Lecture 3 - Properties of Organic Molecules: Representation, Structure, and Functional Groups

Properties of Molecules

Introduction to Molecular Structure

Organic molecules are composed of atoms connected by chemical bonds, forming distinct three-dimensional structures. Understanding how to represent and interpret these structures is fundamental in organic chemistry, as molecular properties and reactivity are closely tied to their arrangement and functional groups.

Representing Molecules

Types of Molecular Representations

• Condensed Formula: Shows the sequence of atoms and groups, e.g., CH3CH2CH2CH3.

• Condensed Structural Formula: Indicates branching and connectivity, e.g., CH3CH(CH3)CH2CH3.

• Lewis Structure: Displays all atoms, bonds, and lone pairs, useful for visualizing electron distribution.

• Line Angle Drawing: Simplifies organic molecules by representing carbon atoms as vertices and bonds as lines; hydrogens attached to carbons are often omitted for clarity.

• Line Angles with Dashes and Wedges: Used to depict three-dimensional geometry, with wedges indicating bonds coming out of the plane and dashes for bonds going behind the plane.

Example: The molecule CH3CH2CH(OH)CH3 can be represented as a Lewis structure showing all atoms and bonds, or as a line angle drawing highlighting the carbon backbone and functional group.

Main Chain and Branches

Organic molecules often have a main chain (the longest continuous chain of carbon atoms) and branches (side chains or substituents).

• Main Chain: Determines the base name of the molecule.

• Branches: Are named as substituents and affect the molecule's properties and nomenclature.

Example: In CH3CH(CH3)CH2CH3, the main chain has four carbons, and there is a methyl branch on the second carbon.

Bonding Patterns and Hybridization

Common Bonding Patterns

The structural formula reveals whether atoms are single, double, or triple-bonded, which affects hybridization and molecular geometry.

• Carbon: Can form single (sp3), double (sp2), or triple (sp) bonds.

• Nitrogen: Often forms three bonds and has a lone pair; can be positively charged (ammonium) or neutral (amine).

• Oxygen: Typically forms two bonds and has two lone pairs; found in alcohols, ethers, carbonyls, etc.

• Halogens: Usually form one bond and have three lone pairs.

• Hydrogen: Forms one bond.

Example: The Lewis structure of water (H2O) shows two single bonds and two lone pairs on oxygen.

Hybridization

• sp3 Hybridization: Tetrahedral geometry, single bonds.

• sp2 Hybridization: Trigonal planar geometry, double bonds (as in alkenes).

• sp Hybridization: Linear geometry, triple bonds (as in alkynes).

Example: Ethylene (C2H4) has sp2 hybridized carbons due to the double bond.

Isomerism

Structural (Constitutional) Isomers

Isomers are molecules with the same molecular formula but different connectivity of atoms.

• Structural Isomers: Differ in the sequence of bonding; e.g., butane (CH3CH2CH2CH3) vs. isobutane (CH3CH(CH3)CH3).

Example: C4H10 can be drawn as two different structures, each with unique properties.

Functional Groups

Definition and Importance

A functional group is a specific group of atoms within a molecule responsible for characteristic chemical reactions. Functional groups are often the sites of reactivity and determine the physical and chemical properties of organic compounds.

• Examples: Hydroxyl (-OH), carbonyl (C=O), amino (-NH2), carboxyl (-COOH).

Application: Functional groups are highlighted in molecules like cortisol and amoxicillin, indicating regions of chemical activity.

Classification of Functional Groups

Group	Structure	Example
Alkene	sp2 hybridized C=C	Ethylene
Alkyne	sp hybridized C≡C	Acetylene
Alcohol	-OH group	Ethanol
Ether	R-O-R'	Diethyl ether
Aldehyde	R-CHO (C=O with H)	Formaldehyde
Ketone	R-CO-R'	Acetone
Carboxylic Acid	R-COOH	Acetic acid
Ester	R-COOR'	Ethyl acetate
Amide	R-CONH2	Acetamide
Halide	R-X (X = F, Cl, Br, I)	Chloroform

Polarity and Molecular Properties

Electronegativity and Bond Polarity

Differences in electronegativity between atoms cause bonds to be polarized, resulting in partial charges. This affects the overall polarity of the molecule and its interactions.

• Bond Dipole: A measure of the separation of charge in a bond.

• Molecular Dipole Moment: The vector sum of all bond dipoles in a molecule.

Example: Acetone has a significant dipole moment due to the polar C=O bond.

Intermolecular Forces

Functional groups influence the types of intermolecular forces present, which affect boiling point, solubility, and other physical properties.

• London Dispersion Forces: Present in all molecules, especially non-polar ones; arise from temporary dipoles.

• Dipole-Dipole Interactions: Occur between polar molecules.

• Hydrogen Bonding: Strong interaction involving H attached to N, O, or F.

Solubility: "Like dissolves like"—polar molecules dissolve in polar solvents, non-polar molecules dissolve in non-polar solvents.

Summary Table: Functional Groups and Their Properties

Functional Group	General Structure	Polarity	Example
Alcohol	-OH	Polar	Ethanol
Ether	R-O-R'	Slightly polar	Diethyl ether
Aldehyde	R-CHO	Polar	Formaldehyde
Ketone	R-CO-R'	Polar	Acetone
Carboxylic Acid	R-COOH	Highly polar	Acetic acid
Amine	R-NH2	Polar	Methylamine
Amide	R-CONH2	Polar	Acetamide
Halide	R-X	Variable	Chloroform

Key Equations

• Dipole Moment:

$\mu = q \times r$

• Hybridization:

$\text{sp}^3: \text{tetrahedral},\ \text{sp}^2: \text{trigonal planar},\ \text{sp}: \text{linear}$

• General Formula for Alkanes:

$C_nH_{2n+2}$

Additional info: Expanded explanations and tables were added for completeness and clarity, including general properties and examples of functional groups, and key equations relevant to molecular polarity and hybridization.