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Main Notes

Structure of the Carbonyl Group:

• The carbonyl carbon is sp2-hybridized — three sigma bonds and one pi bond with oxygen
• Geometry is planar/trigonal — 120° bond angles
• Oxygen is highly electronegative → electron density pulled away from C
• Result: C carries partial positive charge (C^delta+), O carries partial negative (O^delta-)
• The C=O is strongly polar → makes carbon electrophilic

SMILES representation:

• Formaldehyde: SMILES: C=O
• Acetaldehyde: SMILES: CC=O
• Acetone: SMILES: CC(C)=O
• Benzaldehyde: SMILES: O=Cc1ccccc1

Reactivity Order (Nucleophilic Addition): $\text{HCHO} > \text{CH}_3\text{CHO} > \text{other aldehydes} > \text{ketones}$

Reason: Alkyl groups exert +I effect (donate electrons to C → reduce C^delta+) AND create steric hindrance. Both effects reduce reactivity. HCHO has neither.

Mechanism of Nucleophilic Addition:

1. Nu^- attacks electrophilic C^delta+ of C=O (rate-determining step)
2. C=O pi bond breaks; electrons go to O → sp2 → sp3 transition; alkoxide intermediate (tetrahedral)
3. Protonation of O^- → C-OH (final addition product)

Cue Column (Questions to Self-Test)

• Why is HCHO the most reactive carbonyl compound?
• What hybridization change occurs at C during nucleophilic addition?
• Draw the mechanism for HCN addition to CH3CHO

Summary

The carbonyl group (C=O) is sp2 planar and electrophilic at C. Reactivity order: HCHO > aldehydes > ketones. Mechanism: Nu- attacks C^delta+ → tetrahedral alkoxide → protonation gives addition product.