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Organic Chemistry - Chapter 18: Ketones and Aldehydes

Carbonyl Compounds

  • Carbonyl compounds feature a carbon-oxygen double bond (C=O).

  • Classes of Carbonyl Compounds:

    • Ketones: R-C(=O)-R'
    • Aldehydes: R-C(=O)-H
    • Carboxylic Acids: R-C(=O)-OH
    • Acid Chlorides: R-C(=O)-Cl
    • Esters: R-C(=O)-O-R'
    • Amides: R-C(=O)-NH₂

Structure of the Carbonyl Group

  • Ketones: Two alkyl groups bonded to a carbonyl group.

  • Aldehydes: One alkyl group and one hydrogen atom bonded to a carbonyl group.

  • Carbon is sp² hybridized, resulting in a trigonal planar geometry around the carbonyl carbon.

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

Resonance

  • The carbonyl group can be represented by two resonance structures. The first resonance structure (with all atoms completing the octet and no charges) is the more stable and significant contributor.

  • The carbonyl carbon has a partial positive charge (δ+) and acts as an electrophile, making it susceptible to nucleophilic attack.

Dipole Moments

  • Due to the polar C=O bond, ketones and aldehydes have significant dipole moments.

  • Examples:

    • Acetaldehyde: μ = 2.7 D
    • Acetone: μ = 2.9 D
    • Chloromethane: μ = 1.9 D
    • Dimethyl ether: μ = 1.30 D

Ketone Nomenclature

  • Number the carbon chain such that the carbonyl carbon has the lowest possible number.

  • Replace the alkane suffix "-e" with "-one".

Cyclic Ketone Nomenclature

  • For cyclic ketones, the carbonyl carbon is assigned the number 1.

  • When a compound has both a carbonyl group and a double bond, the carbonyl group takes precedence in numbering.

Aldehydes Nomenclature

  • The aldehyde carbon is always numbered as 1.

  • IUPAC nomenclature: Replace the alkane suffix "-e" with "-al".

  • If the aldehyde group is attached to a ring, the suffix "-carbaldehyde" is used.

Carbonyl as Substituent

  • When a molecule contains a higher-priority functional group, a ketone is named as an "oxo" substituent and an aldehyde as a "formyl" substituent.

  • Aldehydes have higher priority than ketones in nomenclature.

Priority of Functional Groups in Naming Organic Compounds

  • Priority (highest to lowest): acids > esters > aldehydes > ketones > alcohols > amines > alkenes, alkynes > alkanes > ethers > halides

Common Names for Ketones

  • Ketones are often named by identifying the alkyl groups attached to the carbonyl group (—C=O).

  • Greek letters (α, β, γ, etc.) are used instead of numbers to indicate the position of substituents.

Common Names of Acids and Aldehydes

  • Examples of common names and their derivations:
    • Formic acid (methanoic acid): Derived from formica (ants); corresponding aldehyde is formaldehyde (methanal).
    • Acetic acid (ethanoic acid): Derived from acetum (sour); corresponding aldehyde is acetaldehyde (ethanal).
    • Propionic acid (propanoic acid): Derived from protos pion (first fat); corresponding aldehyde is propionaldehyde (propanal).
    • Butyric acid (butanoic acid): Derived from butyrum (butter); corresponding aldehyde is butyraldehyde (butanal).
    • Benzoic acid: Derived from gum benzoin ("blending"); corresponding aldehyde is benzaldehyde.

Historical Common Names

  • Acetone (CH₃-C(=O)-CH₃)

  • Acetophenone

  • Propiophenone

  • Benzophenone

  • Examples of historical names and IUPAC names:

    • β-bromobutyraldehyde (common) / 3-bromobutanal (IUPAC)
    • α-methoxypropionaldehyde / 2-methoxypropanal

Boiling Points

  • Ketones and aldehydes are more polar than alkanes or ethers due to the C=O bond, resulting in higher boiling points.

  • Ketones and aldehydes cannot hydrogen-bond to each other, so their boiling points are lower than those of comparable alcohols.

Solubility of Ketones and Aldehydes

  • Ketones and aldehydes are good solvents for alcohols because the carbonyl oxygen can accept hydrogen bonds from O—H or N—H groups.

