Comprehensive Notes: Equilibria, pKa, and Nomenclature (Lecture Transcript)

Equilibrium concepts and pKa values

• Equilibria are read as forward and backward processes simultaneously; both sides exist at the same time, but one side may be favored depending on conditions.
• In acid–base equilibria, the stronger acid/conjugate acid on one side drives the equilibrium toward that side.
• Case study: protonating an alcohol in equilibrium with an acid:
  • If the acid on the right is five pKa units less acidic than the left, the right side is favored by roughly five orders of magnitude. This corresponds to a shift toward the right if the right-hand acid is stronger.
  • Example comparison: left side acid is hydronium with ext{p}Ka( ext{H}3 ext{O}^+) \,\approx\,-1.74. The right side conjugate acid has a higher pKa (weaker acid), e.g., ext{p}K_a \approx +4.75. Thus the left side is favored in this particular setup.
• Five orders of magnitude difference in acidity corresponds to a factor of 10^5 in equilibrium constant; the side with the lower pKa (stronger acid) dominates.
• Trifluoroacetic acid (TFA) is much stronger than acetic acid due to electron-withdrawing fluorines; this shifts acidity significantly:
  • Trifluoroacetic acid has a very low ext{p}Ka, roughly around 0.0.\mathrm{p}Ka(\text{TFA}) \approx 0. The multiple −CF_3 groups dramatically increase acidity.
• Protonation of ketones and related species: protonated ketones tend to be very acidic, placing their conjugate acids in the negative pKa range:
  • Protonated ketones: \text{p}K_a \approx -2 \text{ to } -3. Hence, in many cases, this equilibrium lies to the left (less protonation) but can still involve some protonation (e.g., ~70% on one side and ~30% on the other).
• Carbocations: carbocations are so acidic that they are essentially beyond the scope of the standard pKa table; they’re not listed because their stability and acidity are extreme in comparison to typical organic acids.
• Ether protonation is possible and will be revisited in the context of structure–property relationships; one might even recall practical references (e.g., a conjectural note about OCAM) to remind students that rapid data processing is expected when handling large data sets.
• Take-home messages on acidity and protonation:
  • Small pKa differences (e.g., near zero difference) do not guarantee complete transfer; equilibria can be significantly biased but not 100% complete.
  • The presence of strongly electron-withdrawing groups (e.g., CF_3) greatly increases acidity; protonation equilibria shift accordingly.
  • Protonation can be partial (e.g., 70/30) depending on pKa values of the participating acids.
• Course logistics and study strategy notes:
  • A suggested study target: if Friday at 5 PM is the deadline, aiming for Thursday at 5 PM is a strong planning advantage; good study techniques help with large data loads.
  • Storytelling technique suggestion: turn particles into characters in a play to improve recall and narrative understanding of mechanisms.
  • Chapter planning note: the current focus is not for the first exam (which covers chapters 1–3), but Chapter 4 introduces more complex naming; the goal is to be able to deduce structure from a name and vice versa.

Chapter 4: Nomenclature, naming, and the computational angle

• The naming problem in organic chemistry becomes highly complex; rules are systematic enough to be programmed into computer algorithms.
• Practical tip: maintain a vocabulary booklet with two pages per concept:
  • Left page: the chemical structure.
  • Right page: the corresponding name.
• Extend this idea to reactions: create four chapters corresponding to reaction categories and record in the booklet:
  • Substitutions, Eliminations, Additions, Rearrangements.
  • For each reaction, write: starting materials (left) → products (right) with reagents listed in the back.
• Hierarchical representation metaphor: compare molecules to LEGO pieces – identify the basic building blocks and how many of each are present; this helps when addressing constitutional isomerism.
• Constitutional isomerism arises from different connectivities of the same molecular formula; crude oil is a real-world source illustrating diverse hydrocarbon structures.
• Practical chemistry workflow considerations:
  • In separations, consider immiscible solvents and the separatory funnel; identify which layer is organic and whether the organic layer sits on top or bottom.
  • A fundamental lab caution: do not discard material in waste containers until the final product is isolated, since valuable product may be lost in the early stages.

