Organic Chemistry: Structure and Bonding

Chapter 1: Structure and Bonding I

1.1 The Purpose of Organic Chemistry

• Goal: Understanding how carbon forms complex molecules essential for life.

• Core Idea: Organic chemistry studies carbon-based compounds, inclusive of both natural (biological) and synthetic substances.

• Importance:

  • Carbon's ability to form four bonds facilitates infinite molecular diversity, manifesting in chains, rings, and branches.

1.2 Key Elements in Organic Chemistry

• The most common elements include: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Sulfur (S), Phosphorus (P), and halogens.

• Carbon: forms 4 covalent bonds.

• Nitrogen: typically forms 3 bonds and has 1 lone pair.

• Oxygen: usually forms 2 bonds and has 2 lone pairs.

• Halogens: form 1 bond and have 3 lone pairs.

• Phosphorus & Sulfur: can utilize expanded octets, allowing for 5 and 6 bonds respectively.

Memorization Aid:

1.3 Drawing and Interpreting Structures

Lewis Structures

• Represent all bonds and lone pairs in a molecule.

• Must follow the octet rule, with the exception of hydrogen which can hold 2 electrons ($H = 2e⁻$).

• Use formal charge calculations to verify accuracy of structures.

  • Formal Charge Formula:
    $ext{FC} = ( ext{valence e}^-) - ( ext{nonbonding e}^- + rac{1}{2} ext{bonding e}^-)$

  • Key Rule: The sum of all formal charges must equal the net molecular charge.

Line Structures

• Carbon atoms are implied at every vertex in the structure.

• Hydrogen atoms bonded to carbon are typically omitted unless part of a functional group.

Condensed Structures

• Represent more compact forms of compounds (e.g., $ext{CH}<em>3 ext{CH}</em>2 ext{OH}$ for ethanol).

1.4 Functional Groups

• Functional groups critically define molecular reactivity and physical properties.

Memorization List of Major Functional Groups

1.5 Isomerism

• Constitutional Isomers: Molecules with the same molecular formula but different connectivity of atoms.

• Stereoisomers: Molecules that have the same connectivity but differ in their 3D arrangement of atoms.

• Note: This concept will be elaborated in Chapter 3.

1.6 Biomolecule Overview

• Four major classes of biomolecules include:

  1. Lipids: Composed of fatty acids; serve in energy storage.

  2. Carbohydrates: Made of sugars; functions include energy provision and structural support.

  3. Proteins: Composed of amino acids; play roles as enzymes and in transport.

  4. Nucleic Acids: DNA and RNA; responsible for information storage.

Chapter 2: Structure and Bonding II

2.1 Covalent Bonding & Valence Bond Theory

• Concept: Bonds form through the overlap of orbitals that contain single electrons.

• Types of Bonds:

  • Sigma (σ) Bonds: End-to-end orbital overlap.

  • Pi (π) Bonds: Side-to-side overlap, present in double and triple bonds.

Memorization Aid: Bond Hierarchy

2.2 Hybridization and Geometry

• Carbon Hybrid Orbitals:
  

  Hybrid	Shape	Bond Angle	Example
  sp³	Tetrahedral	$109.5°$	CH₄
  sp²	Trigonal planar	$120°$	C=C (ethene)
  sp	Linear	$180°$	C≡C (acetylene)
  Rule: Each σ bond or lone pair corresponds to 1 hybrid orbital.
  Shortcut Cue: Count electron groups (sum of σ bonds + lone pairs):

  • 4 groups → sp³

  • 3 groups → sp²

  • 2 groups → sp

2.3 Resonance

• Real molecules can be considered hybrids of multiple valid Lewis structures.

• Electrons can move, but atoms remain fixed in position.

• Rules:

  • Move π electrons or lone pairs when drawing resonance structures.

  • Do not break σ bonds.

  • Maintain the overall charge of the molecule.

• Stability Order: Resonance structures with more octets are favored over those with fewer formal charges, with a negative formal charge located on electronegative atoms being more stable.

2.4 Intermolecular Forces

• 
  

• Forces that impact physical properties such as boiling/melting points, solubility, and phase.
  