  • Acetone and acetaldehyde are miscible in water.

Physical Properties of Ketones

  • Table of various ketones, including IUPAC name, common name, structure, melting point (°C), boiling point (°C), density (g/cm³), and solubility in H₂O (%).

Physical Properties of Aldehydes

  • Table of various aldehydes, including IUPAC name, common name, structure, melting point (°C), boiling point (°C), density (g/cm³), and solubility in H₂O (%).

Formaldehyde

  • Formaldehyde is a gas at room temperature.

  • Formalin is a 40% aqueous solution of formaldehyde.

  • Trioxane is a cyclic trimer of formaldehyde; heating trioxane generates formaldehyde.

Infrared (IR) Spectroscopy

  • Strong C=O stretch around 1710 cm^{-1} for ketones and 1725 cm^{-1} for simple aldehydes.

  • Additional C—H stretches for aldehydes: two absorptions at 2710 cm^{-1} and 2810 cm^{-1}.

IR Spectra

  • Conjugation lowers the carbonyl stretching frequencies to about 1685 cm^{-1}.

  • Rings with ring strain have higher C=O stretching frequencies.

Proton NMR Spectra

  • Aldehyde protons (—CHO) typically absorb between δ 9 and δ 10 ppm.

  • Protons on the α-carbon usually absorb between δ 2.1 and δ 2.4 ppm if no other electron-withdrawing groups are nearby.

¹H NMR Spectroscopy

  • Protons closer to the carbonyl group are more deshielded and appear at higher δ values.

  • The α, β, and γ protons appear at δ values that decrease with increasing distance from the carbonyl group.

Carbon NMR Spectra of Ketones

  • The spin-decoupled carbon NMR spectrum of 2-heptanone shows the carbonyl carbon at 208 ppm and the α-carbon at 30 ppm (methyl) and 44 ppm (methylene).

Mass Spectrometry (MS)

  • Fragmentation patterns in mass spectrometry can provide structural information.

  • Example: Fragmentation of a ketone (CH₃-C(=O)-CH₂CH₃)

    • Acylium ion (CH₃C=O⁺) at m/z 43 (base peak)
    • Loss of ethyl radical (29 amu): radical cation at m/z 72
    • Other fragmentations and corresponding m/z values are possible.

MS for Butyraldehyde

  • Important fragment ions:
    • m/z 72 (M⁺)
    • m/z 57 due to β, γ cleavage, stabilized cation
    • m/z 44 (base peak) + loss of 28 via McLafferty rearrangement
    • m/z 29

McLafferty Rearrangement

  • A characteristic rearrangement in the mass spectra of ketones and aldehydes.

  • Involves the breaking of the α, β bond and the transfer of a proton from the γ-carbon to the oxygen.

  • Results in the formation of an alkene through tautomerization of the enol.

Ultraviolet Spectra of Conjugated Carbonyl Compounds

  • Conjugated carbonyl compounds exhibit characteristic π → π* absorption in the UV spectrum.

  • An additional conjugated C=C bond increases λmax by about 30 nm; an additional alkyl group increases it by about 10 nm.

Electronic Transitions of the C=O

  • Small molar absorptivity reflects a "forbidden" transition occurring less frequently.

Grignards As a Source for Ketones and Aldehydes

  • A Grignard reagent can be used to make an alcohol, which can then be easily oxidized to a ketone or aldehyde.

Oxidation of Primary Alcohols to Aldehydes

  • Pyridinium chlorochromate (PCC) is selectively used to oxidize primary alcohols to aldehydes.

  • The Swern oxidation is an alternative method.

Ozonolysis of Alkenes

  • The double bond is oxidatively cleaved by ozone, followed by reduction.

  • Ketones and aldehydes can be isolated as products under these conditions.

Friedel–Crafts Reaction

  • The reaction between an acyl halide and an aromatic ring produces a ketone.

Hydration of Alkynes

  • The initial product of Markovnikov hydration is an enol, which quickly tautomerizes to its keto form.

  • Internal alkynes can be hydrated, but mixtures of ketones often result.

Hydroboration-Oxidation of Alkynes

  • Hydroboration-oxidation of an al