wednesday, may 28 2025

Orbital of Lone Pair on Oxygen

• To determine the orbital of a lone pair on oxygen, find the hybridization of the oxygen atom.

• Example: If oxygen is $sp$ hybridized, it has two $sp$ orbitals, one for bonding and the other for a lone pair.

Quick Practice Questions

• Question 14: Which compound has the weakest carbon-halide bond?

  • Consider carbon-fluorine, carbon-chlorine, carbon-bromine, and carbon-iodine bonds.

  • Atoms further down the periodic table (shells 4, 5, 6) result in weaker bonds.

  • Iodine is below the other halides, so carbon-iodine bond is the weakest.

• Question 17: Which compound has a divalent oxygen (formal charge of zero)?

  • Divalent oxygen is in its normal valency state, resulting in a zero charge.

Pacing and Time Management

• Review sheets are good practice for ensuring a good pace.

• Practice working at a decent pace for quizzes, exams, and standardized tests (MCAT, etc.).

• Example: Six problems should take between 6 to 12 minutes.

Staying Ahead in Class

• Look at the syllabus and schedule to see upcoming topics.

• Review lecture notes and review sheets before class.

• Identify concepts you understand and those you don't.

• Prepare questions to ask during the lecture.

Benefits of Previewing Material

• Previewing material helps prepare you to soak in information during lectures.

• Even 20-30 minutes of review can be beneficial.

Resonance Structures

• Being able to draw resonance structures is critical for success in organic chemistry.

• Resonance structures are used throughout the course and in subsequent courses (e.g., Organic Chemistry 124).

• Understanding resonance structures is like understanding the language of organic chemistry.

Importance of Resonance

• Single drawings of molecules are limited in showing the complete electronic picture.

• Resonance structures provide more detail and understanding, similar to using visible and IR light to view a planet nebula.

Resonance Structures and Electron Delocalization

• Resonance structures are different ways to draw a molecule because our formalism can't fully capture the molecule in one picture.

• Electrons (especially pi electrons) can move around the molecule and be delocalized.

• Delocalization involves spreading out charge (positive or negative).

Representing Resonance

• Use double-headed arrows to indicate resonance forms.

• Use curved arrows to illustrate electron reorganization between resonance structures.

• Curved arrows help visualize electron flow.

• The actual compound exists as a hybrid or combination of all resonance structures.

When Resonance Occurs

• Resonance structures come into play with combinations of lone pairs, pi bonds, and/or charges that are next to each other in the molecule.

• If these elements are separated by one or more carbons, resonance structures cannot be drawn.

Curved Arrows

• Curved arrows show electron flow/movement, not atom or charge movement.

• Be careful where curved arrows start; they must start at an area of high electron density (lone pair or bond).

Rules for Drawing Resonance Structures

• Atoms never change location.

• Electrons in sigma bonds never move.

• Only pi electrons (electrons in pi bonds or p orbitals) move.

• Electrons must stay on an atom that they were originally part of.

• Every resonance structure must have the same overall net charge.

• Atoms in the second row of the periodic table (C, N, O) cannot have more than eight electrons around them.

• Avoid +2 or -2 charges (typically incorrect).

Major vs. Minor Resonance Contributors

• Some resonance structures are better (more stable) than others and contribute more to the overall structure.

• Unlikely resonance structures (e.g., with separated charges) contribute very little.

Determining Major Resonance Contributors

• Most Important: Maximize octets. Structures with all atoms having full octets are major contributors.

• Second: Place charges on atoms with compatible electronegativity values (negative charge on more electronegative atoms, positive charge on less electronegative atoms).

• Third: Avoid having charges if possible.

Valid Resonance Structures

• All resonance structures must have the same net charge.

• Be careful when lone pairs aren't drawn in skeletal structures

Practice Problems

• Practice drawing all likely resonance structures for a compound.

• You may be asked to draw one or more additional resonance structures, better resonance structures, or worse resonance structures.

• Homework sets will assess the ability to draw correct additional resonance structures.

