CHM242 Midterm 1 Cards (Mechanisms, Acid Base, Rxn)
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96 Terms
What does Ka measure?
Acid dissociation
What does pKa measure?
Acid strength
How does Ka relate to pKa?
High Ka = low pKa = strong acid.
How does pKa relate to acid strength?
Lower pKa means a stronger acid
What is the relationship between acid and conjugate-base strength?
Stronger acid = weaker conjugate base, and vice versa.
What is the equilibrium direction for HA + B⁻ ⇌ A⁻ + HB?
Favors the side with the weaker acid (higher pKa).
How do you calculate ΔpKa for HA + B⁻ ⇌ A⁻ + HB?
ΔpKa = pKa(HB) − pKa(HA). Positive values favor products.
What is a strong acid's typical Ka and pKa?
Very large Ka and very low (often negative) pKa.
What makes a base strong?
It readily accepts H⁺
How do you identify the stronger base using conjugate-acid pKa?
The base whose conjugate acid has the higher pKa is stronger.
What is the pKa of H₂SO₄ in the provided table?
Approximately −3.
What is the pKa of HCl in the provided table?
Approximately −3.
What is the pKa of carboxylic acids in the provided table?
Approximately 5.
What is the pKa of NH₄⁺ in the provided table?
Approximately 9.
What is the pKa of H₂O in the provided table?
Approximately 14.
What is the pKa of R–OH in the provided table?
Approximately 17.
What is the pKa of H–C≡C–H (a terminal alkyne) in the provided table?
Approximately 25.
What is the pKa of H₂C=CH₂ in the provided table?
Approximately 43.
What is the pKa of H₃C–CH₃ in the provided table?
Approximately 50.
Which is the stronger acid: a carboxylic acid (pKa ≈ 5) or an alcohol (pKa ≈ 17)?
The carboxylic acid
Which is the stronger acid: water (pKa ≈ 14) or ethane (pKa ≈ 50)?
Water
What is the general rule for acid–base equilibrium?
Equilibrium favors the side with the weaker acid and weaker base.
What does SN1 mean?
Substitution nucleophilic unimolecular
What does SN2 mean?
Substitution nucleophilic bimolecular
What does E1 mean?
Elimination unimolecular
What does E2 mean?
Elimination bimolecular
What is the rate law for SN1 and E1?
Rate = k[substrate].
What is the rate law for SN2 and E2?
Rate = k[substrate][nucleophile or base].
Which mechanisms form a carbocation intermediate?
SN1 and E1.
Which mechanisms occur in one concerted step?
SN2 and E2.
Which substrate favours SN1?
3° > 2°
Which substrate favours SN2?
Methyl > 1° > 2°
Which substrate favours E1?
Usually 3° > 2°
Which substrate favours E2?
3° and 2° commonly
Why do 3° substrates favour SN1 and E1?
They form relatively stable tertiary carbocations.
Why are 3° substrates poor for SN2?
Steric hindrance blocks backside attack.
What is the role of the leaving group in all four mechanisms?
A good leaving group makes the reaction easier.
What are common good leaving groups?
I⁻, Br⁻, Cl⁻, and sulfonate groups such as OTs.
Is OH⁻ usually a good leaving group?
No. OH⁻ is a poor leaving group unless activated, e.g. protonated to H₂O.
What nucleophile favours SN2?
A strong, unhindered nucleophile.
What nucleophile is needed for SN1?
A strong nucleophile is not required
What reagent favours E2?
A strong base, such as OH⁻, OR⁻, or bulky tert-butoxide.
What base is sufficient for E1?
A weak base can remove a β-hydrogen after carbocation formation.
What solvent favours SN1 and E1?
Polar protic solvents, which stabilize ions and carbocations.
What solvent often favours SN2?
Polar aprotic solvents, e.g. DMSO, DMF, or acetone.
Why do polar aprotic solvents favour SN2?
They leave anionic nucleophiles relatively reactive.
What does a polar protic solvent contain?
An O–H or N–H bond
What product stereochemistry is typical of SN1?
Often racemization at a chiral centre, potentially with some excess inversion.
What product stereochemistry is typical of SN2?
Inversion of configuration at the attacked chiral centre.
What alkene stereochemistry is possible in E1?
E/Z (cis/trans where applicable) alkene isomers
What geometric requirement applies to E2?
The β-hydrogen and leaving group must be anti-periplanar
What is the key difference between substitution and elimination?
Substitution replaces a leaving group
What happens when a substrate can form a stable carbocation and the solvent is polar protic?
SN1/E1 become more favourable, depending on nucleophile, base, and conditions.
What favours elimination over substitution?
Higher temperature and a strong base often favour elimination.
What favours substitution over elimination?
A good nucleophile with little basicity favours substitution, especially SN2 on unhindered substrates.
What is the usual major alkene in E1/E2 with a small base?
Often the more substituted (Zaitsev) alkene, unless other factors override.
How can a bulky base affect E2 product selection?
It often favours the less substituted (Hofmann) alkene.
What is a quick rule for a 3° substrate with a strong base?
E2 is favoured
What is an exergonic reaction?
A reaction with ΔG < 0, Keq >1, favour products
What is an endergonic reaction?
A reaction with ΔG > 0, Keq < 1, favour reactants
What is ΔG for a reaction at equilibrium?
ΔG = 0.
What does an exergonic energy diagram look like?
Products lie lower in free energy than reactants.
What does an endergonic energy diagram look like?
Products lie higher in free energy than reactants.
What is the relationship between ΔG° and Keq?
ΔG° = −RT ln Keq.
What Keq value indicates an exergonic reaction under standard conditions?
Keq > 1
What Keq value indicates an endergonic reaction under standard conditions?
Keq < 1
What is ΔG° when Keq = 1?
ΔG° = 0.
Does an exergonic reaction necessarily occur quickly?
No. Reaction rate depends on activation energy, not just ΔG.
What is activation energy?
The energy barrier between reactants and the transition state.
What is the difference between ΔG and ΔG°?
ΔG is for actual conditions