CHEM 331, Organic Chemistry I

Concepts of Atomic Orbitals and Bonding

1. Electrons are located in Atomic Orbitals (AO's) shaped by the central potential of a nucleus and by the energy of an electron. The electron distribution is described by:

a). Wavefunction (), a mathematical description of the electron. Wavefunctions have amplitude, energies (indicated by 1, 2 and s or pz etc. in the atomic electronic configuration), and signs (+ or -). The region of the wavefunctions that separates parts with differing signs is called a node.

b). The square of the wavefunction, (2), the probability of locating an electron (or electron pair) with a given precision, within a specified volume of space.

2. Ionic bonds. Formed by total (or near total) electron transfer resulting in formation of an ion pair. These bonds are not directional.

3. Covalent bonds are formed by sharing of electrons between the bonding atoms.

a. Characterized by directionality, bond length, angle and strength.

b. Specified as (cylindrically symmetric along the line joining the atoms) or (electron density located above and below the plane of the molecule). Multiple (i.e. double or triple) bonds exist.

4. Hybrid Orbitals.

a. Formed by interaction of AO's on the same atom.

b. Allow greater directionality of covalent bonds.

c. Enhance overlap between Atomic Orbitals within Molecular Orbitals.

d. Relative disposition of orbitals and hence the geometry of the hybridized atom is governed by the tendency of electrons (and therefore their orbitals) to keep as far apart as possible.

5. Hybrid Orbitals on Carbon.

A. sp3 hybrid.

Designed for : Making C-H (1s-sp3) and C-C (sp3-sp3) bonds

Molecules : Saturated cyclic and acyclic alkanes.

Source atomic orbitals : 2s + 2px + 2py + 2pz

Coordination number : 4

Geometry : Tetrahedral, ~ 109° H-C-H bond angle

3-D orbital picture :

B. sp2 hybrid.

Designed for : Making C-H (1s-sp2), C-C (sp2-sp2) bonds and C-C (2p-2p) bonds

Molecules : Alkenes, molecules with other double-bonded carbons (carbonyl groups etc.)

Source atomic orbitals : 2s + 2px + 2py; 2pz left over to make a p C-C bond

Coordination number : 3

Geometry : Trigonal planar, ~ 120° H-C-H bond angle

3-D orbital picture :

C. sp hybrid.

Designed for : Making C-H (1s-sp), C-C (sp-sp) bond and two C-C (2p-2p) bonds

Molecules : Alkynes

Source atomic orbitals : 2s + 2px; 2py and 2pz left over to make two p C-C bond

Coordination number : 2

Geometry : Linear, exactly 180° H-C-C-H bond angle

3-D orbital picture :

6. Hybrid Orbitals on other atoms.

Both nitrogen and oxygen may be pictured as possessing hybridized orbitals. While nitrogen may be sp3-hybridized (as in :NH3 or NH4+), sp2-hybridized (as in R2C=NH) or sp-hybridized (as in cyanide anion CN-), oxygen is always pictured as sp3-hybridized, as in H2O.