Organic Chemistry Chapter 2: Molecular Representations and Functional Groups

Organic Chemistry Chapter 2: Molecular Representations And Functional Groups

2.1 Representing Molecules / Introduction

  • Different methods exist to represent molecules in organic chemistry.
  • Key considerations for representation:
    • What essential information is needed to accurately describe a molecule?
    • Which representation methods are easiest to draw?
    • Which forms provide the most informative data about the molecule?

2.1 Representing Molecules / Structural Examples

  • Molecular Formula Limitations:
    • The molecular formula alone (e.g., C3H8O) may be inadequate for distinguishing between different structures.
  • Examples of Structures:
    • Isopropanol:
    • Structural depiction:
      H H H | | H—C—C—C—O—H | | H H
    • Propanol:
    • Structural depiction:
      H H | | H—C—C—O—H | | H H
    • Ethyl methyl ether:
    • Structural depiction is also important.

2.2 Bond-Line Structures

  • Inadequacy of Lewis Structures:
    • Lewis structures are often impractical for large compounds (e.g., Amoxicillin) due to complexity.
    • Condensed formulas provide minimal information about molecular shape.
  • Bond-Line Structure Advantages:
    • Easier to read and draw than Lewis structures.
    • Recognized as benchmark representations for organic compounds.
    • Mastery in drawing bond-line structures is crucial for success in organic chemistry courses.

2.2 Bond-Line Structures / Reading and Interpretation

  • Carbon Atoms:
    • Each corner or endpoint in the bond-line structure represents a carbon atom.
    • Example counts: Hexane contains six carbon atoms; 2-butenes and 2-butyne contain four carbon atoms each.
  • Bond Angles:
    • Zigzag patterns effectively represent bond angles for sp3 and sp2 hybridized atoms.
    • Linear geometry is illustrated for sp hybridized atoms.

2.2 Bond-Line Structures / Hydrogen Atoms

  • Carbon atoms are unlabelled in the structure, but hydrogen atoms bonded to carbon are not shown.
    • It is understood that enough H atoms exist to satisfy the octet rule for carbon (4 bonds).
    • Reference to implicit hydrogens within bond-line structures is crucial for understanding saturation and bonding.

2.2 Bond-Line Structures / Practice Exercises

  • Counting Atoms:
    • Engage in practice exercises to enhance recognition of carbon and hydrogen atoms in bond-line structures.
  • Molecular Formula Generating:
    • Exercise examples include giving molecular formulas based on provided bond-line structures.

2.2 Drawing Bond-Line Structures

  • Rules for Drawing:
    1. Straight Chains: sp2 and sp3 hybridized carbons should be drawn using a zigzag format.
    2. Double Bonds: Position double bonds maximally apart.
    3. Single Bonds: The direction of drawn single bonds is not significant.
    4. Heteroatoms Requirement: All heteroatoms (non-carbon/non-hydrogen) and their attached hydrogens must be explicitly drawn.
    5. Octet Rule Compliance: No carbon should exhibit more than four bonds, adhering to the octet rule.

2.2 Bond-Line Structures: Practice Examples

  • Conversion Exercises:
    • Task: Convert condensed structures to corresponding bond-line structures.
    • Example provided for practice: H3C-CH2-CH_3 and others to be transformed.

2.3 Bond-Line Reactions

  • Relationship in Reactions:
    • Bond-line structures simplify visualization of bonds formed or broken during chemical reactions.
    • Comparing condensed formulas to bond-line structures enhances understanding of functional group transformations.

2.3 Functional Groups

  • Functional groups comprise specific arrangements of atoms that exhibit similar properties and undergo comparable reactions.
    • Understanding functional groups is essential in organic chemistry. Students should learn to identify various functional groups within larger structures (refer to Table 2.1).

2.3 Carbonyl Functional Group

defines

  • Carbonyl:
    • Definition: A carbonyl is a carbon atom double bonded to an oxygen atom.
    • Carbon must form four bonds; hence, true structures of carbonyls need further consideration of their R groups.
  • R Group Role:
    • Different R groups determine the nomenclature of the functional group:
    • Hydroxyl (OH) signifies carboxylic acid.
    • Hydrogen in R denotes an aldehyde.
    • Both R groups as carbon signify a ketone.

