OChem: Electronegativity and PKa Vocabulary

Electronegativity and pKa Concepts (Notes from Transcript)

Electronegativity Trends

• Theme: Increasing electronegativity across a periodic table row and its implications for polarity, bond character, and acidity/basicity.
• Visual cue in transcript: “Electronegativity Table for OChem” with a label “Increasing electronegativity.”
• Key qualitative trend:
  • Electronegativity generally increases across a period from left to right.
  • Electronegativity generally increases up a group (top elements are more electronegative than bottom elements).
• Elements mentioned explicitly in the transcript:
  • Hydrogen (H) noted with a value around 2.2 on the displayed scale.
  • Main-group elements listed across the left-to-right progression: Li, Be, B, C, N, O, F, then alkali/near-alkali metals Na, Mg, K (with values shown for some of these).
  • The table segments indicate typical periodic trends alongside element symbols: H, Li, Be, B, C, N, F, Na, Mg, K, etc.
• Approximate numerical context (typical Pauling scale values, as commonly taught in OChem):
  • H: $2.20$
  • C: $2.55$
  • N: $3.04$
  • O: $3.44$
  • F: $3.98$
  • Li: ≈ $0.98$
  • Na: ≈ $0.93$
  • K: ≈ $0.82$
  • Be, B, Al, Si, P, S, Cl, Br, I: values vary along the period/group; the transcript lists these elements in the same row/column context to emphasize trend.
• Notes on the figure quality:
  • The slide explicitly says “NOT TO SCALE,” so numerical spacing may not reflect exact values.
  • There are some alignment artifacts (e.g., stray numbers near Br, I, TsOH) but the main idea is the relative increase in electronegativity across the period and up the group.
• Significance of electronegativity in OChem:
  • Determines bond polarity: more electronegative elements attract bonding electrons more strongly, creating partial negative charges on the more electronegative atom.
  • Affects acidity/basicity of conjugate acids/bases, strength of acids in water, and likelihood of proton transfer in reactions.
  • Helps predict reaction mechanisms (nucleophilicity vs. basicity, stability of intermediates).

pKa values you need to know for OChem (transcript highlights)

• The transcript includes a section labeled “PKAs you need to know for OChem” and lists several acids/bases with their relative strength cues.
• General umbrella: pKa is a measure of acidity in water, defined by $pK<em>a = -\log</em>{10}(K<em>a)$ where $K</em>a$ is the acid dissociation constant.
• Some representative acids and approximate pKa values (in water) based on common OChem knowledge (listed to match the spirit of the slide content and common classroom values):
  • Hydrochloric acid: $pK_a \,\approx\, -7$
  • TsOH (p-toluenesulfonic acid): $pK_a \,\approx\, -2.8\text{ to }-2.8$
  • H2SO4 (first dissociation): $pK_a1 \approx -3$
  • H2SO4 (second dissociation): $pK_a2 \approx 2$
  • H2O (water): $pK_a \approx 15.7$
  • H3O+ (acid in water): $pK_a \approx -1.74$
  • H2S (diprotic acid): $pK<em>a1 \approx 7$; $pK</em>a2 \approx 12.9$
  • NH4+ (ammonium): $pK_a \approx 9.25$
  • ROH (alcohols, general): $pK_a \approx 15.5\text{ to }17$ (range depending on exact structure; commonly cited value ≈ 16)
  • CH3OH (methanol): similar to other alcohols, $pK_a \approx 16$
• What these values imply in OChem practice:
  • Strong acids have very low (negative) pKa; their conjugate bases are very weak bases (stable anions).
  • Weak acids have high pKa; their conjugate bases are relatively stronger bases.
  • Acid strength correlates with the stability of the conjugate base; highly electronegative atoms and resonance stabilization generally stabilize the conjugate base.
  • In organic reactions, proton transfer equilibria are guided by pKa differences; if a stronger acid is present, deprotonation by a weaker base is favorable.
• Examples and implications (conceptual, tied to the transcript’s list):
  • HCl vs TsOH: both are strong acids in many contexts; HCl is stronger (more negative pKa) in water than TsOH.
  • H2SO4: extremely strong first-proton donor (pKa1 ≈ -3); second proton donation is much weaker (pKa2 ≈ 2).
  • H2O as a solvent participates in autoprotolysis: $2\,\mathrm{H<em>2O} \rightleftharpoons \mathrm{H</em>3O^+} + \mathrm{OH^-}$ with $K<em>w = 10^{-14}$, hence $pK</em>w = 14$ at 25°C.
  • H2S: weak diprotic acid; its conjugate bases HS⁻ and S²⁻ are relatively weak bases; pKa values around 7 and 12.9 reflect this.
  • Ammonium: weak acid; deprotonation to NH3 requires a base of modest strength; pKa ≈ 9.25 for NH4+.
• Notational and presentation notes from the transcript:
  • Some items in the slide are labeled as “NOT TO SCALE” to remind readers that the precise distances/values on the figure are not meant to be exact.
  • The transcript lists items with somewhat garbled typography (e.g., “HOCHS,” “=H,” and scattered numbers). These are interpreted here as common OChem acids/bases and standard pKa values to provide a coherent study reference.
• Practical connections to foundational principles:
  • Bronsted-Lowry acid-base theory: acids donate protons; bases accept protons. pKa values quantify the tendency of a species to donate a proton in aqueous solution.
  • Relationship between electronegativity and acidity: more electronegative conjugate bases tend to stabilize negative charge, often lowering pKa and increasing acidity when appropriate.
  • Solvent effects: pKa values are solvent-dependent; most values listed here pertain to aqueous solutions and may shift in nonaqueous media.

Practical implications and study tips

• Use the electronegativity trend to predict bond polarity and reactivity in organic molecules (e.g., electron-withdrawing vs. donating groups).
• Use pKa values to estimate acid-base equilibria in reactions, predict deprotonation routes, and assess the feasibility of proton transfers in mechanisms.
• Remember common strong acids (HCl, TsOH, H2SO4) have very low pKa values in water; common weak acids (alcohols, NH4+) have high pKa values.
• When in doubt about a specific pKa value, compare relative acidity using conjugate base stability and resonance/inductive effects, keeping in mind that solvent and temperature shift numbers slightly.

Connections to broader OChem topics

• Links to acid-base catalysis and mechanism steps (deprotonation/protonation events).
• Relationship between electronegativity, resonance stabilization, and acidity/basicity.
• Foundations for understanding reaction equilibria, pH effects in solutions, and solvent choice in synthesis.