serving as a core structure in aldehydes, ketones, carboxylic acids, and their derivatives. Its sp2 hybridization results in a trigonal planar structure with bond angles close to 120°, facilitating planar resonance and reactivity.

• The polarity of the C=O bond arises from oxygen's higher electronegativity, creating a dipole that makes the carbonyl carbon electrophilic and the oxygen nucleophilic. This polarity underpins the mechanism of nucleophilic addition reactions characteristic of aldehydes and ketones.

• Resonance stabilization involves the delocalization of electrons between the lone pairs on oxygen and the π-bond electrons of the C=O group, represented by neutral and dipolar resonance structures. This resonance explains the partial double-bond character and the polarity of the carbonyl group.

• In aldehydes, the bonding of the carbonyl carbon to hydrogen and carbon influences their reactivity, making aldehydes typically more reactive than ketones, which have two alkyl groups attached, providing steric hindrance and electron donation that reduce electrophilicity.

💡 Key Takeaway

The carbonyl group (>C=O) features a planar, polar double bond with resonance structures that explain its electrophilic nature and reactivity, with structural differences between aldehydes and ketones influencing their chemical behavior.

📖 2. Aldehydes and Ketones Nomenclature

🔑 Key Concepts & Definitions

• Common nomenclature of aldehydes: Derived from corresponding carboxylic acids by replacing -ic acid with -aldehyde. Greek letters (α, β, γ) indicate substituent positions, with α directly attached to the aldehyde group (source content).
• Common nomenclature of ketones: Named by identifying the two alkyl or aryl groups bonded to the carbonyl carbon, with Greek letters (α, β, γ) used to specify substituent positions relative to the carbonyl. Some ketones have historical names, e.g., acetone (source content).
• IUPAC nomenclature of aldehydes: Formed by replacing -e in the alkane name with -al, with the chain numbered from the aldehyde carbon (source content).
• IUPAC nomenclature of ketones: Formed by replacing -e in the alkane name with -one, with numbering starting from the end nearer to the carbonyl group (source content).
• Naming cyclic aldehydes and ketones: Cyclic aldehydes use the suffix carbaldehyde, with numbering starting at the carbonyl carbon; cyclic ketones are named by adding the suffix -one with the ring numbered starting at the carbonyl (source content).
• Naming aromatic aldehydes: The simplest aromatic aldehyde is benzenecarbaldehyde, with the common name benzaldehyde also accepted; aromatic aldehydes are named as substituted benzaldehydes (source content).

📝 Essential Points

• Common names of aldehydes are based on the corresponding carboxylic acids, with -ic acid replaced by -aldehyde, and Greek letters indicating substituent positions. For example, formic acid becomes formaldehyde.
• Ketone names involve identifying the two attached groups, with Greek letters (α, β, γ) used to specify their positions relative to the carbonyl. The historical name acetone is widely used for propanone.
• IUPAC aldehyde nomenclature: The longest chain is numbered from the aldehyde carbon, which is assigned position 1, and the suffix -al is used.
• IUPAC ketone nomenclature: The chain is numbered from the end nearer to the carbonyl group, with -one as the suffix. Substituents are indicated with numerals and alphabetically.
• Cyclic aldehydes are named with the suffix carbaldehyde, and numbering begins at the aldehyde carbon. Cyclic ketones are named by adding -one, with the ring numbered starting at the carbonyl carbon.
• Aromatic aldehydes: The simplest is benzenecarbaldehyde; the common name benzaldehyde is also accepted. Substituted derivatives are named accordingly, e.g., p-bromobenzaldehyde.

💡 Key Takeaway

Aldehydes and ketones are systematically named using common and IUPAC nomenclature, with specific rules for chain numbering, substituent positioning, and cyclic or aromatic structures, ensuring clarity and consistency in chemical communication.

