Lecture Notes on Carbon Compounds and Reactions

Hybridization of Carbon

• Types of Hybridization:
  • sp3 Hybridization: Carbon forms four single bonds, resulting in a tetrahedral shape with bond angles of 109.5°.
  • sp2 Hybridization: Found in alkenes with double bonds.

Benzene Structure

• Benzene Characteristics:
  • Six carbon atoms arranged in a hexagonal planar structure.
  • Each carbon forms three single covalent bonds and has one unused p-orbital.
  • Trigonal Planar Structure: Bond angles of 120°.
  • Formation of a delocalized π-electron cloud due to overlapping p-orbitals.
• Representation:
  • Cambridge expects a representation showing delocalized electrons.
  • Bond energy is partially localized due to the electron cloud.

Differences between Alkenes and Benzene

• Alkenes:
  • Sp2 hybridized with concentrated π-electron clouds leading to higher electron density.
  • Reacts through electrophilic addition reactions.
• Benzene:
  • Lower electron density than alkenes, leading to different reactions (electrophilic substitution).

Reactions of Alkenes

• Electrophilic Addition:
  • Example: Bromination
    • Bromine becomes polarized due to high electron density.
    • Formation of a carbocation when bromine interacts with the double bond.
• Markovnikov's Rule:
  • Predicts the formation of more stable carbocations during reactions with unsymmetric alkenes.
• Hydrogenation:
  • Addition of H2 across double bonds using heat and pressure.
• Hydration:
  • Addition of water (H2O) using phosphoric acid as a catalyst.
• Oxidation:
  • Mild Oxidation: Addition of two OH groups.
  • Strong Oxidation: Complete breakage of double bonds leads to carboxylic acids or ketones.

Reactions of Benzene

• Electrophilic Substitution Reactions:
  • Example: Nitration using concentrated HNO3 and H2SO4.
  • Mechanism involves the generation of a nitronium ion (NO2+).
  • Bromine can also be substituted in a similar fashion.
• Hydrogenation of Benzene:
  • Converting benzene to cycloalkanes using nickel and H2.
• Derivatives of Benzene:
  • Phenol, Benzaldehyde, Benzoic Acid, etc.
  • Electron-donating groups: (OH, NH2, alkyl groups) activate the benzene ring, allowing for substitution at positions 2, 4, and 6.
  • Electron-withdrawing groups: (NO2, COOH) deactivate the ring, favoring substitution at positions 3 and 5.

Comparison of Acids

• Carboxylic Acids:
  • Strongest due to electron-withdrawing effects of the carbonyl moiety.
• Phenols:
  • Stronger than alcohols; lone pair overlaps with the benzene electron cloud.
• Alcohols:
  • Weakest; alkyl groups have an electron-donating effect.

Nucleophilic Substitution Reactions

• Halogenoalkanes: SN1 and SN2 mechanisms for nucleophilic substitution.
• Hydrolysis Comparisons: Different reaction rates for chlorobenzene, halogenoalkanes, and alkyl halides.

Polymerization and Amino Acids

• Polyester Formation: Combination of diols and dicarboxylic acids.
• Amino Acids: Formation of zwitterions and polypeptides, including secondary and tertiary structures due to hydrogen bonds and interactions between R groups.

Summary

• Hydrolysis of Esters: Back to carboxylic acids and alcohols.
• Azotization: Formation of azo dyes using phenylamine and diazonium ions.
• Polyamides: Formed from amino acids and exhibit zwitterionic behavior.

Important Notes

• Focus on mechanisms for electrophilic substitution and compare reactivity of different functional groups attached to benzene.
• Remember Markovnikov's rule and carbocation stability for alkenes.