Chemical reaction Study Guide

📖 Core Concepts Chemical reaction – transformation of reactants into products, accompanied by an energy change. Reactants / reagents – substances you start with; products – substances formed. Exothermic vs. endothermic – exothermic releases heat (ΔH < 0); endothermic absorbs heat (ΔH > 0). Redox – reactions that involve electron transfer; oxidation = loss of electrons, reduction = gain. Equilibrium – forward and reverse rates equal, ΔG = 0; represented by ⇌. Catalyst – lowers activation energy (Eₐ) without being consumed; can be homogeneous or heterogeneous. Stoichiometry – balancing equations so each element has the same integer count on both sides. Rate law basics – rate generally ↑ with higher concentration, temperature, surface area, or pressure. Organic reaction categories – substitution (SN1, SN2), elimination (E1, E2, E1cb), addition (Markovnikov, anti‑Markovnikov), pericyclic (Diels–Alder). 📌 Must Remember Balancing rule: total atoms of each element must be identical on both sides; use smallest whole‑number coefficients. Arrhenius equation: $k = A \exp\!\left(-\frac{E{\mathrm{a}}}{RT}\right)$. ΔG relation: $\Delta G = \Delta H - T\Delta S$. Le Chatelier: ↑ P → shift toward side with fewer gas moles. ↑ T → favors endothermic direction. Markovnikov’s rule: in HX addition, H attaches to the carbon with more H’s (more substituted carbocation). Anti‑Markovnikov (hydroboration‑oxidation): B adds to less‑substituted carbon, later converted to OH. SN1 vs. SN2: SN1 – unimolecular, carbocation intermediate, racemic; SN2 – bimolecular, concerted backside attack, inversion. E1 vs. E2 vs. E1cb: E1 – carbocation intermediate, unimolecular. E2 – concerted, base anti‑periplanar to leaving group. E1cb – carbanion intermediate, strong base, poor leaving group. Redox half‑reaction balance: electrons lost = electrons gained. 🔄 Key Processes Balancing a redox reaction Write separate oxidation and reduction half‑reactions. Balance O by adding $\text{H}2\text{O}$, H by adding $\text{H}^+$ (acidic) or $\text{OH}^-$ (basic). Balance charge with electrons. Multiply to equalize electron count, add halves, cancel species. Arrhenius temperature effect Determine $k2/k1 = \exp\!\left[-\frac{Ea}{R}\left(\frac{1}{T2}-\frac{1}{T1}\right)\right]$. SN2 substitution Identify strong nucleophile, primary substrate, polar aprotic solvent. Nucleophile attacks backside → inversion of configuration. E2 elimination Strong base, secondary/tertiary substrate, anti‑periplanar geometry required. Diels–Alder cycloaddition Combine a conjugated diene (4 π) with a dienophile (2 π). Thermally allowed [4+2] reaction; produces a cyclohexene ring with predictable regio‑/stereochemistry. 🔍 Key Comparisons SN1 vs. SN2 SN1: unimolecular, carbocation intermediate, rate ∝ [substrate] only, racemic mix. SN2: bimolecular, concerted, rate ∝ [nucleophile][substrate], inversion. E1 vs. E2 E1: carbocation intermediate, rate ∝ [substrate]; favors weak base, more substituted alkenes. E2: single concerted step, rate ∝ [base][substrate]; requires anti‑periplanar geometry. Homolytic vs. Heterolytic bond cleavage Homolytic → two neutral radicals. Heterolytic → ion pair (cation + anion). Exothermic vs. Endothermic Exothermic: ΔH < 0, releases heat, ΔG can be negative if entropy not too unfavorable. Endothermic: ΔH > 0, absorbs heat, favored at high T if ΔS > 0. Homogeneous vs. Heterogeneous Catalysis Same phase as reactants (often liquid) vs. different phase (solid catalyst with gas/liquid reactants). ⚠️ Common Misunderstandings “All precipitation reactions are irreversible.” – Many are reversible; solubility product (Ksp) determines direction. “Catalysts are consumed.” – Catalysts are regenerated; only promoters/poisons affect them. “Higher temperature always speeds a reaction.” – Endothermic reactions may become less favorable if ΔG becomes positive. “All radicals are highly reactive and always lead to chain reactions.” – Some radicals quickly terminate; initiation step is crucial. “E2 always gives the most substituted alkene.” – E2 follows anti‑periplanar geometry; steric constraints can favor the less substituted product. 