📖 3. Preparation Methods

🔑 Key Concepts & Definitions

• Preparation of aldehydes by oxidation of primary alcohols: Primary alcohols are oxidized to aldehydes using mild oxidizing agents such as pyridinium chlorochromate (PCC) or chromium-based reagents, stopping at the aldehyde stage without further oxidation to carboxylic acids.

• Preparation of ketones by oxidation of secondary alcohols: Secondary alcohols are oxidized to ketones through oxidation with reagents like potassium dichromate (K₂Cr₂O₇) or potassium permanganate (KMnO₄), which selectively oxidize secondary alcohols to ketones.

• Dehydrogenation of alcohols over metal catalysts: Volatile alcohols undergo dehydrogenation in the presence of metal catalysts such as Ag or Cu, resulting in aldehydes (from primary alcohols) or ketones (from secondary alcohols), as described by AUTHOR (date).

• Ozonolysis of alkenes: Alkenes react with ozone (O₃) to cleave the carbon-carbon double bond, yielding aldehydes or ketones depending on the substitution pattern of the alkene, a process critical for structural elucidation and synthesis.

• Hydration of alkynes: Alkynes react with water in the presence of catalysts like H₂SO₄ and HgSO₄, leading to the formation of aldehydes (terminal alkynes) or ketones (internal alkynes), depending on the position of the triple bond.

• Rosenmund reduction: A selective hydrogenation process where acyl chlorides are reduced to aldehydes using Pd/BaSO₄ catalyst, allowing for controlled synthesis of aldehydes from acyl chlorides, as described by AUTHOR (date).

📝 Essential Points

• Aldehydes are prepared from primary alcohols via oxidation using mild oxidizing agents such as PCC, which prevents further oxidation to acids. This method is preferred for obtaining pure aldehydes in laboratory settings.

• Ketones are obtained by oxidizing secondary alcohols with strong oxidants like K₂Cr₂O₇ or KMnO₄, which oxidize secondary alcohols efficiently while leaving primary alcohols unaffected.

• Dehydrogenation over metal catalysts (Ag, Cu) is an industrial method for converting volatile alcohol vapors into aldehydes or ketones, providing a route for large-scale production.

• Ozonolysis of alkenes involves breaking the double bond with ozone, followed by reduction with zinc dust or water, resulting in aldehydes or ketones depending on the alkene's structure.

• Hydration of alkynes involves adding water across the triple bond in the presence of H₂SO₄ and HgSO₄, producing aldehydes if the alkyne is terminal, or ketones if internal.

• Rosenmund reduction offers a controlled method to synthesize aldehydes from acyl chlorides, avoiding over-reduction to the corresponding alcohols or hydrocarbons.

💡 Key Takeaway

Aldehydes and ketones can be efficiently prepared through specific oxidation, dehydrogenation, ozonolysis, and hydration reactions, with each method tailored to produce the desired carbonyl compound while maintaining selectivity and control over the reaction pathway.

📖 4. Physical Properties

🔑 Key Concepts & Definitions

• Boiling points of aldehydes and ketones (see source content): Higher than hydrocarbons and ethers of comparable molecular mass due to dipole-dipole interactions, but lower than alcohols because they lack intermolecular hydrogen bonding (source).
• Physical state variation: Methanal exists as a gas at room temperature, ethanal as a volatile liquid, while higher aldehydes and ketones are liquids or solids depending on their molecular size (source).
• Solubility in water: Lower aldehydes and ketones are soluble in water owing to hydrogen bonding with water molecules, but solubility decreases as alkyl chain length increases (source).
• Odour characteristics: Lower aldehydes have sharp, pungent odours; larger molecules tend to be more fragrant and are often used in perfumes and flavourings (source).

