Chemistry Quick Reference: Arrow Rules, Leveling, and Nomenclature

  • Electron-pushing arrows (rules you must memorize)
    • The tail of the arrow starts at the location with electron density (lone pair or negative charge).
    • The head shows where those electrons go (to the atom that can accept them or a bond that forms).
    • Arrows represent a two-electron movement.
    • Always start from the electron-rich site (negative or lone pair) and move toward the electron-poor site.
    • Convention to memorize now: the arrow points away from the negative place and toward the positive/empty orbital; getting this wrong costs points on exam.
    • In acid–base steps, arrows typically move from base to proton (H⁺). Example: deprotonation by OH⁻:
    • Full equation: ext{OH}^- + ext{H}^+
      ightarrow ext{H}_2 ext{O}
    • Net ionic version (spectator ions omitted): ext{OH}^- + ext{H}^+
      ightarrow ext{H}_2 ext{O}
  • Bronsted–Lewis acid–base concepts
    • Bronsted:酸 donates a proton (H⁺); base accepts a proton.
    • Lewis: acids accept electron pairs; bases donate electron pairs.
    • Every Bronsted acid/base is a Lewis acid/base; focus on electron-pair donation/acceptance for mechanisms.
    • Example: a base with a lone pair on oxygen donates that pair to a proton to form a new O–H bond.
  • Leveling effect and solvent pKa (thermodynamics vs kinetics)
    • The solvent defines the strongest acid/base that can exist in that solvent (its pKa range).
    • In water: pKa range is roughly from 0 (strong acids like H₃O⁺) to ~14 (water as conjugate base/acid context). That limits protonation/deprotonation events.
    • In non-aqueous solvents (e.g., THF): higher/undefined pKa windows exist (e.g., amide pKa ~36, butane ~28), allowing stronger acids/bases to operate.
    • Rule of thumb: in a given solvent, the equilibrium favors the side with the larger pKa (weaker acid on that side).
    • Practical takeaway: choose solvent so its conjugate acid/base pKa is lower than the species you want to protonate/deprotonate; otherwise, proton transfer is unfavored.
    • Example data mentioned:
    • HCl in water is strong acid; conjugate base Cl⁻ is very weak base in water.
    • Water as solvent: protonation of an alcohol with pKa > 14 is unfavorable thermodynamically in water.
    • Hydronium pKa ≈ 0; water pKa ≈ 14; amide ~36; butane ~28.
  • Solvent choice and reaction outcome (illustrative reasoning)
    • If you mix a strong acid/base with water, the reaction tends to go toward species that fit the water pKa window (thermodynamic control).
    • Kinetics: in solution, collisions with solvent molecules can steer the fastest path; water molecules are abundant, so interactions with water often dominate first.
    • Summary: leveling explains why some proton transfers are possible in non-aqueous solvents but not in water.
  • Net vs full equations and spectator ions
    • Sometimes you write the full equation with counterions (e.g., Na^+ as spectator):
    • Full: ext{NaOH} + ext{C}6 ext{H}5 ext{CO}2 ext{H} ightarrow ext{Na}^+ ext{C}6 ext{H}5 ext{CO}2^- + ext{H}_2 ext{O}
    • Often, you omit spectator ions to write the net ionic equation:
    • Net ionic: ext{OH}^- + ext{C}6 ext{H}5 ext{CO}2 ext{H} ightarrow ext{C}6 ext{H}5 ext{CO}2^- + ext{H}_2 ext{O}
  • Nomenclature: basics you’ll need for early chapters
    • Parent chain: defined by the longest continuous carbon chain in the molecule.
    • Prefixes/substituents: alkyl groups (e.g., methyl, ethyl); position numbers indicate where substituents attach.
    • Rule: the parent is the longest carbon chain; different chains with the same length may yield different substituent patterns, so pick the one that gives the lowest set of locants.
    • Examples of naming patterns:
    • 2-methylpentane vs 3-methylpentane illustrate how substituent positions matter.
    • Two chains of the same length can exist; choose the chain that gives the substituents lowest numbers overall.
    • Saturated vs unsaturated hydrocarbons:
    • Saturated: only single bonds (alkanes).
    • Unsaturated: contain double (alkenes) or triple (alkynes) bonds.
  • Key terminology and general rules for alkanes, alkenes, and alkynes
    • Endings indicate bond types:
    • -ane: single bonds (alkanes)
    • -ene: one C=C double bond (alkenes)
    • -yne: one C≡C triple bond (alkynes)
    • Cyclo- prefix indicates rings (e.g., cyclohexane is a six-carbon ring).
    • Saturated straight/branched chains have the general hydrogen formula:
    • For alkanes: extC<em>nextH</em>2n+2ext{C}<em>n ext{H}</em>{2n+2}
    • Examples mentioned: waxes (first waxy hydrocarbon approximated by tetradecane, C{14}H{30}); gasoline/diesel discussion touches on condensations and properties but the key naming takeaway is the parent chain and substituents, not fuel specifics.
  • Quick recap: how to approach naming (practical checklist)
    • Find the longest continuous carbon chain (the parent).
    • Identify and number substituents to give them the lowest possible locants.
    • Apply the correct suffix for saturation/bond type (-ane, -ene, -yne).
    • For rings, count the ring carbons as part of the parent when appropriate (cycloalkanes).
  • Example takeaway from the board discussion
    • Longest chain example ended with an eight-carbon chain (octane) in the shown skeleton; the same molecule could be described with different but equivalent parent chains depending on how you count substituents.