Organic Molecules: Covalent Bonding

• The chapter covers properties of organic molecules.
• Students will learn about covalent bonds, hybridization, and molecular geometry.

Covalent Bonding

• A covalent bond forms when atomic orbitals from two atoms overlap.
• Overlapping orbitals create a region of shared electron density, forming a molecular orbital.
• Molecular orbitals, like atomic orbitals, can accommodate two electrons.

Bonding and Antibonding Orbitals

• Constructive overlap (waves in phase) forms bonding molecular orbitals.
• Destructive overlap (waves out of phase) forms antibonding molecular orbitals and a node.

Molecular Orbital (MO) Theory

• Covalent bonds arise when atomic orbitals combine into molecular orbitals.
• A molecular orbital indicates the space around a molecule where finding an electron is probable.
• Molecular orbitals have specific sizes, shapes, and energy levels.

Types of Covalent Bonds

• There are two main types of covalent bonds: sigma (σ) and pi (π) bonds.

Sigma (σ) Bonds

• Sigma bonds result from linear or end-to-end orbital overlap.
• Sigma bonds can form from the overlap:
• Two s orbitals
• An s and a px orbital
• Two p orbitals

Pi (π) Bonds

• Pi bonds result from parallel or side-to-side overlap of p orbitals.

Conditions for Covalent Bonding

• Covalent bonds form if the combining orbitals are half-filled.
• The orbitals must align properly for effective overlap.
• Electrons in the bonding orbitals must have opposite spins.

Rules for Drawing Organic Molecules

• Carbon (C) always forms four bonds by:
• Forming 4 single bonds
• Forming 1 double and 2 single bonds
• Forming 1 triple and 1 single bond
• Forming 2 double bonds
• Hydrogen (H) always forms one bond.
• Oxygen (O) always forms two bonds.
• This can be 2 single or 1 double bond
• Halogens (X) always form one bond. X can be F, Cl, Br, or I
• Nitrogen (N) always forms three bonds.
• These include 3 single bonds
• Forming 1 single and 1 double bond
• Forming 1 triple bond
• Sulfur (S) typically forms 2 bonds, but can form 4 or 6

Hybridization

• Orbital hybridization combines individual wave functions for s and p orbitals.
• Hybrid atomic orbitals have properties of the original orbitals.

Hybridization - Definition

• The mixing of atomic orbitals of similar energy levels produces the same number of hybrid orbitals with equal energy and identical properties.

Types of Hybridization

• Carbon atoms typically undergo three types of hybridization: sp3, sp2, and sp.

Quantum Mechanics and Hybridization

• Wave mechanics by Schrödinger and quantum mechanics by Heisenberg are the basis of bonding theories.
• Wave functions (ψ) represent energy states for electrons.
• Solutions to wave functions can be positive, negative, or zero.

Phase Sign

• A wave equation's phase sign determines whether the solution is positive or negative relative to the nucleus.

sp3 Hybrid Orbitals

• Mixing 2s, 2px, 2py, and 2pz orbitals creates four equivalent sp3 hybrid orbitals.
• The designation sp3 indicates one part s orbital character and three parts p orbital character.

sp3 Hybridization and Methane

• Methane's tetrahedral structure arises from sp3 hybridization.
• Carbon forms single covalent bonds with four hydrogen atoms in methane.
• Each C-H bond involves the overlap of an sp3 orbital from carbon and a 1s orbital from hydrogen.
• C-H bonds in methane are sigma bonds, with bond angles of 109.5°.

Characteristics of sp3 Orbitals

• Each sp3 orbital contains one electron.
• Each sp3 orbital has 25% s-character and 75% p-character.
• Four identical sp3 orbitals are oriented differently in space.

Shape of sp3 Orbitals

• sp3 orbitals are arranged with their axes directed towards the corners of a regular tetrahedron.
• The angle between any two sp3 orbitals is 109.5°.
• The terahedral allows the sp3 orbitals to stay as far away from each other as possible to reduce electron repulsion.

sp2 Hybrid Orbitals

• Mixing one 2s and two 2p orbitals (px, py) results in three sp2 orbitals.
• The third 2pz orbital remains unhybridized.

Characteristics of sp2 Orbitals

• Each sp2 orbital and the unhybridized p₂ orbital contains one unpaired electron.
• The three new sp2 orbitals contain identical energy and shape bus differ in orientation.
• The 3 sp2 orbitals lie in the same plane with their axes directed towards the corners of an equilateral triangle.
• Their angle between any two orbitals is 120 degrees.
• The trigonal arrangement is favored because it allows the sp2 orbitals to stay as far away from each other as possible, thereby reducing the electron-electron repulsion.
• Ethylene is is planar.
• All bond angles (H-C-H and H-C-C-) are 120 degrees.
• The C-H bond length is 1.09 A. The C-C bond length is 1.34 A..

sp Hybrid Orbitals

• Mixing one 2s and one 2p orbital (px) results in three sp orbitals
• The unhybridized orbitals are 2py and 2pz orbitals
• The sp orbitals stay as far apart from each other as possible to reduce the electron repulsion
• the unhybridized py and pz orbitals are at right angles to the line of the sp orbitals.
• Each sp orbital contains an unpaired electron.
• The sp orbitals obtained are identical in energyy and shape but differ only in their orientation in space with respect to each other
• They lie in the straight line The angle between the two sp orbitals is 180 degrees.

Importance of Hybridization

• The main driving force behind hybridisation is stability (energy minimization).
• Molecules would be significantly more unstable without hybridization.