📝 Essential Points

• The boiling points of aldehydes and ketones are elevated compared to hydrocarbons and ethers because of dipole-dipole interactions arising from the polar carbonyl group (source).
• Despite their higher boiling points relative to hydrocarbons, aldehydes and ketones have lower boiling points than alcohols of similar molecular mass, due to the absence of intermolecular hydrogen bonding (source).
• The solubility of lower aldehydes and ketones in water is significant because they can form hydrogen bonds with water molecules, but as the alkyl chain lengthens, the nonpolar hydrocarbon portion dominates, reducing solubility (source).
• Methanal (formaldehyde) is gaseous at room temperature, ethanal (acetaldehyde) is a volatile liquid, and higher aldehydes and ketones are typically liquids or solids depending on their molecular weight and structure (source).
• The odour profile shifts from sharp and pungent in lower aldehydes to more fragrant in larger molecules, making them valuable in flavor and fragrance industries (source).

💡 Key Takeaway

Aldehydes and ketones exhibit higher boiling points than hydrocarbons and ethers due to dipole-dipole interactions, but lower than alcohols because they lack hydrogen bonding; their solubility in water diminishes with increasing alkyl chain length, and their odour characteristics vary from pungent to fragrant depending on molecular size.

📖 5. Chemical Reactions

🔑 Key Concepts & Definitions

• Nucleophilic addition reactions characteristic of aldehydes and ketones: Reactions where a nucleophile attacks the electrophilic carbon of the carbonyl group (>C=O), leading to the addition of the nucleophile across the double bond, forming an alcohol derivative (see mechanism below).

• Mechanism of nucleophilic addition to carbonyl carbon: The nucleophile approaches the electrophilic carbon atom of the carbonyl group from perpendicular to the plane of the sp2 hybridized orbitals, resulting in the formation of a tetrahedral alkoxide intermediate, which then captures a proton to give the neutral addition product (see orbital diagram in source).

• Electrophilic nature of carbonyl carbon and nucleophilic nature of oxygen: The carbon atom in the carbonyl group is electron-deficient due to the polarity of the C=O bond, making it an electrophile, while the oxygen atom, bearing lone pairs, acts as a nucleophile in certain reactions, especially in resonance structures (source).

• Comparison with electrophilic addition in alkenes: Unlike alkenes, where electrophiles add across the C=C double bond, aldehydes and ketones undergo nucleophilic addition at the electrophilic carbon of the carbonyl group, involving attack by nucleophiles rather than electrophiles (source).

• Reactions involving addition of nucleophiles to the carbonyl group: These include addition of hydrogen cyanide (HCN) to form cyanohydrins, addition of sodium hydrogensulphite for purification, and addition of Grignard reagents to form alcohols, all proceeding via nucleophilic attack on the carbonyl carbon (source).

📝 Essential Points

• Nucleophilic addition reactions are fundamental to the chemistry of aldehydes and ketones, enabling the formation of a wide variety of derivatives such as cyanohydrins, alcohols, and acetals.

• The mechanism involves the nucleophile attacking the electrophilic carbon of the >C=O group, which is polarized due to the electronegativity difference between carbon and oxygen, with the carbon carrying a partial positive charge.

• The orbital diagram illustrates the approach of the nucleophile perpendicular to the plane of the sp2 hybridized orbitals, leading to the formation of a tetrahedral intermediate.

• Aldehydes are generally more reactive than ketones in nucleophilic addition due to less steric hindrance and higher electrophilicity of the carbonyl carbon, which is influenced by the electron-donating or withdrawing nature of substituents.

• The addition of nucleophiles to the carbonyl group is a key step in many synthetic pathways, including the formation of alcohols via reduction and cyanohydrins via cyanide addition.

💡 Key Takeaway

Nucleophilic addition reactions are central to aldehyde and ketone chemistry, where the electrophilic carbonyl carbon is attacked by nucleophiles, forming various derivatives that are crucial in organic synthesis and biochemical processes. The mechanism involves attack perpendicular to the sp2 orbital plane, resulting in tetrahedral intermediates that lead to diverse functional groups.

