REACTIVITY 3
Mechanisms of Chemical Change
Proton Transfer • Redox • Radicals • SN1 & SN2 • Electrophilic Addition
3.1 Proton Transfer (Acids)
1. Brønsted-Lowry Theory
We define reactions by the movement of protons ($H^+$).
Proton ($H^+$) Donor
Proton ($H^+$) Acceptor
2. Conjugate Pairs
Acids turn into Conjugate Bases; Bases turn into Conjugate Acids.
3. pH & Concentration
4. Strength vs Concentration
- Strong Acid: Dissociates completely (HCl → H+ + Cl-). Reaction goes to completion.
- Weak Acid: Dissociates partially (CH3COOH). Equilibrium forms favoring reactants.
3.2 Electron Transfer (Redox)
1. Definitions (OIL RIG)
Oxidation Is Loss. Reduction Is Gain.
2. Electrochemical Cells
| Feature | Voltaic (Primary) | Electrolytic |
|---|---|---|
| Function | Chemical → Electrical | Electrical → Chemical |
| Spontaneity | Spontaneous (Exo) | Non-spontaneous |
| Anode (Ox) | Negative (-) | Positive (+) |
| Cathode (Red) | Positive (+) | Negative (-) |
Voltaic cells MUST have a Salt Bridge to balance ions. Without it, voltage is zero.
3. Organic Redox
Reagent: Acidified K2Cr2O7 (Orange → Green).
→ Aldehyde → Acid
→ Ketone
No Reaction
Advanced Theory
The following section is for HL Students ONLY.
Buffers • Standard Potentials • Organic Mechanisms
Advanced Acids & Buffers
1. Lewis Theory
- Lewis Acid: Electron Pair Acceptor (Electrophile).
- Lewis Base: Electron Pair Donor (Nucleophile).
- Basis for Complex Ions (Ligands are Lewis Bases).
2. Buffer Solutions
Resist pH change. (Weak Acid + Conj Base).
3. pH Curves
pH < 7
pH > 7
Advanced Redox
1. Standard Potentials ($E^\ominus$)
Measured relative to Hydrogen (0.00V). More positive = Stronger Oxidizing Agent.
2. Gibbs Energy Link
Positive $E_{cell}$ → Negative $\Delta G$ (Spontaneous).
3. Electrolysis (Aqueous)
Water competes! Electroplating occurs here (metal deposits on cathode).
- Cathode (-): H+ discharged unless metal is lower than H (Cu/Ag).
- Anode (+): OH- discharged unless conc. Halide.
Organic Mechanisms
1. Nucleophilic Substitution ($S_N1$ vs $S_N2$)
| Feature | $S_N2$ (Bimolecular) | $S_N1$ (Unimolecular) |
|---|---|---|
| Substrate | Primary | Tertiary |
| Steps | 1 Step (Concerted) | 2 Steps (Carbocation) |
| Stereochem | Inversion | Racemic Mixture |
| Rate Law | $k[RX][Nu]$ | $k[RX]$ |
*Tertiary forms stable carbocations due to the Inductive Effect.
Electrophilic Addition (Alkenes)
Electrophilic Substitution (Benzene)
The Examiner's Vault
Strictly assessed on Reactivity 3 content.
Weak acid $HA$ ($pK_a = 4.8$). Solution contains $0.1M$ HA and $0.1M$ NaA. pH?
$[Acid] = [Salt]$, so $\log(1)=0$. $pH = pK_a$.
$Ag^+ (+0.80V)$ and $Zn^{2+} (-0.76V)$. Spontaneous $E^\theta_{cell}$?
$E_{cell} = 0.80 - (-0.76) = 1.56V$. Must be positive.
Which reacts fastest via $S_N1$?
Tertiary substrate ($S_N1$) + Iodide is better leaving group (weaker bond).
Explain how Ethanoic Acid/Ethanoate buffer resists pH change when $H^+$ is added.
- Added $H^+$ reacts with Conjugate Base ($CH_3COO^-$). [1]
- Eq: $CH_3COO^- + H^+ \rightarrow CH_3COOH$. [1]
- Equilibrium shifts to remove added $H^+$, keeping pH constant. [1]
Outline steps, curly arrows, and major intermediate.
- Arrow from C=C to H of H-Br. [1]
- Arrow from H-Br bond to Br. [1]
- Structure: Secondary Carbocation ($CH_3-CH^+-CH_3$). [1]
- Arrow from Br- lone pair to C+. [1]