CHE 2511: Basic Organic Chemistry Lecture Notes

THE UNIVERSITY OF ZAMBIA SCHOOL OF NATURAL SCIENCES DEPARTMENT OF CHEMISTRY LECTURE NOTES BY PETER MUBANGA CHEUKA

Alkenes and Geometric Isomerism

  • Definition of Alkenes:

    • Alkenes are unsaturated hydrocarbons.
    • Their general formula is C<em>nH</em>2nC<em>nH</em>{2n}.
    • They contain at least one carbon-carbon double bond somewhere in their structure.

  • Examples of Alkenes:

    • Ethene: C<em>2H</em>4C<em>2H</em>4
    • Propene: C<em>3H</em>6C<em>3H</em>6
    • But-1-ene: C<em>4H</em>8C<em>4H</em>8 (with a double bond at the first position)
    • But-2-ene: C<em>4H</em>8C<em>4H</em>8 (with a double bond at the second position)

  • Structure of Alkenes:

    • The C=C double bond consists of one sigma (σ) bond and one pi (π) bond.
    • Formation of σ bond:
      • Formed from the head-on overlap of two sp² hybrid orbitals from two carbon atoms, which leads to the formation of a C-C sigma bond.
    • Formation of π bond:
      • The π bond consists of the sideways overlap of remaining unhybridized p orbitals on each carbon atom.
    • π bonds are exposed and have high electron density, making them vulnerable to attacks by electrophiles.

  • Rotation:

    • Rotation can occur around the sigma bond.

Stereoisomerism

  • Definition of Stereoisomers:

    • Stereoisomers have the same molecular and structural formula but differ in their spatial arrangement of atoms.

  • E-Z Stereoisomerism:

    • Alkenes can exhibit E-Z stereoisomerism under two conditions:
    1. There is restricted rotation around the C=C double bond.
    2. Two different groups/atoms are attached to each carbon of the double bond.

  • Naming E-Z Stereoisomers:

    • To name E-Z stereoisomers, the priority groups on both sides of the double bond must be determined based on atomic number.
    • The atom with the higher atomic number is classified as the priority atom.
    • If the priority atoms are on the same side of the double bond, it is labeled Z (from the German word 'zusammen', meaning 'together').
    • If the priority atoms are on opposite sides of the double bond, it is labeled E (from 'entgegen', meaning 'opposite').

  • Cis-Trans Isomerism:

    • Cis-trans isomerism is a special case of E-Z isomerism where two of the substituent groups on the double bond are identical.
    • Example:
      • Z-but-2-ene can also be called cis-but-2-ene.
      • E-but-2-ene can also be called trans-but-2-ene.

Conformational Isomers

  • Definition of Conformational Isomers:

    • Conformational isomers (or rotational isomers) are different spatial arrangements of a molecule generated by rotation about C-C single bonds (σ bonds).

  • Conformational Analysis of Ethane:

    • Ethane has two extreme conformations:
    • Eclipsed
    • Staggered
    • In a staggered conformation, the C-H bonds are as far apart from each other as possible, while in an eclipsed conformation, C-H bonds overlap producing torsional strain.
    • Energy Difference:
    • The eclipsed conformation of ethane is 12 kJ/mol less stable than the staggered conformation.
    • At room temperature, approximately 99% of ethane is in the staggered conformation.

  • Potential Energy Diagram of Ethane:

    • Diagram shows energy variations according to the dihedral angle of the rotation from staggered (0°) to eclipsed (60°) conformers.

Torsional Strain

  • Definition of Torsional Strain:

    • Torsional strain is the increase in potential energy of a molecule due to repulsion between electrons in bonds that do not share an atom.

  • Factors of Torsional Strain:

    • Rotation about the C-C σ bond is not entirely free due to energy differences between staggered and eclipsed conformers.
    • The energy barrier is small enough to allow millions of interconversions at room temperature.