📖 6. Reactivity and Mechanisms

🔑 Key Concepts & Definitions

• Structure of the Carboxyl Group (-COOH): The carboxyl group consists of a carbonyl carbon (C=O) bonded directly to a hydroxyl group (-OH). The carbon atom is sp2 hybridized, forming a planar structure with bond angles approximately 120°, and the group exhibits resonance stabilization, delocalizing the negative charge over the oxygen atoms (source content).

• Resonance Stabilization of the Carboxylate Ion: When a carboxylic acid loses a proton, it forms a carboxylate ion, which is stabilized by resonance. The negative charge is delocalized over the two oxygen atoms, distributing the charge evenly and increasing stability (source content).

• Acidity of Carboxylic Acids Influenced by Structure and Substituents: The acidity of carboxylic acids depends on the ability of substituents to stabilize the conjugate base (carboxylate ion). Electron-withdrawing groups (like -NO2, -Cl) increase acidity by stabilizing the negative charge, while electron-donating groups (like -CH3) decrease acidity (source content).

• Bonding and Hybridization in the Carboxyl Group: The carbon atom in the carboxyl group is sp2 hybridized, forming a planar structure with the carbonyl carbon and the attached oxygen atoms. The resonance delocalization involves p-orbitals, which stabilize the carboxylate ion and influence reactivity (source content).

• Derivatives of Carboxylic Acids: These include amides, acyl halides, esters, and anhydrides. They are formed by replacing the hydroxyl group (-OH) of the carboxyl group with other functional groups, affecting reactivity and properties. Their formation involves nucleophilic acyl substitution mechanisms (source content).

📝 Essential Points

• The structure of the carboxyl group with a carbonyl bonded to hydroxyl imparts acidity due to the ability of the conjugate base to be stabilized by resonance. The resonance stabilization of the carboxylate ion is a key factor in the acidity of carboxylic acids, as it disperses the negative charge over two oxygen atoms, making deprotonation more favorable (source content).

• The acidity of carboxylic acids is significantly affected by substituents attached to the carbon chain. Electron-withdrawing groups increase acidity by stabilizing the conjugate base, while electron-donating groups decrease acidity (source content).

• The bonding and hybridization in the carboxyl group involve the carbon being sp2 hybridized, with the p-orbitals participating in resonance. This delocalization stabilizes the negative charge in the carboxylate ion and influences the reactivity of derivatives (source content).

• Derivatives of carboxylic acids are formed through nucleophilic acyl substitution, where the hydroxyl group is replaced by other nucleophiles, leading to compounds like esters, amides, acyl halides, and anhydrides, each with distinct reactivity profiles (source content).

💡 Key Takeaway

The acidity and reactivity of carboxylic acids are primarily governed by their structure, especially the resonance stabilization of the carboxylate ion, which is influenced by substituents. The bonding and hybridization in the carboxyl group facilitate the formation of various derivatives, making carboxylic acids versatile in organic synthesis.

📖 7. Carboxylic Acids Structure

🔑 Key Concepts & Definitions

• Reactions of carboxylic acids (see section 8): Chemical processes involving carboxylic acids, including acid-base reactions, formation of derivatives, and reduction reactions, which alter or utilize the carboxyl functional group.

• Conversion to acyl chlorides: A reaction where carboxylic acids are transformed into acyl chlorides (acid chlorides) typically using reagents like thionyl chloride (SOCl₂). This process increases reactivity, facilitating further derivatization.

• Formation of esters: The reaction of carboxylic acids with alcohols in the presence of acid catalysts (e.g., sulfuric acid) to produce esters, which are important in fragrances and flavors.

• Formation of amides: Carboxylic acids react with ammonia or amines to produce amides, involving nucleophilic substitution at the carbonyl carbon, often requiring activation due to the low reactivity of the acid.

• Formation of anhydrides: When two carboxylic acid molecules react, they form anhydrides, which are more reactive derivatives used in acylation reactions and as intermediates in organic synthesis.

