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Chapter 2: Molecular Representations and Resonance in Organic Chemistry

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Molecular Representations

Overview of Molecular Representations

Organic molecules can be represented in several ways, each providing different levels of detail and utility. Understanding these representations is fundamental for communication and problem-solving in organic chemistry.

  • Lewis Structure: Shows all atoms, bonds, and lone pairs explicitly.

  • Partially Condensed Structure: Bonds are often omitted, but atoms are still shown.

  • Condensed Structure: Atoms are grouped, and bonds are generally omitted.

  • Molecular Formula: Only the number and type of atoms are indicated.

Key Considerations:

  • Choose a representation that balances clarity and simplicity for the task at hand.

  • Lewis structures provide the most information, while molecular formulas provide the least.

Converting Between Representations

  • Lewis to Condensed: Omit bonds, group identical groups using parentheses and subscripts.

  • Condensed to Lewis: Draw all atoms and bonds, ensuring correct valency and octet completion.

Bond-Line (Skeletal) Structures

The bond-line structure is a simplified way to represent organic molecules, omitting most hydrogen and carbon atoms for clarity.

  • Drawn in a zigzag format; each vertex or endpoint represents a carbon atom.

  • Double and triple bonds are shown with two or three lines, respectively.

  • Hydrogen atoms bonded to carbon are not shown; those bonded to heteroatoms (N, O, halogens) must be shown.

  • Heteroatoms and their attached hydrogens are always shown.

Rules for Drawing Bond-Line Structures

  • Rule 1: Draw sp2 and sp-hybridized atoms in a zigzag format.

  • Rule 2: Draw double bonds as far apart as possible.

  • Rule 3: The direction of single bonds is irrelevant.

  • Rule 4: Show all heteroatoms and hydrogens attached to them.

  • Rule 5: Never draw more than four bonds to a carbon atom (octet rule).

Steps for Drawing Bond-Line Structures

  1. Delete hydrogen atoms (except those connected to heteroatoms).

  2. Arrange the carbon skeleton in a zigzag pattern.

  3. Delete carbon atoms (they are implied at vertices and endpoints).

Functional Groups

Definition and Importance

Functional groups are characteristic groups of atoms/bonds that determine the chemical behavior of molecules. They react in predictable ways and are the most important part of a molecule for reactivity.

  • The symbol R is used to represent an alkyl or other unreactive group.

Common Functional Groups

Type

Example Structure

Alkyl halide

R-X (X = F, Cl, Br, I)

Alkene

R-CH=CH-R

Alkyne

R-C≡C-R

Aromatic

Benzene ring

Alcohol

R-OH

Ether

R-O-R'

Thiol

R-SH

Sulfide

R-S-R'

Ketone

R-CO-R'

Aldehyde

R-CHO

Carboxylic Acid

R-COOH

Ester

R-COOR'

Acid Chloride

R-COCl

Anhydride

R-CO-O-CO-R'

Amide

R-CONH2

Amine

R-NH2

Bonding Patterns and Formal Charges

Carbon Atom

  • Neutral carbon: 4 bonds, no lone pairs.

  • Carbocation (positively charged): 3 bonds, no lone pairs.

  • Carbanion (negatively charged): 3 bonds, 1 lone pair.

Oxygen Atom

  • Negative charge: 1 bond, 3 lone pairs.

  • Neutral: 2 bonds, 2 lone pairs.

  • Positive charge: 3 bonds, 1 lone pair.

Nitrogen Atom

  • Negative charge: 2 bonds, 2 lone pairs.

  • Neutral: 3 bonds, 1 lone pair.

  • Positive charge: 4 bonds, 0 lone pairs.

Formal Charge Calculation

Formal Charge = Valence electrons – (number of bonds) – (number of lone pair electrons)

Three-Dimensional Bond-Line Structures

Organic molecules are three-dimensional. To represent 3D structure on paper:

  • Dashed lines indicate bonds going back into the plane.

  • Solid wedges indicate bonds coming out of the plane.

Introduction to Resonance

Concept of Resonance

Pi bonds and/or formal charges are often more delocalized than a single bond-line structure can show. Resonance describes the delocalization of electrons across multiple atoms, stabilizing the molecule.

  • Each atom involved is typically sp2 hybridized with unhybridized p orbitals.

  • Electrons can move throughout the overlapping area, creating resonance.

Resonance Structures and Resonance Hybrid

  • Resonance structures are different Lewis structures for the same molecule, connected by resonance arrows.

  • The actual molecule is a resonance hybrid, an average of all significant contributors.

  • Resonance hybrids are more stable due to electron delocalization.

Resonance Stabilization

  • Delocalization of electrons minimizes repulsions and maximizes attractions, lowering energy and increasing stability.

  • Delocalization of charge spreads partial charges over multiple atoms, making the molecule more stable than if the charge were localized.

Curved Arrows in Resonance

Using Curved Arrows

  • Curved arrows show the movement of electron pairs (from a lone pair or bond to another atom or bond).

  • The tail shows where electrons start; the head shows where they end up.

Rules for Curved Arrows in Resonance

  • Rule 1: Do not break single bonds when drawing resonance structures.

  • Rule 2: Never exceed an octet for second-row elements (B, C, N, O, F).

Formal Charges in Resonance

  • Always show formal charges when using curved arrows to derive resonance structures.

  • Second-row elements may have less than an octet, but never more.

Resonance Pattern Recognition

Five General Bonding Patterns for Resonance

  1. Allylic lone pair

  2. Allylic carbocation

  3. Lone pair adjacent to a carbocation

  4. Pi bond between atoms of different electronegativities

  5. Conjugated pi bonds in a ring

Vinyl and Allylic Positions

  • Vinyl: Carbons directly involved in a double bond.

  • Allylic: Atoms directly attached to the vinyl carbons.

Patterns in Detail

  • Pattern #1: Lone pair next to a pi bond (requires two curved arrows).

  • Pattern #2: Allylic carbocation (one curved arrow from pi bond to carbocation).

  • Pattern #3: Lone pair adjacent to a carbocation (one curved arrow from lone pair to form a pi bond).

  • Pattern #4: Pi bond between atoms of different electronegativities (move pi bond to more electronegative atom).

  • Pattern #5: Conjugated pi bonds in a ring (pi bonds can be shifted around the ring).

Assessing the Relative Importance of Resonance Structures

Rules for Determining Major Resonance Contributors

  1. The most significant forms have the greatest number of filled octets.

  2. Structures with fewer formal charges are more significant.

  3. A negative charge on a more electronegative atom is more significant (and vice versa).

Equivalent Resonance Structures

  • Structures that are equivalent contribute equally to the resonance hybrid.

  • Insignificant resonance forms do not contribute meaningfully.

Delocalized and Localized Lone Pairs

  • Localized electrons are not involved in resonance.

  • Delocalized electrons participate in resonance and increase stability.

  • For a lone pair to be delocalized, it must be adjacent to an atom with an unhybridized p orbital.

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