Introductory Concepts and Review

Structure and Properties of Organic Molecules

Understanding the structure and properties of organic molecules is fundamental in organic chemistry. This includes drawing correct Lewis structures, identifying hybridization, and recognizing the geometry and bond angles of atoms within molecules.

• Lewis Structure: A diagram that shows the bonding between atoms of a molecule and the lone pairs of electrons that may exist.

• Hybridization: The concept of mixing atomic orbitals to form new hybrid orbitals suitable for the pairing of electrons to form chemical bonds.

• Bond Angles: The angles between adjacent lines representing bonds. These are determined by the electron-pair geometry around the central atom.

Structure, Nomenclature, and Conformation

Lewis Structures of Organic Molecules

Drawing correct Lewis structures is essential for understanding molecular geometry and reactivity.

• Example 1: CH4 (Methane)

  • Central carbon atom with four single bonds to hydrogen atoms.

  • All valence electrons are paired; no lone pairs on carbon.

  • Geometry: Tetrahedral.

• Example 2: HOCH2CHO (Glycolaldehyde)

  • Structure: HO–CH2–CHO

  • Contains an alcohol (-OH) and an aldehyde (-CHO) functional group.

  • Each atom's valence is satisfied according to the octet rule.

Three-Dimensional Representation: Hash-Wedge Convention

Organic chemists use the hash-wedge convention to depict three-dimensional structures on paper. This is important for visualizing stereochemistry and molecular shape.

• Wedge: A solid triangle indicates a bond coming out of the plane toward the observer.

• Hash: A dashed line indicates a bond going behind the plane away from the observer.

• Example: For a molecule like CH3COCH2NHCH3, the nitrogen atom is tetrahedral, so one N–H bond must be shown as either wedged or hashed to indicate its 3D orientation.

Additional info: The lone pair on nitrogen affects the geometry, but is not always drawn explicitly in these representations.

Hybridization, Electron-Pair Geometry, and Bond Angles

The hybridization of an atom in a molecule determines its electron-pair geometry and the approximate bond angles.

• sp3 Hybridization: Tetrahedral geometry, bond angles ≈ 109.5°.

• sp2 Hybridization: Trigonal planar geometry, bond angles ≈ 120°.

• sp Hybridization: Linear geometry, bond angles ≈ 180°.

Example Table: Hybridization, Geometry, and Bond Angles

Formal Charges

Assigning formal charges is important for understanding the stability and reactivity of molecules. The formal charge on an atom is calculated as:

• Atoms in their most stable state usually have a formal charge of zero.

• Formal charges help identify reactive sites in molecules.

Biopolymers: Proteins and Amino Acids

Monomers of Proteins

Proteins are biopolymers constructed from 20 different amino acid monomers. Each amino acid contains an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group) attached to a central carbon atom (the α-carbon).

• General Structure of an Amino Acid:

  • Central α-carbon

  • Amino group (-NH2)

  • Carboxyl group (-COOH)

  • Hydrogen atom

  • Side chain (R group)

• Peptide Bond: Amino acids are linked by peptide bonds to form proteins.

Additional info: The sequence and properties of the side chains determine the structure and function of proteins.