Hành trình Phản ứng Hóa học
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Hiệu ứng Điện tử
Hiệu ứng điện tử giúp giải thích vì sao các nhóm thế đẩy hoặc hút electron qua các liên kết σ- và π, kiểm soát độ bền, tính acid, tính base, và cả tính chất vật lý của các hợp chất hữu cơ.
Hiệu ứng cảm là sự truyền mật độ electron qua liên kết σ. Nhóm có độ âm điện mạnh hút electron (−I), trong khi nhóm alkyl đẩy (cho) electron (+I).
Nhóm −I tiêu biểu:
I < Br < Cl < F < C=O < C≡N < CF₃ < NO₂
Nhóm +I tiêu biểu: CH₃ < C₂H₅ < i-Pr < t-Bu
Hệ quả: Nhóm −I ổn định điện tích âm nhưng lại gây bất ổn trên carbocations; Nhóm +I có tác động ngược lại.
• Carbocation:
• Carbanion:
Cộng hưởng là để chỉ sự bất định vị của electron trên nhiều nguyên tử, thể hiện qua nhiều cấu tạo tương đương khác nhau.
Ý chính: Bất định vị càng nhiều → độ bền càng tăng.
- Phenoxide (C₆H₅O⁻) bền hơn một alkoxide đơn giản (RO⁻).
- Carboxylate (RCOO⁻) cũng bền hơn alkoxide vì điện tích âm được chia sẻ bởi hai oxygen.
Sự cộng hưởng trong trường hợp của nitrate và carboxylate được gọi là cộng hưởng p-π, do có sự tham gia và chuyển đổi qua lại giữa electron p và electron π.
Với phenoxide (phenolate): giữa O và C là cộng hưởng p-π, còn cộng hưởng trong vòng benzene xảy ra giữa các liên kết π nên được gọi là cộng hưởng π-π.
Siêu liên hợp (siêu tiếp cách) là chuyển mật độ electron từ liện kết σ C–H đến một orbital p trống (hay coi như trống) kế cận.
Độ bền của carbocation: 3° > 2° > 1° > CH₃⁺
Hiệu ứng này giúp giải thích tại sao các dẫn xuất bậc ba thường ưu đãi phản ứng SN1/E1 (do chất carbocation trung gian bền hơn).
Electronic effects influence polarity and intermolecular forces, which affect physical properties.
| Property | Trend |
|---|---|
| Dipole moment | Increases with −I substituents (more polar bonds) |
| Boiling point | Increases with polarity and especially with H-bonding |
| Melting point | Often increases with symmetry and efficient packing |
Example: CH₃CH₂OH has a higher boiling point than CH₃CH₃ due to hydrogen bonding.
Acids–Bases
Acid–base behavior is the “first filter” for many mechanisms: proton transfers, nucleophile/electrophile strength, leaving-group activation, and whether a reaction is driven by equilibrium.
Brønsted–Lowry: acid donates H⁺, base accepts H⁺.
Conjugate pairs: HA ⇌ H⁺ + A⁻ (A⁻ is the conjugate base).
Quick examples:
-
Water with itself:
H₂O + H₂O ⇌ H₃O⁺ + OH⁻ -
Strong acid in water:
HCl + H₂O → H₃O⁺ + Cl⁻ -
Carboxylic acid:
CH₃CO₂H + H₂O ⇌ H₃O⁺ + CH₃CO₂⁻ -
Ammonia as base:
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
Illustration (Reaction mechanism):
NH₃ (base), NH₄⁺ (conjugated acid)
H₂O (acid), OH⁻ (conjugated base).
pKa measures acidity: lower pKa = stronger acid.
Rule of thumb: reactions favor formation of the weaker acid/base pair.
Henderson–Hasselbalch equation: links solution pH to the ratio of conjugate base to acid and explains buffer behavior.
• With: HA ⇌ H⁺ + A⁻
| Species | pKₐ |
|---|---|
| HCl | –7.0 |
| H₂SO₄ (first dissociation) | –3.0 |
| H₃O⁺ | –1.74 |
| Formic acid (HCO₂H) | 3.75 |
| Acetic acid (CH₃CO₂H) | 4.76 |
| Carbonic acid (H₂CO₃, first) | 6.35 |
| Phenol (C₆H₅OH) | 10.00 |
| Water (H₂O) | 15.74 |
| Ammonium (NH₄⁺) | 9.25 |
| Ammonia (NH₃, conjugate base of NH₄⁺) | 35.0 |
• Lower pKₐ → stronger acid.
• Higher pKₐ → weaker acid (stronger conjugate base).
