Organic Chemistry Valence Bond Theory

Chemical Bond

• Transfer or sharing of electrons between 2 atoms to achieve a stable (low Energy) arrangement

Theories of Bonding

• Thomson Theory: Plumb pudding model (19th Century)

• Lewis Theory: Bonds form by transferring or sharing electrons: Octet Rule (1916)

• Valence Bond Theory (VB): approaches chemical bonding based on an extension of the quantum-mechanical model (1970s)

Valence Bond Theory

• Chemical bonding based on combining atomic orbitals.

• Explains how atomic orbitals (s, p, d) of one atom combine with another to produce a bond.

Carbon Hybridization

• Carbon has 4 valence electrons and forms 4 bonds.

• Types: sp3, sp2, and sp.

sp3 Hybridized Carbon

• Tetrahedral geometry with 109.5° angles.

• Forms four sigma bonds, e.g., in methane (CH4).

sp2 Hybridized Carbon

• Forms one sigma ($\sigma$) bond and one pi ($\pi$) bond.

• Ethene is a flat planar molecule with 120° bond angles.

sp Hybridized Carbon

• Forms one sigma ($\sigma$) bond and two pi ($\pi$) bonds.

• Example: Acetylene.

Sigma ($\sigma$) and Pi ($\pi$) Bonds

• Sigma ($\sigma$) bond: Bonding electrons directly between 2 nuclei.

• Pi ($\pi$) bond: Bonding electrons NOT between the 2 nuclei.

• $\sigma$ bonds are stronger than $\pi$ bonds.

Consequences of Pi Bonds

• Restricted rotation around C=C bond.

• Can lead to different molecules with different spatial orientations.