Organic Chemistry Basics

Study Notes

Organic chemistry deals with molecules primarily composed of carbon and hydrogen, often incorporating oxygen, sulfur, and nitrogen as well.
Carbon can form stable covalent bonds with various elements, allowing for the creation of long, straight or branched chains.
Example of isomers: C6H14 has five structural isomers, showcasing distinct molecular structures with identical formulas.

Molecular geometry, determined by bond arrangements, affects molecular polarity, influencing reactivity and interactions.
Polar molecules, due to their structure, can exist in polar environments like blood, while nonpolar molecules are stored in fat tissues.
Electronegativity differences dictate bond polarity, requiring memorization of specific values for elements such as H, Li, F, and others.

Covalent and ionic bonds can be polar or nonpolar, with bond polarity determined by electronegativity differences (ΔEN).
Polarity guidelines:
- ΔEN < 0.5 indicates nonpolar
- 0.5 < ΔEN < 1.9 suggests polar
- ΔEN > 1.9 means ionic bonds.
Even if molecules contain polar bonds, overall molecular polarity may differ based on geometry.

Begin with counting total valence electrons, adjusting for anions (+1 for extra electrons) or cations (-1 for fewer electrons).
Create a molecular skeleton, avoiding H and F as central atoms, and aiming for symmetry.
Distribute remaining valence electrons, forming octets, prioritizing outer atoms first.
Central atoms (C, N, O, F) should never exceed the octet rule; lone pairs can be converted to bonding pairs as needed.
Calculate formal charges for molecular stability using the formula:
- Formal Charge = Group # - (Lone Electrons + Bonds).

Favor structures that minimize size and formal charge, with negative charges assigned to more electronegative atoms.
Examples of structures to evaluate include SCN-, N3-, SO32-, and PO33-.

Third-period elements may form expanded octets, allowing more than eight electrons around the central atom (e.g., phosphorus can form five bonds).
Group 3A elements (e.g., boron, aluminum) have diminished octets, typically forming only three bonds.

Oxyacids follow the general formula HaXOb, contain polyatomic ions, and focus on Brønsted-Lowry acids.
The hydrogen atom in oxyacids should bond to a more electronegative oxygen atom, not the central atom X.
Smaller organic molecules can have their Lewis structures drawn using the previously discussed steps, while larger molecules may require additional complexity.
Carbon typically forms four bonds and almost never possesses lone pairs; it participates in double or triple bonds when necessary.