Acid/base strength tracks stability of the conjugate base/acid.
- More electronegative atom stabilizes negative charge.
- Resonance delocalization stabilizes charge.
- Inductive withdrawal stabilizes anions.
- Larger atoms stabilize charge better (down a group).
Solvents can change “effective” strength.
- Leveling effect: very strong acids/bases appear similar in a given solvent.
- Protic vs aprotic: affects nucleophile/basicity trends.
| Feature | Protic solvents | Aprotic solvents |
|---|---|---|
| H-bonding | Can donate and accept H-bonds | Accept only (no N–H or O–H) |
| Common examples | H₂O, ROH (alcohols), NH₃ | DMSO, DMF, acetone, acetonitrile |
| Effect on nucleophiles | Strong solvation → weaker nucleophiles | Less solvation → stronger nucleophiles |
| Best for SN2 | Generally poor | Generally good |
| Best for SN1 | Generally good (stabilizes ions) | Generally poorer for carbocation formation |
| Leveling effect | Present (strong acids/bases appear similar) | Minimal or absent |
Key takeaway:
• aprotic solvents enhance nucleophilicity.
• protic solvents stabilize ions.
Reaction Mechanisms
Mechanisms explain how bonds break and form. We’ll build this as interactive “pathways”: choose substrate + conditions → see the most likely steps and intermediates.
Core question: one-step backside attack (SN2) or two-step via carbocation (SN1)?
- SN2: strong nucleophile, primary substrate, aprotic solvent.
- SN1: tertiary substrate, weak nucleophile OK, protic solvent; carbocation stability matters.
SN1 vs SN2 — quick comparison
| Feature | SN1 | SN2 |
|---|---|---|
| Substrate (carbocation) | 3° preferred (2° possible) | 1° preferred (2° possible) |
| Nucleophile | Weak nucleophile acceptable | Strong nucleophile required |
| Solvent | Protic (stabilizes ions) | Aprotic (enhances nucleophilicity) |
| Leaving group | Good leaving group needed | Good leaving group needed |
| Rate law | Rate = k[substrate] | Rate = k[substrate][nucleophile] |
| Stereochemistry | Racemization (planar carbocation) | Inversion (backside attack) |
| Mechanism steps | Two steps (ionization → attack) | One step (concerted) |
Decision tree
Predict SN1 vs SN2
Elimination competes with substitution.
- E2: strong base, concerted; needs anti-periplanar geometry.
- E1: via carbocation; often with SN1 conditions.
E1 vs E2 — Quick comparison
| Feature | E1 | E2 |
|---|---|---|
| Mechanism type | Stepwise (2 steps) | Concerted (1 step) |
| Carbocation | 3° best (2° possible) | No carbocation formed |
| Base | Weak base preferred | Strong base required |
| Solvent | Protic favored | Aprotic favored |
| Substrate | 3° > 2° > 1° | 3° > 2° > 1° |
| Major product | More substituted alkene (Zaitsev) | Usually more substituted (Zaitsev), but bulky base → Hofmann |
| Stereochemistry | Mixture possible | Anti-periplanar requirement |
Decision tree
Predict E1 vs E2
Zaitsev vs Hofmann + Anti-periplanar check
1) Predict major alkene
2) Anti-periplanar requirement (E2)
✔ Anti-periplanar — E2 allowed
Additions to C=C involve attack on an electrophilic π bond of an alkene.
- Hydrohalogenation: HBr, HCl (Markovnikov vs anti-Markovnikov)
- Hydrogenation: H₂, cat. Ni (syn addition)
- Halogenation: Br₂, Cl₂ (anti addition)
- Hydration: H₂SO₄ / H₂O → alcohol
- Radical pathway: HBr + peroxides
2-bromopropane (Markovnikov rule)
>
Hydrogenation: syn addition with Ni catalyst
Additions to C=C — quick comparison
| Reaction | Reagent | Product / Key feature |
|---|---|---|
| Hydrohalogenation (ionic) | HBr, HCl | Markovnikov alkyl halide (carbocation pathway; rearrangements possible) |
| Hydrohalogenation (radical) | HBr + peroxides | Anti-Markovnikov alkyl halide (radical chain; no rearrangement) |
| Hydrogenation | H₂, Ni (or Pd, Pt) | Alkane via syn addition of H₂ |
| Halogenation | Br₂ or Cl₂ | Vicinal dihalide via anti addition (halonium ion) |
| Acid-catalyzed hydration | H₂SO₄ / H₂O | Markovnikov alcohol (carbocation pathway) |
| Hydroboration–oxidation | BH₃/THF → H₂O₂, OH⁻ | Anti-Markovnikov alcohol, syn, no rearrangement |
| Dihydroxylation | KMnO₄ (or OsO₄) | Vicinal diol (glycol), concerted syn addition |
Decision tree
(Later: add curved-arrow mechanism diagrams.)