2.3 Identifying Functional Groups

  • Classification Techniques:
    • Approach by separating groups into those containing carbonyls and those that do not.

2.3 Functional Groups Listing

  • The following are examples of functional groups and their classifications:
    • Ketone: R-C(=O)-R
    • Aldehyde: R-C(=O)-H
    • Carboxylic Acid: R-C(=O)-OH
    • Additional examples include amides, esters, nitriles, alkynes, etc., along with their respective structures and classifications.

2.4 Carbon Atoms with Formal Charges / Common Scenarios

  • Understanding Formal Charges:
    • A carbon atom has 4 bonds when it possesses no formal charge.
  • Carbocations & Carbanions:
    • In a carbocation, carbon shows 3 bonds and one empty orbital; while a carbanion has 3 bonds and a lone pair.

2.4 Formal Charge and Stability

  • Formal charges must be clearly identified on bond-line structures as they impact molecular stability and reactivity.

2.5 Bond-Line Structures: Identifying Lone Pairs

  • Presence of Lone Pairs:
    • While essential to indicate formal charges, drawing lone pairs on structures is optional.
    • Formal charges inform about the implied presence of lone pairs adjacent to atoms.
  • Oxygen Atom Examples:
    • Single-bond scenarios denote valence configurations:
    • 6 valence electrons for neutral, 7 for anionic conditions with one bond.
    • Lone pairs are implied based on affiliations of atoms in structures.

2.6 Bond-Line Structures in 3-D / Dashes and Wedges

  • To depict 3D molecular representations:
    • Use dashed lines for bonds pointing back into the paper and solid wedges for those out of the paper.

2.7 Introduction to Resonance

  • Resonance Definition:
    • Pi bonds and/or formal charges can be more distributed than shown in a bond-line structure.
  • Example: The allyl carbocation and its inadequate representation by a single bond-line structure.
  • p Orbitals Interaction:
    • Overlapping p orbitals crucial for discussing electron delocalization, affirming resonance stability.

2.7 Resonance Representation

  • Drawing Resonance Structures:
    • Depict multiple resonance structures to express the positive charge effectively, utilizing resonance arrows and brackets as indicators.

2.7 Resonance / Analogy

  • Resonance Structural Analogy:
    • Example: A nectarine represents a fixed hybrid of peach and plum, akin to how resonance structures exist as a hybrid without switching between forms.

2.7 Resonance / Stabilization Effects

  • Stabilization via Delocalization:
    • Delocalized electrons prefer lower energy states and greater stability due to minimized repulsion and amplified attraction to nuclei.
    • Molecules with increased resonance structures tend to exhibit greater stability.

2.8 Curved Arrows / Introduction

  • Curved Arrow Usage:
    • Employed to depict electron movement in organic reactions.
    • Rules for curved arrow representation focus on start and end positions indicating electron flow.

2.8 Curved Arrows / Rules of Usage

  • Five Specific Rules:
    1. Do not depict single (sigma) bonds as delocalized.
    2. Do not exceed octets for elements from the second row (B, C, N, O, F).

2.9 Formal Charges in Resonance / Importance

  • Significance of Proper Formal Charge Indication:
    • Formal charges are essential to form valid resonance structures; incorrect assignment can mislead structural representations.

2.10 Resonance Pattern Recognition / Five Patterns

  • Recognizing patterns aids in predicting and identifying resonance structures:
    • Allylic lone pair.
    • Allylic carbocation.
    • Lone pair adjacent to carbocation.
    • Pi bond between different electronegativities.
    • Conjugated pi bonds in a ring are the main resonance features.

2.10 Pattern Summary

  • Each identified pattern allows for informed prediction regarding resonance circumstances, necessitating significant practice for mastery.

2.11 Assessing Resonance Structures / Introduction

  • Identifying Major Contributors:
    • The real structure is a combination of several resonance structures, emphasizing that not all contribute equally to the hybrid.
  • Key Evaluation Rules for assessing stability:
    1. Octet Rule Compliance: Favor structures with filled octets.
    2. Fewest Formal Charges: Preference is given to structures with the least number of formal charges.
    3. Electronegative Charge Stability: Negative charge stability is maximized on the most electronegative atom.

2.12 The Resonance Hybrid

  • The hybrid reflects delocalized pi bonds across carbon atoms, consistent with molecular orbital theory, signifying shared electron density rather than localized bonding.