Building a hierarchical picture of organic molecules

• Longest carbon chain determines the parent name; the remaining pieces are substituents.
• Examples of branching and packing effects:
  • A straight-chain hydrocarbon packs efficiently like a cylinder; branching disrupts packing and lowers the energy required to reach a liquid state.
  • Branched molecules generally have lower melting and sometimes lower boiling points than their straight-chain isomers.
  • As molecules become near-spherical, packing improves again, affecting physical properties in a non-linear way.
• Alkyl substituents and attachment points:
  • Primary carbon attachment (attached to a carbon that is bonded to only one other carbon) gives the substituent name that starts with “n-”: e.g., n-propyl.
  • Secondary carbon attachment (attached to a carbon bonded to two other carbons) yields the isomeric form: e.g., isopropyl.
  • For longer chains (butane and beyond), more isomeric forms appear: primary (n-), secondary, tertiary attachments.
  • Common names and their caveats: isopropyl vs isopropyl naming sometimes used casually; precise nomenclature distinguishes substituent connectivity (e.g., n-, sec-, tert- vs systematic names).
  • For butane substituents, common forms include: n-butyl, sec-butyl, tert-butyl, and isobutyl (the isopropyl style has analogous isomers, though “isopyr…” terminology is often used informally and can be confusing).
• Naming conventions for substituents on the longest chain:
  • Identify the longest carbon chain first; that chain becomes the root name (e.g., hexane, heptane, octane).
  • The remaining pieces are substituents (e.g., methyl, ethyl, propyl, etc.).
  • When the chain has multiple substituents, assign locants (numbers) to indicate where each substituent is attached:
  • Choose the numbering that gives the lowest set of locants in the substituent list.
  • If multiple substituents are present, the set of locants should be as small as possible when read in order.
  • Alphabetical order is used to list substituents in the name, but numerical prefixes di-, tri-, etc. are ignored for ordering purposes (e.g., 2,3-dimethylhexane, not 3,2-dimethylhexane).
  • When identical substituents appear, use di-, tri-, tetra-, etc. to indicate quantity (e.g., 2,2-dimethyl-3-ethylhexane).
• Examples to illustrate the process (conceptual, not exhaustive):
  • If the longest chain is eight carbons and a methyl substituent is present, the parent is octane and a methyl substituent yields something like 2-methyl-octane or 3-methyl-octane, depending on the attachment position.
  • If there are two methyl substituents on different carbons, the name would be something like 2,4-dimethylhexane, chosen to minimize the locant set.
  • When there are multiple distinct substituents, list them in alphabetical order (ignoring di/tri prefixes) within the full name.
• Practice note: the goal of these rules is to provide a consistent, unambiguous method for naming any given structure; students should be able to deduce a structure from a name and vice versa using the same rules.
• Conceptual takeaways about naming and structure from the lecture:
  • Always start with the longest continuous carbon chain as the root.
  • Use the smallest possible locants for substituents.
  • List substituents alphabetically in the final name, ignoring multiplicative prefixes (di/tri, etc.).
  • Recognize common substituent families (n-, sec-, tert-, isobutyl; n-, iso-, sec-, tert- forms) and where they fit in systematic naming.
• Final reminder on the study approach:
  • It’s unlikely you’ll memorize every specific name from scratch; focus on the underlying rules, then apply them to generate correct names or deduce structures.
  • Practice can be supported by building a personal vocabulary booklet and by routinely translating a structure to a name and a name to a structure in a controlled, repeatable way.

Quick recall and study tips highlighted in the lecture

• Be comfortable with pKa scale values and what they imply for equilibrium direction:
  • Stronger acids have lower pKa values; when comparing two acid strengths, the one with the lower pKa dominates the protonation direction.
  • Small pKa differences do not guarantee complete conversion; equilibria can still be substantial but not quantitative.
• Use real-world analogies (e.g., LEGO pieces) to understand the composition and connectivity of molecules.
• Build a four-chapter reaction vocabulary (substitution, elimination, addition, rearrangement) as a framework for organizing reactions and mechanisms.
• Recognize that some topics (chapter four) introduce systematic rules that are essential for consistent nomenclature and are amenable to algorithmic implementation.
• Emphasize the practical lab mindset: anticipate how solvent properties, layering in separatory funnels, and purification steps influence the outcome and data integrity.
• Adopt a story-telling or character-based approach to memory: turn mechanistic steps or structures into a narrative to improve retention.

Summary of core ideas to memorize for the exam

• Equilibrium direction is dictated by relative acidity; the side with the weaker acid is disfavored.
• Electron-withdrawing substituents drastically increase acidity (e.g., CF3 groups in trifluoroacetic acid).
• Carbocations are extremely acidic and are typically not included in standard pKa tables.
• Protonation of ethers and ketones can occur, with varying extents depending on the acid strength and environment.
• Chapter 4 introduces systematic nomenclature rules that allow deduction of structure from name and vice versa; these rules are the basis for algorithmic processing of chemical structures.
• A practical naming workflow centers on longest carbon chain, substituent identification, locants with minimal values, and alphabetical ordering (with di/tri ignored in the alphabet).
• Understand substituent types (n-, sec-, tert-, isobutyl, etc.) and how attachment points define primary/secondary/tertiary carbons.
• Recognize how branching affects physical properties like melting and boiling points due to packing efficiency and molecular shape.

Important formulas and numerical references in the transcript

• Acid strength differences (example):
  • ext{p}Ka( ext{H}3 ext{O}^+) \,\approx\,-1.74
  • ext{p}K_a ext{ on the right} \approx +4.75
  • Difference implying a five-order-of-magnitude shift: \Delta pK_a \approx 5
• Trifluoroacetic acid: ext{p}K_a \approx 0.0
• Protonated ketones: ext{p}K_a \approx -2 \text{ to } -3
• General idea: multiple high-energy concepts can be distilled into rules that guide equilibrium positions and naming conventions.

Note on exam strategy mentioned in the lecture

• The first exam covers chapters 1–3; chapter 4 content is introduced as preparation for more advanced material and for algorithmic thinking in nomenclature.
• Build a personal vocabulary booklet and keep structure-name pairs organized to enable rapid recall and application under exam conditions.
• Accept that not every name will be memorized; focus on the rules and practice translating between structure and name to build fluency.