Organic Chemistry - Molecular Representations & Resonance

Chapter 2: Molecular Representations

2.1 Molecular Representations / Introduction

  • Various methods exist to represent molecules.
  • Essential considerations include:
    • What information is vital to accurately describe a molecule?
    • Which representations are straightforward to draw?
    • Which representations convey maximum information about the molecule?

2.1 Molecular Representations / Practice

  • Engage in converting between different types of molecular representations utilizing SkillBuilder 2.1 – Converting a Structure.

2.2 Bond-Line Structures / Comparison

  • Lewis Structure: Represents molecules but is less efficient in terms of readability and drawing.
  • Bond-Line Structure (Skeletal Structure):
    • Easier to read and draw, serving as the benchmark representation for organic compounds.

2.2 Bond-Line Structures / Utility

  • Proficiency in drawing bond-line structures for any compound is crucial for success in organic chemistry courses.

2.2 How to Draw Bond-Line Structures

  • Essential rules for drawing bond-line structures include:
    • Rule 1: sp² and sp³ hybridized atoms in a straight chain should be represented in zigzag format.
    • Rule 2: For double bonds, spread all bonds as far apart as possible.
    • Rule 3: The orientation of single bond drawings is irrelevant.
    • Rule 4: Include all heteroatoms (non-carbon, non-hydrogen) and their attached hydrogen atoms.
    • Rule 5: Follow the cardinal rule: Never draw more than four bonds to a carbon atom (Honoring the octet rule).

2.3 Bond-Line Structures in 3D

  • Molecules occupy 3D space, but representing this on a 2D medium (paper or board) poses challenges.
  • Use dashed and solid wedges to illustrate groups extending back or out from the paper, respectively.
  • Understanding molecular shape is critical for biological interaction relevance.

2.4 Identifying Functional Groups / Introduction

  • Bond-line structures facilitate visualization of bonds made and broken during chemical reactions.
  • Bond-line structures enhance clarity regarding functional group transformations during reactions.

2.4 Identifying Functional Groups / Definition

  • Functional groups are characteristic groups of atoms or bonds essential for understanding chemical behavior.
  • Students must learn the terminology for each functional group (as indicated in Table 2.1).

2.4 Identifying Functional Groups / List

  • Common functional groups include (R signifies other atoms, typically carbon or hydrogen):
    • Thiol: R-SH
    • Ester: R-
    • Sulfide: R-S-R
    • Amide: R=NH₂
    • Amine: R=N
    • Alcohols, Amines, Alkyl Halides, Amides

2.5 Bond-Line Structures: Identifying Lone Pairs / Introduction

  • Formal charge must always be indicated.
  • Although drawing lone pairs is optional, the presence or absence is often implied by the formal charge.
  • Example of oxygen's valence electrons:
    • Oxygen in the 6th group of the periodic table requires 6 valence electrons for neutrality.
    • An oxygen anion has seven electrons (one bond and six unshared electrons).

2.5 Bond-Line Structures: Identifying Lone Pairs

  • Oxygen's bonding patterns can involve 5, 6, or 7 valence electrons.
  • Practice applying these concepts using SkillBuilder 2.4 – Identifying Lone Pairs.

2.6 Carbon Atoms with Formal Charges

  • A neutral carbon atom has 4 bonds.
  • In a carbocation, a carbon has 3 bonds and one vacant orbital.
  • A carbanion also has 3 bonds but possesses a lone pair (no vacant orbitals).

2.6 Carbon Atoms with Formal Charges / Practice

  • Understanding formal charges (refer to section 1.4) impacts molecular stability and reactivity.
  • Structures must have formal charges correctly indicated.

2.7 Introduction to Resonance / Practice

  • A single representation may not sufficiently describe a chemical species, thus necessitating resonance structures.

2.8 Curved Arrows / Introduction

  • Curved arrows signify electron movement in organic chemistry.
  • The arrow begins from the current electron location and ends where the electrons relocate.

2.8 Curved Arrows / Rule One

  • Specific rules exist for using curved arrows for representing electron delocalization (resonance):
    • Rule 1: Never depict a single (sigma) bond as delocalized; resonance involves overlapping p orbitals (pi bonds and lone pairs).

2.8 Curved Arrows / Rule Two

  • Rule 2: Do not exceed an octet for second-row elements (B, C, N, O, F); they possess a maximum of 8 electrons.

