This section introduces several common representations to depict organic molecules.
- Condensed structures efficiently show atomic connectivity
- Lewis dot structures use dots to represent electrons that are found in bonds and lone pair electrons
- To simplify structures, two dots representing a bond in a Lewis dot structure can be represented by a line.
- Line bond structures can show large organic molecules efficiently
- The ends and bends of line bond structures are understood to be C atoms bonded to enough H atoms to complete the octet
- Elements other than C and H are always labelled explicitly
- H atoms bonded to atoms other than C are shown explicitly
- Formal charges are always included
- Lone pair electrons are often included, but are optional
- Wedge-dash structures represent the 3D geometry of a molecule
- Wedges represent groups coming out of the plane of the page, and dashes represent groups going into the plane of the page
- Isomers are different molecules with the same molecular formula
- Constitutional isomers have the same molecular formula but the atoms have different connectivity
This section introduces how to name organic molecules using systematic IUPAC nomenclature. It also covers common names for small organic molecules.
- Molecules are named systematically using the IUPAC system of nomenclature
- The longest continuous carbon chain determines the root
- The suffix for alkanes is -ane
- Branched substituents are identified at the front of the name in alphabetical order
- If a hydrocarbon has a double bond, use the suffix -ene
- If a hydrocarbon has a triple bond, use the suffix -yne
- Simple branched substituents are usually referred to by their common names
- IUPAC nomenclature systematically names more complex branched substituents
- To name cyclic hydrocarbons, add cyclo- before the root
- Functional groups are specific groupings of atoms
- Identifying the functional groups in a molecule can help us predict is reactivity
- Ethers, sulfides and halides are lower priority functional groups than hydrocarbon
- Ether, sulfide and halide functional groups on hydrocarbons are named as substituents at the front of the name
- The principle functional group suffix of a molecule's name reflects its highest priority functional group
- If a molecule has multiple functional groups, the highest priority group dictates the suffix and the rest are named as substituents at the front of the name
- For some small molecules, common names are used much more than IUPAC names
This section reviews Valence Shell Electron Pair Repulsion Theory, Valence Bond Theory, Molecular Orbital Theory, and Frontier Molecular Orbital Theory in the context of organic chemistry.
- VSEPR theory predicts molecular geometry by minimizing electron repulsions between bonds and/or lone pair electrons
- Valence Bond Theory describes bonds as resulting from the overlap of orbitals
- Hybrid orbitals involve the combination of two or more atomic orbitals
- In MO theory, all atomic orbitals from every atom in a molecule are combined to create molecular orbitals
- Electrons in molecular orbitals can be spread across the molecule rather than confined to a single bond or atom in atomic orbitals
- MO theory introduces antibonding orbitals
- Frontier Molecular Orbital (FMO) theory considers only interactions between HOMO and LUMO orbitals (i.e. the frontier orbitals)
This section explores how electronegativity, induction, polarizability and resonance affect the distribution of electrons in molecules, and how this affects the molecule's stability.
- More electronegative atoms attract electrons towards themselves more strongly in molecules
- Molecules with an equal distribution of electrons are non-polar, while those with unequal electron distributions are polar
- More electronegative atoms are more stable with negative charges than less electronegative atoms
- Less electronegative atoms are more stable with positive charges than more electronegative atoms
- The electronegativity of an atom changes depending on its hybridization
- The electronegativity of an atom can affect the electron density of atoms two or more bonds away through induction
- More electronegative atoms have stronger inductive effects
- A greater number of electronegative atoms increases the inductive effect
- Inductive effects are stronger at shorter distances
- A highly electronegative atom stabilizes a nearby negative charge and destabilizes a nearby positive charge
- When comparing the relative stabilities of two charged atoms that are in the same row of the periodic table, electronegativity is the most important factor
- When comparing the relative stabilities of two charged atoms that are in the same column of the periodic table, polarizability is the most important factor
- Larger (more polarizable) atoms are better able to stabilize negative charges
- Molecules with resonance have delocalized electrons, so require two or more Lewis structures to describe their bonding
- Resonance can stabilize both positive and negative charges by spreading the charge across multiple atoms
- Only electrons in the π-system can participate in resonance
This section applies the concepts from section 4 to predict relative strengths of organic acids and bases.
- In acid-base reactions, the more stable species are favoured at equilibrium
- If an acid-base reaction has a charged species on each side, then we can use electronegativity, induction, polarizability, and resonance to predict which side is more stable
- Weaker acids/bases are more thermodynamically stable than stronger acids/bases
- To predict the relative strengths of neutral acids, consider the relative strengths of their charged conjugate bases
- To predict the relative strengths of neutral bases, consider the relative strengths of their charged conjugate acids
This section introduces how the way in which molecules twist about their bonds affects their stability.
- Molecules can rotate about their σ-bonds.
- Molecules that differ by a simple bond rotation are called conformers.
- Wedge-dash structures depict the 3D orientation of atoms, so can be used to represent conformers.
- Sawhorse projections represent the 3D structure of a molecule by looking at the molecule at a 45° angle.
- Newman Projections show the 3D orientation of the atoms in a molecule by looking at a particular bond head-on rather than side-on.
- Eclipsed conformations (when the bonds off adjacent carbon atoms line up) are the least stable.
- Staggered conformations (when bonds off adjacent carbon atoms are offset) are the most stable.
- Torsional strain is strain due to the repulsion of electrons in bonds.
- Steric strain is strain due to the repulsion of electrons in atoms.
This section focuses on the most stable conformation of cyclohexane derivatives.
- The most stable conformation of a cyclohexane ring is a chair conformation
- The 2D representation of a chair conformation has 3 sets of parallel lines
- Each carbon in a cyclohexane chair has an axial and an equatorial substituent
- We can draw a double Newman projection of a cyclohexane by looking down two parallel bonds simultaneously.
- Cyclohexane can adopt two different chair conformations.
- These two chair conformations rapidly interconvert at room temperature via a chair flip.
- When a cyclohexane chair flips from one chair conformer to another, all axial substituents become equatorial and vice versa.
- The two chair conformers of substituted cyclohexane have different stabilities.
- Substituents tend to be more stable in the equatorial position than the axial position, since this minimizes steric interactions (1,3-diaxial interactions).
- As substituents become bulkier, they are increasingly unstable in the axial position.
- Substituents with longer bonds experience less 1,3-diaxial interaction.
- Two substituents on the same side of a chair conformation have a cis-relationship
- Two substituents on opposite sides of a chair conformation have a trans-relationship
- It is less stable to have large groups close together due to steric interactions (e.g. 1,3-diaxial interactions)