Bronsted-Lowry Acids & Resonance Review Notes

Zoom office hours follow-up

  • Time: 5:00 PM to 7:00 PM on Zoom tonight
  • Link: on the instructor page under office hours
  • If you can’t attend, email a screenshot/photo showing your mock exam attempt to receive the Zoom recording

Resonance worksheet reminders

  • Be sure you know what your skeletal structures show and what they don’t show
  • Common pitfall: drawing carbon with five bonds (not allowed; carbon cannot exceed its valence)
  • If you have an allylic lone pair, in resonance push electrons with the appropriate arrow to form the other lone pair on the opposite side of the system
  • For resonance patterns: learn what the pattern looks like, how to identify it on a molecule, and which arrows to use to move electrons to reach the next structure
  • If you’re confused about resonance, start by identifying the pattern, then practice applying the arrow-pushing to reach other canonical forms

Bronsted-Lowry acids and bases (to be reviewed before Lewis theory in chapter 6)

  • Bronsted acid = proton donor (HA in general); proton = \(H^+\)
  • Bronsted base = proton acceptor (e.g., water accepting H^+ from a strong acid)
  • Reactions exist in equilibrium: forward and reverse processes; each acid has a conjugate base (HA ⇌ H^+ + A^- in water)
  • Strong acid ⇒ conjugate base is very weak; Weak acid ⇒ conjugate base is very strong
  • Electron density flow drives acid-base chemistry; electrons are the movers in bond-making/breaking
  • Distinction between resonance arrows and reaction arrows:
    • Resonance: curved arrows used to illustrate shifts within a molecule to form alternative valid Lewis structures; typically a single arrow per structure is used to indicate resonance
    • Reactions: curved arrows indicate actual movement of electrons during bond-making/breaking in a reaction, occurring in a mechanism with forward and reverse steps
  • Mechanism: how the reaction actually occurs (step-by-step electron movements and bond changes)
  • Equilibrium context: need to compare structures to determine which side is favored; this connects to pKa and relative acidity/basicity

Equilibrium concepts and pKa basics

  • General acid-base equilibrium (in water):
    [\mathrm{HA} + \mathrm{H2O} \rightleftharpoons \mathrm{A^-} + \mathrm{H3O^+} ]\
  • Equilibrium constant (acid-base):
    K<em>a=[H</em>3O+][A−][HA]K<em>a = \frac{[\mathrm{H</em>3O^+}][\mathrm{A^-}]}{[\mathrm{HA}]}
  • We often compare pKa values instead of Ka for convenience:
    pK<em>a=−log⁡</em>10(Ka)pK<em>a = -\log</em>{10}(K_a)
  • A larger Ka (or smaller pKa) means a stronger acid (more product-favored right side in water)
  • The relationship between acid strength and conjugate base stability:
    • A more stable conjugate base is a weaker base
    • A weaker base corresponds to a stronger acid
  • In practice, when Ka is not known, use structural features to predict relative acidity and base strength

Conjugate base stability and the ARIO framework

  • To compare acidity across related acids, compare the stability of their conjugate bases (A^-)
  • More stable conjugate base = weaker base = stronger acid
  • ARIO acronym for stabilizing factors of the conjugate base:
    • A: Atom electronegativity/electronic character at the site bearing the negative charge
    • R: Resonance stabilization (delocalization of negative charge over multiple atoms)
    • I: Inductive effects (through sigma bonds from electronegative atoms nearby)
    • O: Orbital character (hybridization/s-character of the orbital holding the negative charge)
  • Practical notes:
    • Among counterions discussed, a more electronegative anion like chloride (Cl^-) tends to be more stabilized than hydroxide (OH^-), contributing to HCl being a stronger acid than water
    • Delocalization via resonance (e.g., acetate) stabilizes the conjugate base and increases acidity
    • Inductive effects: nearby electronegative atoms pull electron density through sigma bonds, stabilizing the charge on the conjugate base
    • Orbital/hybridization effects: greater s-character in the orbital holding the negative charge stabilizes that charge; the higher the s-character, the more stabilizing the effect

Resonance, stability, and structure details

  • Resonance helps stabilize conjugate bases by spreading out negative charge across multiple atoms/positions
  • The more extensive the resonance stabilization, the more stable the conjugate base
  • Inductive stabilization depends on the presence and proximity of electronegative atoms that withdraw electron density
  • Hybridization and s-character trend for acidity of C–H bonds:
    • Higher s-character in the carbon-hydrogen bond increases acidity because the conjugate base has better stabilization from a more electronegative-like environment
    • Order of s-character importance in carbon hybrids (highest to lowest): sp > sp2 > sp3
    • As s-character increases, the corresponding carbanion is more stabilized, and the C–H bond is more acidic
  • Example concepts mentioned in the discussion:
    • Ethanol (CH3CH2OH) has a relatively high pKa ≈ 16, indicating a weak acid compared to carboxylic acids; its conjugate base (ethoxide, CH3CH2O^-) is a relatively strong base but is not very stabilized by resonance
    • Acetate (CH3COO^-) is a stabilized conjugate base due to resonance between the two oxygens; this stabilizes the negative charge and increases acidity of acetic acid relative to ethanol
    • Carboxylate conjugate bases benefit from resonance, increasing acidity of their parent carboxylic acids
    • The presence of additional electronegative substituents near the conjugate base can enhance inductive stabilization and thus acidity

Key practical takeaways and patterns

  • When comparing acids, start by examining the conjugate base stability via ARIO:
    • If the conjugate base is stabilized by resonance (R) or inductive effects (I), acidity is increased
    • If the conjugate base resides on an atom with higher electronegativity (A), stability is enhanced
    • If the negative charge is delocalized into orbitals with higher s-character (O), stability is enhanced
  • Remember the conceptual distinction between resonance and reaction mechanisms:
    • Resonance depicts electron distribution in a static set of canonical structures (one arrow indicating delocalization within the molecule)
    • Reactions depict real electron movement during bond breaking/forming with a complete mechanism and forward/backward steps
  • Quick check for carbon-based acidity: increased s-character in the hybrid orbital holding the negative charge correlates with greater acidity for C–H bonds (sp > sp2 > sp3 for acidity)

Quick recap: core formulas and concepts to memorize

  • Acid-base equilibrium in water:
    extHA+extH<em>2extOightleftharpoonsextA−+extH</em>3extO+ext{HA} + ext{H}<em>2 ext{O} ightleftharpoons ext{A}^- + ext{H}</em>3 ext{O}^+
    K<em>a=[H</em>3extO+][A−][HA]K<em>a = \frac{[\text{H}</em>3 ext{O}^+][\text{A}^-]}{[\text{HA}]}
    pK<em>a=−log⁡</em>10(Ka)pK<em>a = -\log</em>{10}(K_a)
  • Strong acid vs conjugate base strength: stronger acid → weaker conjugate base; weaker acid → stronger conjugate base
  • ARIO stability factors: A (electronegativity), R (resonance), I (inductive), O (orbital s-character)
  • Higher s-character in a conjugate base’s orbital generally stabilizes the negative charge and increases acidity of the corresponding C–H bond
  • Exotic example notes include the relative acidity of HCl vs water (Cl^− vs OH^− stability) and the resonance stabilization in acetate vs ethanol