Crash Course Organic Chemistry: Alkenes and Reactions

Introduction

• Presenter: Deboki Chakravarti
• Platform: Crash Course app available on Android and iOS.
• Context: Understanding small molecules like isoprene in organic chemistry.
  • Isoprene is a volatile, biogenic organic compound.
  • Reacts with ozone and other pollutants.
  • Polymerizes to make natural rubber.

Alkene Nomenclature

• Alkenes: Molecules with carbon-carbon double bonds.
  • Double bonds are rigid and can't rotate easily.

Cis-Trans Isomerism

• Definition: Describes different geometric isomers around a double bond.
  • Cis-pent-2-ene: Methyl and ethyl groups on the same side.
  • Trans-pent-2-ene: Methyl and ethyl groups on opposite sides.

Limitations of Cis-Trans System

• Fails when double-bond carbons have different groups.
• Example: 2-chloropent-2-ene.

E-Z System

• Method: Prioritize groups on each carbon of the double bond.
  • Higher atomic number = higher priority.
  • Z-Isomer: High priority groups on the same side ("Ze Zame Zide").
  • E-Isomer: High priority groups on opposite sides ("Ecross").
  • Example: (Z)-2-chloropent-2-ene and (E)-2-chloropent-2-ene.

Reactions of Alkenes

• Addition Reactions: Pi electrons attract electrophiles, adding groups to carbons at the double bond.

Hydrogen Bromide and Alkenes

• Reaction with cis-but-2-ene: Results in a straightforward nucleophilic attack.
  • Symmetrical alkenes: Single product regardless of which carbon bonds to bromine.

Carbocations

• Introduction: Positively charged carbon atoms created during reactions.
  • Tertiary carbocations are more stable than secondary or primary.

Markovnikov’s Rule

• Predicts products of addition reactions:
  • Proton adds to the side of the double bond with more hydrogens.
  • Results in more substituted, stable carbocations.

Example Reactions

• Hydrogen Bromide with 1-methylcyclohex-1-ene:

  • Forms tertiary carbocation, leads to 1-bromo-1-methylcyclohexane.

• Hydrogen Bromide with 3,3-dimethylbut-1-ene:

  • Unexpected secondary product due to 1,2-alkyl shift.
  • 1,2-Alkyl Shift: Methyl group moves, forming a more stable carbocation.

Rearrangements

• 1,2-Hydride Shift: Hydrogen shifts and forms a more stable carbocation.
  • Example: Hydrogen Bromide with 3-methyl-pent-1-ene results in 3-bromo-3-methylpentane.
  • Stability is key for such reactions.

Conclusion

• Key Learnings:
  • E/Z system enhances alkene nomenclature.
  • Markovnikov’s rule aids in predicting addition reaction products.
  • Carbocations are stabilized by inductive effects and hyperconjugation.
  • Rearrangements may occur to achieve more stable carbocations.
• Next Episode Preview: Focus on thermodynamics, free energy, and kinetics in predicting reaction products.

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