2.8 Curved Arrows / Second Row Elements

  • Second-row elements may occasionally have fewer than eight electrons but must never exceed this limit.

2.9 Formal Charges in Resonance / Importance

  • Indicating formal charges is essential for drawing valid resonance structures.

2.9 Formal Charges in Resonance / Must Be Shown

  • When drawing resonance structures using curved arrows, formal charges must be clearly represented.

2.10 Resonance Pattern Recognition / Five Patterns

  • Five general bonding patterns for resonance:
    1. Allylic lone pair
    2. Allylic carbocation
    3. Lone pair adjacent to carbocation
    4. Pi bond between atoms with different electronegativities
    5. Conjugated pi bonds in a ring
  • Practicing these patterns through examples is essential to mastering resonance concepts.

2.10 Resonance Pattern Recognition / Pattern One

  • Pattern #1 focuses on "Allylic Lone Pair" (p orbitals next to a C=C double bond).
  • Requires two curved arrows to illustrate delocalization.
    • Upon delocalization, the atom with the lone pair becomes positively charged while the atom accepting the lone pair becomes negatively charged.

2.10 Resonance Pattern Recognition / Pattern Two

  • Pattern #2: Deals with allylic carbocations where a single curved arrow illustrates resonance across multiple structures.

2.10 Resonance Pattern Recognition / Pattern Three

  • Pattern #3: Involves a lone pair adjacent to a carbocation, requiring only one curved arrow for resonance demonstration.

2.10 Resonance Pattern Recognition / Pattern Four

  • Pattern #4: Involves a pi bond between different electronegativity atoms, resulting in unequal sharing of pi electrons, showcasing extreme resonance structures.

2.10 Resonance Pattern Recognition / Pattern Five

  • Pattern #5: Involves conjugated pi bonds in a ring where each atom possesses an unhybridized p orbital allowing resonance delocalization.

2.11 Assessing Resonance Structures / Introduction

  • The hybrid structure resulting from multiple resonance forms indicates the actual compound structure.
  • Not all resonance structures contribute equally to the hybrid; assess their significance using set rules.

2.11 Assessing Resonance Structures / Rule One

  • Most significant resonance forms feature the maximum number of filled octets.

2.11 Stability of Contributors / Rule Two

  • Structures with fewer formal charges are considered more significant contributors than those with multiple formal charges.

2.11 Stability of Contributors / Rule Two, Example

  • For charged compounds, focus on drawing resonance forms that illustrate the charge's delocalization.

2.11 Stability of Contributors / Rule Three

  • A structure with a negative charge on a more electronegative atom is assessed as more significant in resonance contributions.

2.11 Stability of Contributors / Practice

  • Engage in SkillBuilder 2.27 to draw significant resonance forms and identify the major contributor.

2.12 – Resonance Hybrid / MO Theory

  • Resonance often depicts pi bonds or formal charges that may not be accurately represented by bond-line structures.
  • An allyl carbocation serves as an example of complex charge depiction.

2.12 – Resonance Hybrid / p Orbitals

  • Overlapping p orbitals allow for increased electron delocalization, leading to resonance structure representation.

2.12 The Resonance Hybrid

  • The allylic carbocation may be described by resonance structures depicting electron localization across multiple carbon atoms, aligning with MO theory.

2.12 – Resonance Hybrid / MO Viewpoint

  • Upon overlapping p orbitals, three new MOs are formed, impacting the charge distribution of the allyl carbocation positively across the molecular structure.

2.13 Delocalized Lone Pairs

  • Localized lone pairs are electrons not in resonance, while delocalized lone pairs undergo resonance stabilization.
    • Delocalization is feasible if lone pairs are adjacent to an atom possessing an unhybridized p orbital.

2.13 Localized Lone Pairs / Definition

  • A lone pair that cannot overlap with a neighboring p orbital is termed a localized lone pair.
  • Example: The lone pair in pyridine does not allow overlap requisite for resonance participation.

2.13 Localized Lone Pairs / General Guidelines

  • Localized lone pairs are not to be assumed as delocalized merely due to proximity to pi bonds.
    • It can be determined that if an atom possesses both pi bonds and lone pairs, generally they will not participate in resonance.

Questions and Practice

  • Review and practice questions regarding the contribution of various electron configurations to resonance and chemical stability.