• Factors affecting reactivity of carboxylic acids: Structural features such as electron-withdrawing groups, hydrogen bonding, and steric hindrance influence acidity and reactivity, with resonance stabilization of the carboxylate ion playing a key role (see section 8).

📝 Essential Points

• Reactions of carboxylic acids include acid-base reactions where they act as acids donating protons to bases, forming carboxylate ions and water. They also undergo reactions to form derivatives like acyl chlorides, esters, amides, and anhydrides, each with specific mechanisms involving nucleophilic acyl substitution.

• Conversion to acyl chlorides is a common method to increase the electrophilicity of the carbonyl carbon, facilitating subsequent reactions such as esterification or amide formation. This transformation is typically achieved using thionyl chloride (SOCl₂).

• Formation of esters occurs via esterification, a reversible acid-catalyzed reaction with alcohols. Esters are widely used in industry for flavors, fragrances, and solvents.

• Amide formation involves nucleophilic attack by ammonia or amines on the activated carboxylic acid or its derivatives, often requiring prior conversion to acyl chlorides for efficiency.

• Anhydrides are prepared by dehydration of two carboxylic acids and are more reactive than acids, useful in acylation reactions. They are characterized by the presence of the -CO-O-CO- linkage.

• Factors influencing reactivity include the presence of electron-withdrawing groups that stabilize the carboxylate ion, hydrogen bonding that affects acidity, and steric hindrance that can slow down nucleophilic attack.

💡 Key Takeaway

Carboxylic acids undergo a variety of reactions to form derivatives such as acyl chlorides, esters, amides, and anhydrides, with their reactivity heavily influenced by structural and electronic factors, enabling their versatile use in organic synthesis and industry.

📖 8. Carboxylic Acids Reactions

🔑 Key Concepts & Definitions

• Side chain chlorination and hydrolysis (source content): A method to synthesize benzaldehyde by chlorinating the methyl group attached to a benzene ring, followed by hydrolysis to yield benzaldehyde. This process involves selective chlorination of the methyl side chain and subsequent hydrolysis of chlorinated intermediates.

• Use of chromyl chloride (Etard reaction): Etard (1902): A reaction where chromyl chloride (CrO2Cl2) oxidizes methyl groups attached to aromatic rings to form benzaldehyde. The methyl group is converted into a chromium complex, which hydrolyzes to give benzaldehyde.

• Use of chromic oxide in acetic anhydride: Chromic oxide (Cr2O3) in acetic anhydride reacts with aromatic methyl compounds to produce benzylidene diacetate, which upon hydrolysis yields benzaldehyde. This method allows selective oxidation of methyl groups on aromatic rings.

• Gatterman-Koch reaction: Gatterman and Koch (1904): A process where benzene or substituted benzenes react with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride to produce aromatic aldehydes, especially benzaldehyde.

• Applications of aldehydes, ketones, and carboxylic acids: These compounds are extensively used in flavors, fragrances, solvents, adhesives, paints, resins, perfumes, plastics, and fabrics due to their aromatic and reactive properties.

📝 Essential Points

• Preparation of benzaldehyde:

  • Side chain chlorination and hydrolysis is a common industrial method, where methyl groups on aromatic rings are chlorinated and then hydrolyzed to form aldehydes.
  • Etard reaction involves chromyl chloride oxidizing methyl groups on aromatic rings to benzaldehyde, offering a selective and efficient synthesis.
  • Chromic oxide in acetic anhydride converts methyl groups to benzylidene diacetate, which hydrolyzes to benzaldehyde, providing a controlled oxidation process.
  • Gatterman-Koch reaction employs carbon monoxide and hydrogen chloride with catalysts to produce aromatic aldehydes directly from benzene derivatives.

• Applications:

  • Used in flavoring agents (vanillin, cinnamaldehyde), fragrances, and as solvents.
  • Integral in manufacturing plastics, resins, perfumes, and dyes due to their reactive aldehyde groups.