Conformational Analysis of Butane

  • Description of Butane Conformations:

    • Various conformations of n-butane exist, with free rotation about all C-C bonds.
    • The C-2-C-3 bond can exhibit staggered and eclipsed conformations.

  • Conformational Analysis Results:

    • Different conformers have different energy levels due to steric repulsion between atoms.
    • Examples:
      1. Eclipsed Conformation: Highest energy (max. strain).
      2. Anticlinal Conformer: Medium energy.
      3. Staggered Conformation: Lowest internal energy with the least steric repulsion.

Steric Strain

  • Definition of Steric Strain:
    • Steric strain is the strain experienced by a molecule when atoms or groups are too close, causing electron clouds to repel each other.
    • Steric strain increases with the size of the interacting atoms or groups.

Cycloalkanes

  • Definition:

    • Cycloalkanes are alkanes in which carbon atoms form a ring (a type of alicyclic compounds).
    • General formula for cycloalkanes is also C<em>nH</em>2nC<em>nH</em>{2n}.
    • Examples:
    • Cyclopropane, Cyclobutane, Cyclopentane, Cyclohexane

  • Cis-Trans Isomerism in Cycloalkanes:

    • The ring structure limits rotation about C-C bonds, creating stereoisomers.
    • Substituents on cycloalkanes are labeled for their relative facial positions (cis or trans).

Stability of Cycloalkanes: Ring Strain

  • Definitions of Types of Strain:

    • Angle Strain: Expansion or compression of bond angles away from most stable state.
    • Torsional Strain: Eclipsing of bonds on neighboring atoms.
    • Steric Strain: Repulsive interactions between nonbonded atoms in close proximity.
    • Ring Strain: Combination of angle and torsional strain.

  • Summary of Ring Structure Strains:

    • Cyclopropane is planar with 60° bond angles, causing significant strain.
    • Cyclobutane experiences lesser angle strain but higher torsional strain due to eclipsing interactions.
    • Cyclopentane has a non-planar conformation, reducing torsional strain.
    • Cyclohexane is free from angle strain and torsional strain with a chair conformation at 109.5° bond angles.

Chair and Ring-Flipping Constructs

  • Chair Conformation:
    • All bond angles in a chair conformation are 109.5°, all hydrogens are staggered, leading to minimized strain.
  • Axial and Equatorial Bonds:
    • Chair cyclohexane has two types of hydrogen positions: axial and equatorial.
    • Equatorial positions are more stable compared to axial due to reduced steric strain.
    • 1,3-Diaxial Interactions:
    • Steric strain is due to interactions between axial substituents.

Stereochemistry and Chiral Molecules

  • Chirality:

    • A molecule is chiral if it has two non-superimposable mirror images.
    • A carbon atom is chiral if it has four different substituents, leading to non-superimposable stereoisomers (enantiomers).

  • Optical Activity:

    • Chiral compounds are optically active and rotate plane-polarized light.
    • Enantiomers rotate light in opposite directions but by the same magnitude.

Configuration of Stereoisomers

  • R,S Nomenclature:

    • The Cahn-Ingold-Prelog priority rules assign priorities based on atomic number for substituents around chiral centers.
    • Clockwise arrangement of priority gives R configuration; counterclockwise gives S configuration.

  • Important Notes:

    • Do not confuse (+)/(-) with (R)/(S); they describe different properties.
    • Enantiomers have different physical properties despite sharing some characteristics (e.g., boiling point, melting point).

Summary of Important Terms

  • Conformational Isomers: Cannot be isolated; continuous rotation occurs due to small energy differences.
  • Steric strain: Increases with the size of groups.
  • 1,3-Diaxial interactions: A significant source of steric strain in cyclohexanes.
  • Optical purity: Indicates enantiomeric excess in mixtures and relates to physical properties.

Conclusion

  • These fundamental concepts in organic chemistry, including alkenes, stereoisomerism, conformational analysis, and chirality, guide understanding of molecular interactions and stability, directly influencing physical and chemical behaviors in organic compounds.