Organic Chemistry Lecture I: Structure and Bonding

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Introduction to Organic Chemistry

Definition and Importance

Organic chemistry is the branch of chemistry that studies carbon-containing molecules and their reactions. Organic compounds are fundamental to daily life and are found in:

Food
Gasoline
Clothes
Pharmaceuticals
Plastics

The study of organic chemistry involves understanding the structure, properties, synthesis, and characterization of organic compounds.

Atoms, Molecules, and Materials

Organic materials (such as proteins and plastics) are composed of organic molecules, which in turn are made up of atoms (commonly C, H, O, N, etc.). For example, ethanol (CH3CH2OH) is an ingredient in beer and wine and consists of carbon, hydrogen, and oxygen atoms.

Chapter 1: Structure and Bonding

Atoms in Organic Molecules

An organic molecule is composed of atoms, which are the building blocks of all matter. The most common atoms in organic compounds are listed below, along with their position in the periodic table:

Group Number	1A	2A	3A	4A	5A	6A	7A	8A
First Row	H							He
Second Row	Li	Be	B	C	N	O	F	Ne
Third Row	Na	Mg	Al	Si	P	S	Cl	Ar

Elements in the same row are similar in size, while elements in the same column have similar electronic and chemical properties.

Valence Electrons

The number of valence electrons can be determined by the group number for Group A elements:

Group number = Number of valence electrons

For example, carbon is in group 4A and has four valence electrons.

Atomic Structure of Carbon

The carbon atom consists of a nucleus (protons and neutrons) and orbitals occupied by electrons. The electron configuration for carbon is:

Two electrons in the n=1 shell (1s orbital)
Four electrons in the n=2 shell (2s and 2p orbitals)

Electron configuration:

Valence electrons are those in the outermost shell and are crucial for bonding.

Atomic Orbitals

Atomic orbitals describe the wave properties of electrons and are visualized by their shapes:

s-orbital: Spherical shape
p-orbital: Dumbbell shape; three p-orbitals are perpendicular to each other

Bonding in Organic Molecules

Covalent Bonds

A chemical bond acts like a spring connecting two atoms. In a covalent bond, atoms share a pair of electrons, which acts as the "glue" holding them together. For example:

Bond length for H—H is 0.74 Å

Formation of a covalent bond:

Two atoms share a pair of electrons
Bond has a defined length and strength

Types of Bonds

Covalent bonds: Sharing of electrons between two nuclei (e.g., C—C, C—H, C—N)
Ionic bonds: Transfer of electrons from one atom to another (e.g., NaCl)

A compound with covalent bonds is called a molecule.

Bonding Preferences of Common Elements

Element	Typical Number of Bonds
C (Carbon)	4
N (Nitrogen)	3
O (Oxygen)	2
H (Hydrogen) and Halides (F, Cl, Br, I)	1

These bonding patterns are essential for predicting molecular structure.

Lewis Structures

Lewis structures are diagrams that show the bonding between atoms and the arrangement of valence electrons. Steps to draw a simple Lewis structure:

Draw individual atoms using dots for valence electrons
Connect atoms to share pairs of electrons and complete octets
Check that each atom (except hydrogen) has an octet

Example: Ethanol (CH3CH2OH)

Formal Charge

Formal charge indicates whether an atom in a molecule has an excess or deficiency of electrons. It is calculated as:

A negative formal charge indicates an excess of electrons (anion)
A positive formal charge indicates a deficiency of electrons (cation)

Isomers

Compounds with the same molecular formula but different connectivity of atoms are called constitutional isomers. Isomers can have different physical and chemical properties.

Example: C2H6O can be ethanol or dimethyl ether, which have different boiling points and states at room temperature.

Resonance

Some molecules cannot be adequately described by a single Lewis structure. Resonance structures are multiple valid Lewis structures that differ only in the arrangement of electrons, not atoms. The true structure is a resonance hybrid, a composite of all resonance forms.

Resonance structures are not real, nor are they in equilibrium
Resonance stabilizes molecules by delocalizing electrons

Curved Arrows in Resonance

Curved arrows are used to show the movement of electron pairs in resonance structures:

The tail starts at the electron source (lone pair or pi bond)
The head points to where the electrons move
Do not break single (sigma) bonds in resonance
Do not exceed the octet for second-row elements (C, N, O, F)

Molecular Geometry

Bond Length and Bond Angle

Two variables define molecular structure:

Bond length: Distance between nuclei of bonded atoms
Bond angle: Angle between adjacent bonds

Bond length decreases across a row (as atom size decreases) and increases down a column (as atom size increases).

Bond	Bond Length (pm)
H—F	92
H—Cl	127
H—Br	141
H—I	161

VSEPR Theory and Molecular Shapes

The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular geometry based on the number of groups (atoms or lone pairs) around a central atom:

Number of Groups	Geometry	Bond Angle
2	Linear	180°
3	Trigonal planar	120°
4	Tetrahedral	109.5°

Examples:

Linear: CO2, C2H2
Trigonal planar: BF3, ethylene (C2H4)
Tetrahedral: CH4 (methane)

Bond angles may be slightly less than the ideal values due to lone pair repulsion (e.g., NH3 and H2O).

Drawing 3D Structures

Solid line: Bond in the plane
Wedge: Bond in front of the plane
Dashed line: Bond behind the plane

All these conventions help represent the three-dimensional structure of molecules on paper.

Summary Table: Determining Geometry Based on Number of Groups

Groups	Geometry	Bond Angle
2	Linear	180°
3	Trigonal planar	120°
4	Tetrahedral	109.5°

Additional info: These notes provide foundational concepts for understanding organic molecules, their bonding, and geometry, which are essential for further study in organic chemistry.