1 Structure and Bonding in Organic Molecules
1-2 Coulomb Forces
Unlike charges attract
Like charges repel
Charges represented as + and – with circles, e.g.
Attraction/repulsion varies with 1/r2
Attraction > repulsion until valence shell is filled
Atoms share electrons until valence shell is filled
In molecules, shared electrons represented as dots between atoms
1-3 Ionic and Covalent Bonds
Electronegativity is a measure of electron attraction
Electronegativity increases to the right in the periodic table
Polarized bonds between unlike atoms
Ionic bonds between atoms on opposite sides of table
Form molecules by adding atoms with lower valence to higher valence atoms
VSEPR (Valence Shell Electron Pair Repulsion) can be used to predict the shape of molecules based on maximal seperation of electron pairs
1-4 Lewis Structures
Lewis structures represent molecules with pairs of electrons shown as dots drawn between atoms
1-5 Resonance Forms
Resonance is a formalism that is used when a single Lewis structure does not properly account for structure and properties of a molecule.
Resonance structures differ from each other only in the position of electrons.
Curved arrows are used to indicate the motion of electrons that would be necessary to covert one resonance structure into another.
Sometimes, resonance structures are considered to be equivalent and thus each contributes equally to a combination called a hybrid. More often, each resonance structure has unique features. Those factors used to evaluate which structure(s) will be a greater contributor to the hybrid are:
Number of bonds (the more bonds the better);
Completion of valance shells (the more filled valence shells, the better);
Charges---separation of charge is bad as is accumulation of like charges (e.g. two negative charges) on the same atom.
1-6 Atomic Orbitals
Electrons in atoms and molecules are quantized and have specific energies
Atomic orbitals are mathematical descriptions of electron density about an atom
The atomic orbitals (in order of increasing energy) are:
1s; 2s; 2px 2py 2pz; 3s; 3px 3py 3pz; five 3d
The 1s orbital has no nodes
The 2s orbital has an internal spherical node
The 2p orbitals have nodal planes (orthogonal to each other)
1-7 Molecular Orbitals
Molecular Orbitals are formed by the combination of atomic orbitals
In the simplest case, two atomic orbitals combine to form two molecular orbitals, one bonding and one antibonding orbital.
1-8 Hybrid Orbitals
Hybrid orbitals are formed by combining 2nd level atomic orbitals
s + 3p -> 4 sp3
s + 2p -> 3 sp2 + p
s + p -> 2 sp + 2p
sp2 trigonal planar
1-9 Structures and Formulas of Organic Molecules
Bond-line formula notations is the most straightforward way of representing the connectivity in a molecule. Lines represent bonds between atoms and
the end of every line and all the junctions of lines are implied to be carbon atoms unless another element is represented. All atoms are assumed to have
valence shells filled by hydrogens which are not shown.