Steps for Drawing Resonance Structures

• Move one thing. If you move two things, it's easier to miss another resonance structure.

Resonance Hybrids

• Resonance hybrids are a way to try to draw all resonance structures in one form, but they can be confusing.

• They involve showing partial charges and dotted lines for partial pi bonds.

Resonance and Stability

• Adding resonance stabilizes a molecule.

• The more resonance structures possible, the greater the stabilization.

• Major resonance contributors best describe the structure, while minor resonance contributors better describe reactivity. This helps determine why one nitrogen is more reactive than another.

Common Pitfalls

• Do not hybridize an SP3 into an SP2/resonance. If an atom looks like SP or SP2 in any resonance structure, it has to be that hybridization in all subsequent structures.

Resonance Practice Questions

• Practice drawing degenerate (equivalent) resonance structures.

• Distinguish between better and worse resonance structures.

• Understand which is more stable when charges exist.

• Correctly identify hybridization.

Acid-Base Chemistry

• Organic chemistry handles acid-base chemistry differently from general chemistry.

• No titration curves or buffer calculations.

• Focus on understanding core concepts from an organic perspective.

Bronsted-Lowry Acid-Base Reactions

• Bronsted-Lowry reactions involve proton (H+) shuffling.

• Acid: H+ donor

• Base: H+ acceptor

• Organic molecules are used as acids and bases.

Electron Flow in Acid-Base Reactions

• Draw curved arrows to show electron flow during the reaction.

• Base donates a lone pair to form a bond with the hydrogen from the acid.

• The bond between the hydrogen and the acid breaks, and the electrons go to the initial acid atom.

Acid Strength and pKa Values

• Acid-base reactions typically reach equilibrium.

• Acid strength is measured by the dissociation constant $K<em>a$ in water: $HA + H</em>2O <br>ightharpoonup H_3O^+ + A^-$

• $pKa = -log(K_a)$

• Organic chemistry primarily uses pKa values (e.g., 5, 12, -6).

• Lower pKa values indicate stronger acids.

• Memorize key pKa values for different functional groups.

• Will provide a table during exams for elements such as this.

Acidity Trends

• It's more important to understand why something is acidic instead of memorizing values.

• Key factor: stability of the conjugate base.

• A stable conjugate base means a stronger acid.

• Factors stabilizing the conjugate base:

  • Electronegativity

  • Resonance

  • Inductive effects

  • Hybridization

  • Bond strength

Electronegativity and Acidity

• More electronegative atoms handle the negative charge in the conjugate base better.

• Example: Acidity increases from C to N to O to F.

Resonance and Acidity

• Resonance stabilizes the conjugate base by delocalizing the negative charge.

• Example: Carboxylic acids (pKa ~ 5) are more acidic than alcohols (pKa ~ 16) due to the extra resonance structure.

Inductive Effects and Acidity

• Electronegative atoms near the negative charge can pull electron density and stabilize it.

• Strength of inductive effects:

  • More electronegative atoms = better

  • Closer to the negative charge = better

  • More of them = better

• Example: Adding chlorine atoms near an alcohol increases its acidity.

Acidity Table Implications

• Aryl alcohols are resonance structures

Hybridization and Acidity

• Higher s character in the hybrid orbital stabilizes the negative charge.

• sp > sp2 > sp3

Bond Strength and Acidity

• Weaker bonds are easier to break

• $H-I$ a strong acid because $H-I$ bonds are weaker than $H-F$

Key pKa Values

• Learn the specific list of 10-11 functional group pKa's given:

  • Know these and the ability to compare them

  • Identify the most acidic hydrogen in a compound

  • Bond strength, hybridization, inductive effects, electronegativity, resonance effects are used to decide

Resonance for Stabilizing Charges

• Amino acids pKa implications

• Aromatic rings stabilize resonance structues

Equilibrium and Acid-Base Reactions

• Acid-base reactions will favor equilibrium to one side

• Reaction favors the side of the weaker acid

• Look at which negative charge is stabilized by the molecule

• Favored to 10^(pKa difference) for acidity