🧠 Mental Models / Intuition Energy profile diagram – Visualize reactants → activation barrier (Eₐ) → products; catalysts lower the peak, not the overall ΔH. Collision theory – Rate ∝ frequency of effective collisions (↑ concentration, ↑ temperature, ↑ pressure, ↑ surface area). “Carbocation stability ladder” – tertiary > secondary > primary > methyl; predicts where carbocations will form (SN1/E1). “Electron flow arrow” – Curved arrows start at electron sources (lone pairs, π bonds) and point to electron sinks (electrophilic centers). “Pericyclic symmetry” – Remember Woodward–Hoffmann: thermally allowed = suprafacial on both components for [4+2]; photochemical flips the rule. 🚩 Exceptions & Edge Cases E1cb – occurs when the leaving group is poor and the base is strong; the carbanion forms before leaving group departure. SN1 with neighboring group participation – can lead to rearranged products (e.g., bridged intermediates). Zero‑order reactions – rate independent of concentration when catalyst surface is saturated. Reversible redox under non‑standard conditions – equilibrium constant changes with concentration, but Keq (thermodynamic) stays constant. Acid‑base reactions in non‑aqueous solvents – Brønsted definitions still hold, but Ka/Kb values differ dramatically. 📍 When to Use Which Choose SN1 when substrate is tertiary or allylic/benzylic, weak nucleophile, polar protic solvent. Choose SN2 for primary/secondary substrate, strong nucleophile, polar aprotic solvent. Choose E1 if substrate can form a stable carbocation and the base is weak. Choose E2 when a strong base is present and anti‑periplanar geometry is accessible. Use hydroboration‑oxidation for anti‑Markovnikov alcohols on terminal alkenes. Apply Le Chatelier to predict shift when pressure, temperature, or concentration changes. Use Arrhenius equation to estimate how a temperature change will affect k. Select homogeneous catalysis when reactants are liquids/gases that can mix; choose heterogeneous for solid‑phase processes (e.g., catalytic converters). 👀 Patterns to Recognize “More substituted carbocation → more stable” → predicts major product in SN1/E1. “Anti‑periplanar H and leaving group → E2” – look for staggered conformations. “Markovnikov addition → more substituted carbocation” – check alkene substitution pattern. “Radical initiators (light, peroxides) → chain reactions” – presence of UV or peroxides flags radical mechanisms. “Double‑displacement → formation of precipitate or gas” – if an insoluble product or gas appears, a precipitation or gas‑evolution reaction is likely. “Equilibrium arrow ⇌ + catalyst” – catalyst does not shift Keq, only speeds attainment. 🗂️ Exam Traps Confusing ΔH with ΔG – a reaction can be exothermic (ΔH < 0) but non‑spontaneous if ΔS is very negative (ΔG > 0). Assuming all “fast” reactions are zero‑order – zero‑order only when catalyst surface is saturated; most fast reactions are first‑order or higher. Choosing SN2 for a tertiary alkyl halide – steric hindrance blocks backside attack; the correct answer is SN1 (or no substitution). Mistaking “strong base” for “strong nucleophile” – in protic solvents, a strong base may be poorly nucleophilic (e.g., $ \text{HO}^- $). Neglecting anti‑periplanar requirement in E2 – even with a strong base, a syn‑periplanar arrangement yields little elimination. Balancing redox without equal electrons – forgetting to multiply half‑reactions leads to charge imbalance. Assuming precipitation only occurs when a solid is formed – supersaturation can produce a precipitate even if the product is technically a solid in the solid state. --- Use this guide for quick recall right before the exam – focus on the bolded keywords, the decision trees, and the pattern‑recognition cues!