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• SCH4U — Grade 12 Chemistry (Ontario)
• General Chemistry I
  • 1. Stoichiometry
  • 2. Aqueous Reactions
  • 3. Chemical Equations
    • 3.1 Balancing Equations
    • 3.2 Synthesis Reactions
    • 3.3 Decomposition Reactions
    • 3.4 Combustion Reactions
  • 4. Thermochemistry
  • 5. Atomic Structure
  • 6. Electronic Structure & Periodic Trends
  • 7. Chemical Bonding & Molecular Geometry
  • 8. Gases
  • 9. Liquids, Solids & Intermolecular Forces
  • 10. Solutions & Colligative Properties
  • 11. Gases & Atmospheric Chemistry
• General Chemistry II
  • 1. Reaction Kinetics & Rate Laws
  • 2. Chemical Equilibrium
  • 3. Acids, Bases, and Buffers
  • 4. Electrochemistry & Redox Cells
  • 5. Thermodynamics
  • 6. Solubility & the Solubility Product
  • 7. Nuclear Chemistry
  • 8. Coordination Chemistry
• Organic Chemistry I
  • 1. Nomenclature & Functional Groups
  • 2. Stereochemistry & Isomerism
  • 3. Substitution and Elimination
  • 4. Structure & Bonding in Organic Molecules
  • 5. Alkenes
  • 6. Alkynes
  • 7. Alcohols & Ethers
  • 8. Spectroscopy
  • 9. Polymers
• Organic Chemistry II
  • 1. Aromatic Chemistry
  • 2. Aldehydes & Ketones
  • 3. Carboxylic Acids & Derivatives
  • 4. Enolates & the Aldol Reaction
  • 5. Amines
  • 6. Conjugated Systems & Diels-Alder
  • 7. Biomolecules
  • 8. Synthesis Strategy & Retrosynthesis

2. Stereochemistry & Isomerism

Stereoisomers share the same connectivity but differ in 3D arrangement — and that difference can decide whether a molecule is a medicine or a poison. Stereochemistry is the study of that spatial language.

Constitutional Isomers and Cis–Trans

Constitutional isomers differ in connectivity (1-butene vs 2-methylpropene). Cis–trans isomers share connectivity but differ across a rigid double bond: in cis-2-butene both methyl groups sit on the same side; in trans-2-butene they oppose.

Chirality and Enantiomers

A carbon bonded to four different groups is a stereocenter, making the molecule chiral — non-superimposable on its mirror image. The two mirror images are enantiomers (e.g., the two forms of 2-butanol, $\text{CH}_3\text{–CH}(\text{OH})\text{–CH}_2\text{–CH}_3$).

Assigning R and S

Use the Cahn–Ingold–Prelog priorities: 1. Rank the four groups by atomic number of the attached atom (highest = 1). 2. Point the lowest-priority group away. 3. Trace 1 → 2 → 3: clockwise = R, counterclockwise = S.

Diastereomers and Meso Compounds

Diastereomers are stereoisomers that are not mirror images (e.g., cis vs trans 1,2-dimethylcyclopropane). A meso compound has stereocenters but an internal plane of symmetry, making it achiral — 2,3-butanediol's meso form is the classic example.

Practice Problems

1. Which of these is chiral: CH₃CH₂CH₂CH₃, CH₃CHBrCH₂CH₃, or (CH₃)₂CHCH₃?
2. Draw cis- and trans-2-butene and label which is which.
3. Assign R or S to glyceraldehyde drawn in a Fischer projection with –CHO on top, –CH₂OH on the bottom, –OH on the right, and –H on the left.
4. A molecule has two stereocenters and a plane of symmetry. Is it chiral? What is it called?

Answers: 1. CH₃CHBrCH₂CH₃ (2-bromobutane) — C2 has four different groups   2. cis: both CH₃ on the same side of the double bond; trans: opposite   3. R — priorities OH(1) > CHO(2) > CH₂OH(3) > H(4); 1→2→3 (right → top → bottom) runs counterclockwise, but H sits on a horizontal Fischer bond (a wedge, toward you), so invert the reading → R. This is D-glyceraldehyde = (R)-glyceraldehyde   4. Achiral — a meso compound