  Force	Example	Strength	Notes
  London Dispersion	All molecules	Weakest	Temporary dipoles
  Dipole–Dipole	Polar molecules	Medium	Align opposite poles
  Hydrogen Bond	H–O, H–N, H–F	Strong	Forms extensive networks
  Ion–Dipole	Ions + polar solvent	Strongest	Particularly significant in aqueous solutions
  Trends in Intermolecular Forces

  • Stronger intermolecular forces lead to increased boiling and melting points.

  • Increased branching in hydrocarbons leads to decreased surface area and, consequently, lower boiling points.

  • The phrase "like dissolves like" encapsulates solubility tendencies: polar substances will dissolve in polar solvents, while nonpolar substances will dissolve in nonpolar solvents.

  2.5 Lipid Properties

  • Saturated Lipids: Characterized by single bonds in fatty acids, generally solid at room temperature, and have higher melting points.

  • Unsaturated Lipids: Contain one or more double bonds (C=C) causing kinks in their structure; generally liquids at room temperature, possessing lower melting points.

  • This property explains the fluid nature of biological membranes, which incorporate both types of fatty acids.

  Chapter 3: Conformation and Stereochemistry

  3.1 Conformations

  • Varying spatial arrangements due to rotation around σ bonds.

  Newman Projections

  • A method for visualizing conformations by viewing along a bond axis.

  • Energy States:

    • Staggered: Lowest energy form, with groups positioned $60°$ apart.

    • Eclipsed: Highest energy form, with groups in direct alignment ($0°$ apart).

    • Anti: The most stable energy arrangement, with large groups located opposite each other (180° apart).

    • Gauche: Partially stable arrangement, with large groups $60°$ apart.

  • Energy Order: ext{Anti} < ext{Gauche} < ext{Eclipsed B} < ext{Eclipsed A} .

  3.2 Cyclic Conformations

  Cyclohexane

  • Chair Form: Most stable conformation with bond angles of $109.5°$.

  • Boat Form: Higher energy state due to steric and torsional strain.

  • Axial and Equatorial Positions:

    • Axial: Positions are directed up or down.

    • Equatorial: Positions are directed outward from the ring, which is more favorable for bulky groups.

  • Rule: Bulky substituents preferentially adopt equatorial positions to minimize steric strain.

  3.3 Chirality and Stereoisomerism

  • Chirality: A property of a molecule that makes it non-superimposable on its mirror image.

    • Enantiomers: Molecules representing a pair that are mirror images of one another but differ in configuration.

    • Diastereomers: Molecules that are not mirror images but have the same connectivity.

    • Meso Compounds: Molecules with multiple stereocenters that have an internal plane of symmetry, rendering them achiral.

  • Identifying Chiral Centers: A carbon atom with four different substituents is a chiral center.

  3.4 R/S Configuration

  Cahn–Ingold–Prelog (CIP) Rules

  1. Assign Priorities: Based on atomic number; higher atomic number denotes higher priority.

  2. Orient the Lowest Priority Group Away: Position the group of lowest priority in the back.

  3. Trace the Path: From the highest to lowest priority, if the trace is clockwise it is designated as R, if counterclockwise it is designated as S.

  • Shortcut: Using your right hand, point your thumb along the lowest priority; then curl your fingers from 1 to 2 to 3.

  3.5 Optical Activity

  • Enantiomers: Rotate plane-polarized light in opposite directions.

  • Racemate (± Mixture): A 50/50 mixture of both enantiomers that results in an optically inactive substance due to cancellation of light rotation effects.

  3.6 Isomer Classification Summary

  Type	Definition	Example
  Constitutional	Isomers with different connectivity	butane vs isobutane
  Conformational	Isomers formed through rotation about a single bond	staggered vs eclipsed
  Stereoisomers	Same connectivity, different spatial arrangement	R vs S
  Enantiomers	Mirror images	R-lactic acid / S-lactic acid
  Diastereomers	Not mirror images	cis/trans isomers
  Meso	Molecules with an internal plane of symmetry	meso-tartaric acid