Carbonyl C=O is polarized: C is electrophilic, O is nucleophilic.
- Nucleophilic addition to aldehydes/ketones
- Acyl substitution for carboxylic derivatives
Carbonyl carbon is electrophilic due to C=O polarization; nucleophiles attack the carbonyl carbon.
| Reaction | Reagent | Product / Key feature |
|---|---|---|
| Hydride reduction (mild) | NaBH₄ | Aldehyde → 1° alcohol; Ketone → 2° alcohol |
| Hydride reduction (strong) | LiAlH₄ | Reduces aldehydes, ketones, esters, acids → alcohols |
| Cyanohydrin formation | HCN | H⁺ and C≡N⁻ added; forms cyanohydrin RCH(OH)CN |
| Acetal / ketal formation | ROH + acid | Protecting group; forms acetal / ketal (reversible) |
| Imine formation | RNH₂ + acid | C=N (imine); condensation reaction |
| Grignard addition | RMgBr (then H₃O⁺) | New C–C bond; alcohol formed after workup |
| Acyl substitution | Acid chloride / ester + Nu⁻ | Addition–elimination (nucleophilic acyl substitution) |
Predict carbonyl reaction
(Later: add curved-arrow stepper with tetrahedral intermediate.)
SR (radical substitution) proceeds via a radical chain mechanism: initiation → propagation → termination.
- Typical: Alkane halogenation (Cl₂ / Br₂, hν or Δ)
- Key factors: radical stability & selectivity (Br₂ more selective than Cl₂)
- Outcome: mixture of products; controlled by relative H-abstraction rates
| Feature | Description |
|---|---|
| Typical substrate | Alkanes (sp³ C–H bonds) |
| Reagents | Cl₂ or Br₂ + light (hν) or heat (Δ) |
| Mechanism type | Free radical chain reaction |
| Steps |
1) Initiation (radical formation) 2) Propagation (chain continues) 3) Termination (radicals combine) |
| Selectivity |
Br₂ is more selective than Cl₂ (3° > 2° > 1°) |
| Major product rule | Hydrogen is replaced at the position that forms the most stable radical |
| Initiation | Cl₂ → 2 Cl• (homolytic cleavage) |
| Propagation (step 1) |
Cl• + R–H → HCl + R• |
| Propagation (step 2) |
R• + Cl₂ → R–Cl + Cl• |
| Termination |
Cl• + Cl• → Cl₂ R• + Cl• → R–Cl R• + R• → R–R |
Radical halogenation is thermodynamically driven by bond strengths and kinetically influenced by radical stability.
Predict Radical Substitution Product Distribution
| Type | Count |
|---|---|
| 1° (primary) | |
| 2° (secondary) | |
| 3° (tertiary) |
(Later: add an interactive “which H is substituted?” selector + selectivity chart.)
Electrophilic Aromatic Substitution (EAS) preserves aromaticity. An electrophile replaces a hydrogen on the benzene ring.
| Step 1 | Formation of strong electrophile (E⁺) |
| Step 2 | Aromatic ring attacks E⁺ → σ-complex (arenium ion) |
| Step 3 | Deprotonation restores aromaticity |
The σ-complex (arenium ion) is resonance-stabilized but temporarily loses aromaticity.
| Reaction | Reagents | Electrophile |
|---|---|---|
| Halogenation | Br₂ / FeBr₃ or Cl₂ / FeCl₃ | Br⁺ or Cl⁺ |
| Nitration | HNO₃ / H₂SO₄ | NO₂⁺ (nitronium) |
| Sulfonation | SO₃ / H₂SO₄ | SO₃ or HSO₃⁺ |
| Friedel–Crafts alkylation | R–Cl / AlCl₃ | R⁺ (carbocation) |
| Friedel–Crafts acylation | RCOCl / AlCl₃ | RCO⁺ (acylium) |
| Substituent Type | Examples | Direction | Activation |
|---|---|---|---|
| Electron-donating (EDG) | –OH, –OR, –NH₂, –R | Ortho / Para | Activating |
| Weakly deactivating halogens | –Cl, –Br | Ortho / Para | Deactivating |
| Electron-withdrawing (EWG) | –NO₂, –CF₃, –CN, –COOH | Meta | Deactivating |
Rule of thumb:
• EDG → stabilize σ-complex → ortho/para
• Strong EWG → destabilize ortho/para σ-complex → meta