Foundations of Organic Structure and Reactivity: GOB Chemistry Study Notes

Course Goals and Overview

Introduction to GOB Chemistry

This course introduces the foundational principles of General, Organic, and Biological (GOB) Chemistry, focusing on the structure, properties, and reactivity of major classes of organic molecules. The initial portion emphasizes organic chemistry concepts essential for understanding biochemical processes.

• Recognize major classes of organic molecules and their physical and chemical properties.

• Identify principal functional groups and understand their reactivity.

• Understand the chemical basis of living systems and the role of organic molecules in biological processes.

• Develop skills in drawing and interpreting Lewis structures and predicting molecular geometry.

Organic Structure and Reactions

Key Atomic Properties for Organic Chemistry

Two atomic properties are central to predicting the outcome of organic reactions:

• The Octet Rule

• Electronegativity

Most organic and biochemical molecules contain a small subset of elements: carbon, nitrogen, oxygen, sulfur, phosphorous, and halogens.

The Octet Rule

Definition and Application

The octet rule states that atoms tend to combine in such a way that they each have eight electrons in their valence shells, achieving the same electronic configuration as a noble gas.

• Applies mainly to main-group elements: carbon, nitrogen, oxygen, and halogens.

• Phosphorous and sulfur may not always follow the octet rule.

Periodic Table and Valence Electrons

Determining Valence Electrons

The periodic table helps determine how many electrons an atom needs to satisfy the octet rule. In GOB chemistry, focus is placed on elements in organic and a few metal ions.

Lewis Structures

Drawing Lewis (Electron-Dot) Structures

Lewis structures represent paired and unpaired valence electrons in an atom. They are essential for visualizing bonding and lone pairs in molecules.

• Carbon has four unoccupied "slots" for bonding, allowing four covalent bonds.

• Hydrogen forms one covalent bond and has no unshared pairs.

• Nitrogen typically forms three covalent bonds and has one lone pair.

• Oxygen typically forms two covalent bonds and has two lone pairs.

• Halogens (Group 7A) form one covalent bond and have three lone pairs.

Examples of Lewis Structures

• Methane (CH4): Carbon forms four single bonds with hydrogen, satisfying the octet rule.

• Ammonia (NH3): Nitrogen forms three single bonds with hydrogen and has one lone pair.

• Water (H2O): Oxygen forms two single bonds with hydrogen and has two lone pairs.

• Hydrochloric acid (HCl): Chlorine forms one single bond with hydrogen and has three lone pairs.

Practice: Drawing Lewis Structures

• Acetic acid (CH3COOH): Contains both a carboxylic acid group and a methyl group.

• Diethyl ether (CH3CH2OCH2CH3): An ether functional group between two ethyl groups.

• Glycine (NH2CH2COOH): The simplest amino acid, containing both amine and carboxylic acid groups.

Chemical and Structural Formulas

Types of Formulas

• Chemical Formula: Shows the types and number of atoms present in a molecule.

• Structural Formula: Shows how the atoms are connected and the bonds between them.

Each line in a structural formula represents a covalent bond, a pair of electrons shared between two atoms.

Electronegativity

Definition and Trends

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. Atoms in a covalent bond may not share electrons equally.

• Electronegativity values increase as you move up and to the right in the periodic table.

• Most electronegative elements: F > O > N > Cl > Br > I > S > C > H

Bond Polarity and Electronegativity

Polar and Non-Polar Bonds

Bonds with unequal sharing of electrons are polar. The electron cloud is pulled toward the more electronegative atom, creating a dipole (separation of charge).

• More electronegative atom: Attracts the electron cloud more strongly (partially negative charge).

• Less electronegative atom: Attracts the electron cloud less (partially positive charge).

Bond Polarity Visualization:

• Blue: low electron density

• Red: high electron density

• Green: non-polar

Determining Bond Type

Electronegativity Difference ($\Delta EN$)

The difference in electronegativity between two atoms determines the type of bond formed:

Electronegativity Difference ($\Delta EN$)	Bond Type
> 1.8	Ionic bond
0.0	Pure covalent bond
0.0 – 0.4	Non-polar covalent (e.g., C–H bond)
0.5 – 1.6	Polar covalent (e.g., C–O or N–H bond)

Summary of Essential Lessons

• The octet rule: Most atoms (except hydrogen) aim for eight valence electrons.

• Memorize the number of bonds and lone pairs for C, O, N, and halogens.

• Know the order of electronegativity: F > O > N > Cl > Br > I > S > C > H

• In most chemical reactions, the most electronegative atom attracts electrons, often leading to nucleophilic attack.

Predicting 3D Shape of Molecules

VSEPR Theory (Valence-Shell Electron Pair Repulsion)

The VSEPR model predicts the three-dimensional geometry of molecules based on the repulsion between electron pairs around a central atom.

• Electron pairs (bonding and lone pairs) repel each other and arrange themselves as far apart as possible.

• Bonding pairs and lone pairs are each considered one area of electron density.

• Double and triple bonds count as one area of electron density each.

Common Geometries:

• Linear: 2 areas of electron density, bond angle ≈ 180°

• Tetrahedral: 4 areas of electron density, bond angle ≈ 109.5° (e.g., methane)

• Trigonal pyramidal: 3 bonds + 1 lone pair (e.g., ammonia)

• Bent: 2 bonds + 2 lone pairs (e.g., water)

Lone pairs take up more space and compress bond angles.

Functional Groups

Definition and Importance

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

• They determine the reactivity and properties of organic compounds.

• Compounds are often named based on their functional groups.

Examples of Functional Groups: Alcohols, aldehydes, ketones, carboxylic acids, esters, amines, amides, ethers, etc.

Practice and Application

• Draw Lewis structures for a variety of organic molecules.

• Identify functional groups and predict molecular geometry using VSEPR theory.

• Classify bonds as ionic, polar covalent, or non-polar covalent based on electronegativity differences.

Additional info: These notes are based on introductory slides for a GOB Chemistry course, focusing on the foundational concepts needed for further study in organic and biochemistry. Students are encouraged to practice drawing structures and to memorize key trends and properties for success in the course.