Chapter 12: Carbonyl Compounds – Aldehydes & Ketones

Overview of Carbonyl-Containing Compounds

• Carbonyl functional group (C=O) is central to five important classes of organic compounds:
  • Aldehydes
  • Ketones
  • Carboxylic Acids
  • Esters
  • Amides
• All share the same right-hand side (an alkyl chain, $R$) but differ on the left-hand side of the C=O, giving each class unique chemical & physical properties.

Structure & Geometry of the Carbonyl Group

• Lewis structure: R{-}C!
  ||=O with a second substituent on the carbon.
• Carbonyl C is sp$^2$ hybridised → trigonal-planar geometry; all bond angles ≈ $120^\circ$.
• Oxygen is highly electronegative (EN $=3.5$) vs. carbon (EN $=2.5$) → polar C=O bond with a permanent dipole.

Categories Determined by the Left-Hand Substituent

• Aldehyde: $R{-}C(=O)H$ (substituent = H)
• Ketone: $R{-}C(=O)R'$ (substituent = another alkyl)
• Carboxylic Acid: $R{-}C(=O)OH$
• Ester: $R{-}C(=O)OR'$ (note: $R$ and $R'$ may differ)
• Amide: $R{-}C(=O)N(R'')(R''')$ (N can be $NH<em>2$, $NHR$, or $NR</em>2$)

Aldehydes

Definition & General Formula

• Carbonyl carbon bonded to one alkyl chain and one hydrogen.
• Condensed notation: $RCHO$.

Representative Models

• Ball-and-stick examples: formaldehyde, acetaldehyde, propionaldehyde — identical right side (C=O–H), differing alkyl chain length.

IUPAC Nomenclature Rules

1. Identify longest chain containing the aldehyde carbon (it must include C=O).
2. Carbonyl carbon is always position 1; no locant needed in the name.
3. Replace terminal -e of parent alkane with -al.
   • $1$ C: methanal (common: formaldehyde)
   • $2$ C: ethanal (common: acetaldehyde)
   • $3$ C: propanal
   • Branched example: 4-ethyl-3-methylhexanal (alphabetise substituents, lowest possible numbers)

Physical Properties & Intermolecular Forces

• C=O dipole gives dipole-dipole attractions.
• Cannot H-bond internally (no O–H) → weaker forces than alcohols, stronger than hydrocarbons.
• Consequences:
  • Boiling points: \text{hydrocarbon} < \text{aldehyde} < \text{alcohol} of comparable molar mass.
  • Water solubility: moderate when $R$ is small; decreases as $R$ length increases.

Worked Problem: Boiling-Point Ranking

• Compounds: heptane < hexanal < 1-hexanol.
  • Forces: London < dipole-dipole < H-bonding.
  • Therefore $T_b$ trend matches force strength.

Ketones

Definition & General Formula

• Carbonyl carbon bonded to two alkyl chains (may be same or different).
• Condensed notation: $RC(=O)R'$.

Simple & Common Ketones

• Acetone (2-propanone): smallest ketone, solvent, nail-polish remover; formula $CH<em>3COCH</em>3$.
• Other examples: 2-butanone, 3-pentanone, etc.

Biological & Pharmaceutical Examples

• Hydrocodone: potent, addictive opioid; ketone highlighted.
• Codeine: related opiate; alcohol instead of ketone lowers potency to ~15 % of morphine.
• Prednisone: synthetic steroid immunosuppressant containing three ketone groups.
• Fructose (a simple sugar): contains a ketone; glucose instead contains an aldehyde.

IUPAC Nomenclature Rules

1. Longest chain containing the carbonyl.
2. Number from end nearer the C=O.
3. Replace -e of parent alkane with -one and give locant.
   • Example: $CH<em>3CH</em>2COCH<em>2CH</em>3$ is 3-pentanone.
4. Branched chain: list substituents alphabetically with locants; carbonyl locant immediately before parent name.
   • Ex: 5-ethyl-2-methyl-3-heptanone.

Sample Structures with Lone Pairs

• 2-methyl-3-octanone: eight-carbon backbone, C=O at C-3, methyl at C-2; show O with two lone pairs.
• 3-methyl-2-octanone: C=O at C-2, methyl at C-3; lone pairs similarly displayed.

Comparison of Intermolecular Forces

• Hydrogen bonding > dipole-dipole (carbonyl) > London dispersion.
• Explains trends in boiling point, solubility, and volatility among hydrocarbons, aldehydes, ketones, and alcohols.

Functional-Group Recognition Exercise

• D-sorbose (open-chain) contains:
  • Multiple alcohol (O-H) groups.
  • One ketone (C=O) group.
• Highlighted accordingly when analysing structures.

Links to Foundational Principles

• Utilises VSEPR theory for geometry (trigonal planar around sp$^2$ carbon).
• Electronegativity difference drives bond polarity and intermolecular interaction type.
• Physical-property trends connect directly to strengths of IMF covered in general chemistry.

Practical & Ethical Implications

• Solvent choice (e.g., acetone) must balance volatility, flammability, and human exposure.
• Presence of ketone or alcohol groups in drugs drastically alters potency/addiction risk (hydrocodone vs. codeine).
• Steroid immunosuppressants (prednisone) require careful dosing due to systemic effects — understanding functional groups aids rational drug design.

Key Numbers, Symbols, & Equations Recap

• Electronegativity: $\text{EN}<em>O = 3.5$, $\text{EN}</em>C = 2.5$.
• Typical carbonyl bond angle: $120^\circ$.
• General naming transformation: $\text{alkane} \rightarrow \text{aldehyde}$ by replacing -e with -al; $\text{alkane} \rightarrow \text{ketone}$ by replacing -e with -one and adding locant.

Study Tips

• Draw complete structures with lone pairs to visualise possible IMFs.
• Practise naming by identifying the carbonyl first, then longest chain, then substituents.
• Compare physical data (bp, solubility) with predicted IMF strength to solidify concepts.
• Relate functional groups to biological molecules to appreciate real-world relevance.