• Significance:

  • These methods highlight the importance of selective oxidation and chlorination in industrial organic synthesis, especially for aromatic aldehydes like benzaldehyde.

💡 Key Takeaway

Industrial synthesis of benzaldehyde relies heavily on side chain chlorination, chromyl chloride oxidation, and the Gatterman-Koch reaction, which are crucial for producing aromatic aldehydes used across various industries such as flavors, fragrances, and plastics.

📖 9. Industrial Applications

🔑 Key Concepts & Definitions

Preparation of aldehydes from nitriles using selective reduction (DIBAL-H):
A method where nitriles are reduced to aldehydes by diisobutylaluminium hydride (DIBAL-H), a selective reducing agent that stops at the aldehyde stage, preventing further reduction to alcohols or hydrocarbons.

Preparation of ketones from acyl chlorides using dialkylcadmium:
A process involving the reaction of acyl chlorides with dialkylcadmium compounds, resulting in the formation of ketones through nucleophilic acyl substitution, as described in industrial synthesis pathways.

Preparation of ketones from nitriles via Grignard reagent and hydrolysis:
Nitriles react with Grignard reagents (organomagnesium halides) to form magnesium imine intermediates, which upon hydrolysis yield ketones, enabling efficient industrial synthesis of ketones from nitrile precursors.

Friedel-Crafts acylation reaction to prepare aromatic ketones:
A catalytic process where acyl chlorides or anhydrides react with aromatic compounds in the presence of a Lewis acid catalyst (e.g., AlCl₃), producing aromatic ketones used in manufacturing fragrances, pharmaceuticals, and polymers.

📅 Key Dates

(OMITTED: No significant dates or chronological events provided in the content)

📊 Synthesis Tables

⚠️ Common Pitfalls & Confusions

1. Confusing aldehyde and ketone nomenclature; forgetting aldehyde suffix -al and ketone -one.
2. Assuming aldehydes are less reactive than ketones; aldehydes are generally more reactive due to less steric hindrance.
3. Misidentifying the position of substituents in nomenclature, especially in cyclic or aromatic compounds.
4. Overlooking the specific oxidation conditions that stop at aldehyde stage (e.g., PCC) versus those that produce acids.
5. Confusing preparation methods: mistaking oxidation of secondary alcohols for primary alcohols.
6. Misapplying the rules for numbering in IUPAC nomenclature, especially in cyclic structures.
7. Assuming all aldehydes are aromatic or all ketones are aliphatic; both can be aromatic or aliphatic.
8. Overgeneralizing reactivity mechanisms without considering resonance stabilization effects.

✅ Exam Checklist

• Know the structure and hybridization of the carbonyl group (>C=O) as described by orbital diagrams.
• Understand the polarity of the carbonyl group and how resonance structures explain its electrophilicity.
• Master the differences in bonding and reactivity between aldehydes and ketones, including their physical properties.
• Be able to name aldehydes and ketones using both common and IUPAC nomenclature, including cyclic and aromatic derivatives.
• Recall key authors and concepts: e.g., Rosenmund reduction, AUTHOR (date) for dehydrogenation.
• Know the methods for preparing aldehydes: oxidation of primary alcohols with PCC, ozonolysis, hydration of alkynes.
• Understand how ketones are prepared via oxidation of secondary alcohols and other methods.
• Recognize the significance of oxidation states and reagents used in preparation.
• Be familiar with the industrial applications of aldehydes and ketones, such as in fragrances and solvents.
• Be able to distinguish aldehydes from ketones based on their reactivity and structure.
• Understand the role of resonance in stabilizing the carbonyl group and influencing reactivity.
• Know the specific rules for nomenclature of cyclic and aromatic aldehydes and ketones.
• Be prepared to explain the mechanisms of nucleophilic addition reactions involving carbonyl compounds.