Revision Lecture Wk7

ENZ Isomers

• Configurational isomers (stereochemistry):

  • Double bond diastereomers.
  • To move between diastereomers, a bond must be broken.
  • ENZ applies only to double bonds with differing groups.

• Assigning priority:

  • Based on atomic number. Higher atomic number = higher priority.
  • Compare atom by atom away from the double bond.
  • Look for a difference, if no difference, move to the next atom away.

• ENZ Determination:

  • Determine the plane of the double bond.
  • If priority groups are on the same side of the plane, it is "Z" (zussammen - same side).
  • If priority groups are on opposite sides of the plane, it is "E" (entgegen).
  • E and Z are different molecules with different properties.

• Cycloalkanes:

  • Use cis and trans instead of E and Z.
  • cis: groups are on the same side of the ring.
  • trans: groups are on opposite sides of the ring.
  • Current convention: E/Z for double bonds, cis/trans for disubstituted cycloalkanes.

Example:
Configuration of the circled double bond in the provided structure is E.

Isomers - Worksheet 18

• Dimethylcyclohexane isomers:

  • Constitutional Isomers: Differ in bonding arrangements.
  • Diastereomers: Same bonding configuration, different spatial arrangement (cis/trans).

• Identifying Isomer Types:

  • Top pair: Constitutional (different bonding arrangements).
  • Middle pair: Conformational (can rotate around single bonds).
  • Bottom pair: Configurational (cis/trans, requires bond breaking to convert).

Naming Cycloalkanes

• Steps:

  • Count carbons in the ring to determine the base name (e.g., cyclopentane, cyclohexane).
  • Identify and number substituents to give the lowest possible numbers.
  • Order substituents alphabetically.
  • Include stereochemical labels (cis/trans) if stereochemistry is provided.

• Examples:

  • Methylcyclopentane (one substituent).
  • 2-Bromo-1-chlorocyclohexane (numbering and alphabetizing substituents).
  • trans-1,3-Dichlorocyclopentane (numbering and stereochemistry).

Thermal Decomposition of Potassium Chlorate - Sample Quiz Question

• Balanced equation:
  2KClO3(s) \rightarrow 2KCl(s) + 3O2(g)

  • Ratio reminder: For every two potassium chlorates, there is two potassium chlorates formed and three oxygens formed.

• Theoretical yield: Assuming 100% conversion.

  • ICE Tables reminder, listing the initial, the change and the end.

• Steps:

  • Convert mass of oxygen to moles: moles = \frac{mass (grams)}{molar mass (grams/mole)}
    • Molar mass of O_2 is approximately 32 g/mol.
    • moles = \frac{10 grams}{32 g/mol} = 0.3125 moles
  • Use the stoichiometry of the reaction to find the moles of potassium chlorate required.
    • Ratio KClO3 : O2 is 2:3.
    • moles of KClO_3 = 0.3125 * \frac{2}{3} = 0.2083 moles
  • Convert moles of potassium chlorate back to mass.
    • Molar mass of KClO_3 is approximately 122.5 g/mol.
    • mass = moles * molar mass = 0.2083 * 122.5 = 25.5 grams

Constitutional Isomers and Overlapping

• Check if molecules can be rearranged to overlap perfectly.
• Consider rotations around single bonds. If rotation allows perfect overlap, the molecules are the same.
  *

Polarity and Intermolecular Forces

• Polar Bond: Unequal sharing of electrons due to electronegativity differences (e.g., C-O).

• Polar Molecule: Molecule with a net dipole moment due to polar bonds and molecular geometry (e.g., water).

• Nonpolar Molecule: Molecule with no net dipole moment (e.g., methane).

• Intermolecular Forces

  • Types: London dispersion forces, dipole-dipole interactions, hydrogen bonding.