lecture recording on 24 February 2025 at 13.50.11 PM

Stabilization of Conjugate Bases

• Conjugate Base and Stability

  • Negative charges can become stabilized by several factors:

    • Charge distribution over larger atoms in the same group can lead to stability.

    • Resonance structures create additional stability by delocalizing charge.

    • Hybridization: more s character leads to electrons being closer to the nucleus.

    • Inductive effect: atoms or groups within the molecule can pull electron density away, stabilizing the conjugate base.

• Inductive Effect Detailed

  • Example with fluorine: Fluorine's high electronegativity pulls electron density away, stabilizing the conjugate base, lowering pKa (e.g., pKa around 5 with fluorines).

  • Comparison of chlorine vs. fluorine on alcohol shows differences in pKa due to electronegativity influence.

Acid-Base Reactivity

• Reactivity Analogy

  • Analogy of wearing a heavy coat in extreme weather illustrates acid reactivity:

    • In hot conditions (high temperatures), the body wants to shed excess weight (give up a proton), leading to greater stability.

    • In cold conditions, retaining the coat (holding onto a proton) leads to less stability, emphasizing the link between acidity and stability.

• Key Concept

  • Stronger acids correspond with more stable conjugate bases, and vice versa; a weaker base correlates with stronger acids.

Identifying Acids and Bases in Reactions

• Process for Determining Acid-Base Relationships

  • Assess pKa values of potential acids: a lower pKa indicates a stronger acid.

  • Example: Alcohol (pKa 16) vs. HCl (strong acid, pKa < 0).

  • Strong base is identified as the one that reacts with the acid by capturing the hydrogen ion.

Stability and pKa Relationship

• Understanding Stability through pKa

  • Higher pKa indicates a stable conjugate acid: it is less reactive and retains protons more effectively.

  • Conversely, lower pKa denotes a more reactive acid needing to release a proton.

Physical Properties of Alkanes

• Reactivity and Stability of Alkanes

  • Alkanes are generally unreactive with high pKa values (around 50), suggesting they do not easily donate protons.

  • Commonly used as solvents but are flammable; caution required in lab settings.

• Boiling Points of Alkanes

  • Alkanes transition from gases (e.g., methane, ethane, propane) at low carbon counts to liquids (e.g., pentane) and then solids (higher carbon counts).

  • Intermolecular forces determine melting/boiling points: higher surface area leads to higher boiling/melting points due to increased forces of attraction.

Constitutional and Conformational Isomers

• Constitutional Isomers

  • Isomers share the same molecular formula but differ in arrangement.

  • Example: C6H14 can be arranged differently, leading to distinct compounds with unique physical properties.

• Conformational Isomers

  • Same molecular structure can adopt different spatial arrangements through twisting around single bonds (e.g., staggered vs. eclipsed forms).

  • Staggered conformations have lower energy than eclipsed due to less steric hindrance and electron cloud repulsion.

  • Practice distinguishing between different visual representations of molecules (like Newman projections).

Conclusion

• Acid-base chemistry is deeply interconnected with stability and the energy of conformations within molecular structures. The understanding of these principles enhances prediction of molecular interactions and stability in chemical reactions.