General and Organic Chemistry: Practice Exam Study Notes

Atoms, Elements, and Chemical Bonding

Atomic Structure and Formal Charge

Understanding atomic structure and formal charge is essential for predicting molecular properties and reactivity.

• Atomic Number (Z): The number of protons in the nucleus of an atom, which defines the element.

• Formal Charge: Calculated as: $\text{Formal Charge} = (\text{Valence electrons}) - (\text{Non-bonding electrons}) - \frac{1}{2}(\text{Bonding electrons})$

• Application: Used to determine the most stable Lewis structure for a molecule.

• Example: In a structure with all nonbonding electrons shown, assign formal charges to each atom to identify the most stable resonance form.

Chemical Bonding and Hybridization

Types of Bonds and Hybrid Orbitals

Chemical bonds involve the sharing or transfer of electrons between atoms. Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals suitable for the pairing of electrons.

• Single, Double, Triple Bonds: Single bonds are sigma ($\sigma$) bonds; double and triple bonds contain one sigma and one or two pi ($\pi$) bonds, respectively.

• Hybridization: The type of hybridization depends on the number of electron domains around an atom:

  • sp3: 4 electron domains (tetrahedral geometry)

  • sp2: 3 electron domains (trigonal planar geometry)

  • sp: 2 electron domains (linear geometry)

• Example: In ethene (C2H4), each carbon is sp2 hybridized.

Functional Groups and Organic Structures

Identification of Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

• Common Functional Groups:

  • Alcohols (–OH)

  • Amines (–NH2)

  • Aldehydes (–CHO)

  • Ketones (C=O within a chain)

  • Carboxylic acids (–COOH)

  • Esters (–COOR)

  • Amides (–CONH2)

• Example: The structure of penicillin contains amide, carboxylic acid, and thioether functional groups.

Acids, Bases, and pKa

Brønsted-Lowry and Lewis Acids/Bases

Acids and bases can be classified by their ability to donate or accept protons (Brønsted-Lowry) or electron pairs (Lewis).

• Brønsted-Lowry Acid: Proton (H+) donor

• Brønsted-Lowry Base: Proton acceptor

• Lewis Acid: Electron pair acceptor

• Lewis Base: Electron pair donor

• pKa: The negative logarithm of the acid dissociation constant; lower pKa means a stronger acid. $\text{p}K_a = -\log K_a$

• Example: Carboxylic acids have lower pKa values than alcohols, making them stronger acids.

Resonance and Aromaticity

Resonance Structures

Resonance structures are different Lewis structures for the same molecule that show the delocalization of electrons.

• Rules for Resonance:

  • Only electrons in pi bonds or lone pairs can be moved.

  • The arrangement of atoms must remain the same.

  • The most stable resonance structure has the least formal charge and places negative charges on more electronegative atoms.

• Aromaticity: A molecule is aromatic if it is cyclic, planar, fully conjugated, and follows Hückel's rule ($4n+2$ pi electrons).

Isomerism and Stereochemistry

Constitutional and Stereoisomers

Isomers are compounds with the same molecular formula but different structures or spatial arrangements.

• Constitutional Isomers: Differ in the connectivity of their atoms.

• Stereoisomers: Same connectivity but different spatial arrangement (e.g., cis/trans, enantiomers, diastereomers).

• Newman Projections: Used to visualize conformations around single bonds, especially in alkanes.

• Staggered vs. Eclipsed: Staggered conformations are more stable due to minimized electron repulsion.

Reaction Mechanisms and Energy Diagrams

Reaction Coordinate Diagrams

Energy diagrams illustrate the energy changes during a chemical reaction.

• Activation Energy (Ea): The energy required to reach the transition state from the reactants.

• Transition State: The highest energy point along the reaction path.

• Exergonic vs. Endergonic: Exergonic reactions release energy; endergonic reactions absorb energy.

• Example: A two-step reaction will have two transition states and one intermediate.

Acid-Base Equilibria and Buffers

Equilibrium Calculations

Acid-base equilibria involve the transfer of protons between acids and bases, and the position of equilibrium can be calculated using pKa values.

• Henderson-Hasselbalch Equation: $\text{pH} = \text{p}K_a + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right)$

• Buffer: A solution that resists changes in pH upon addition of small amounts of acid or base.

Tables

Periodic Table (Organic Focus)

The periodic table is essential for understanding trends in electronegativity, atomic size, and reactivity.

Element	Symbol	Atomic Number
Hydrogen	H	1
Carbon	C	6
Nitrogen	N	7
Oxygen	O	8
Fluorine	F	9
Phosphorus	P	15
Sulfur	S	16
Chlorine	Cl	17

Acidic Protons Table

This table lists common acidic protons and their approximate pKa values.

Acidic Proton	Structure	pKa
Carboxylic acid	RCOOH	~5
Alcohol	ROH	~16
Ammonium ion	RNH3+	~10
Phenol	ArOH	~10
Alkyne	RC≡CH	~25
Alkane	RH	~50

Additional info:

• Some questions and tables focus on organic chemistry, which is often included in General Chemistry II or as an introduction to Organic Chemistry.

• Topics such as resonance, hybridization, and acid-base theory are foundational for both general and organic chemistry courses.