❓People Also Ask — Alkyl Halides
Questions students most frequently search about this chapter. Answers are direct and exam-focused.
QWhat are alkyl halides?
Alkyl halides (haloalkanes) are organic compounds with general formula R–X where X is F, Cl, Br, or I bonded to an sp³ hybridized carbon. They are versatile synthetic intermediates undergoing nucleophilic substitution (SN1/SN2) and elimination (E1/E2).
QWhat is the difference between SN1 and SN2?
SN1: two-step via carbocation, first-order, tertiary substrates, polar protic solvent, racemization. SN2: one-step backside attack, second-order, methyl/primary substrates, strong nucleophile, polar aprotic solvent, inversion of configuration.
QWhat is the difference between E1 and E2?
E1: two-step via carbocation, first-order, tertiary + heat. E2: one-step anti-periplanar, second-order, strong base + 2°/3° substrate. Both follow Zaitsev's rule unless a bulky base is used (Hofmann product).
QWhy is the C–I bond most reactive?
The C–I bond is the weakest (lowest bond energy), and I⁻ is the best leaving group (largest, most polarizable, weakest conjugate base). Despite C–F being more polar, the strong C–F bond and the poor leaving ability of F⁻ make fluoroalkanes least reactive.
QWhat is Zaitsev's rule?
Zaitsev's rule: in an elimination reaction, the major product is the more substituted (more stable) alkene. For example, 2-bromobutane + alcoholic KOH gives mostly 2-butene (not 1-butene).
QWhat is the Hofmann rule?
The Hofmann rule states that when a bulky base (e.g., tert-butoxide) is used, the major product is the less substituted alkene, because the bulky base cannot reach the more hindered β-hydrogen.
QWhat is Walden inversion?
Walden inversion is the inversion of stereochemical configuration at a chiral carbon during SN2 substitution. The nucleophile attacks from the opposite side of the leaving group, "flipping" the geometry.
QWhich mechanism does tertiary alkyl halide follow?
Tertiary alkyl halides predominantly follow SN1 (with weak nucleophiles) or E1 (with heat) because the tertiary carbocation intermediate is highly stable. With strong bases they shift to E2.
QWhat is the Finkelstein reaction?
Finkelstein reaction prepares alkyl iodides: R–Cl + NaI (in dry acetone) → R–I + NaCl↓. NaCl/NaBr precipitate, driving the reaction forward.
QWhat is the Wurtz reaction?
Wurtz reaction: 2R–X + 2Na (dry ether) → R–R + 2NaX. Couples two alkyl halides into a higher alkane. Works best with primary halides; tertiary halides undergo elimination.
QWhat is the Swarts reaction?
Swarts reaction: R–Cl + AgF (or Hg₂F₂) → R–F + AgCl. Prepares alkyl fluorides.
QWhy does aqueous KOH give substitution but alcoholic KOH gives elimination?
In aqueous KOH, OH⁻ is a good nucleophile, so substitution dominates. In alcoholic KOH, ethoxide is a stronger base and the non-aqueous environment favors elimination.
QWhy are vinyl halides unreactive?
Vinyl halides are unreactive because (1) C–X bond has partial double bond character via resonance, (2) sp² carbon is more electronegative and holds electrons tighter, (3) the π-cloud blocks backside attack.
QWhat is solvolysis?
Solvolysis is a substitution where the solvent itself acts as the nucleophile (e.g., water in hydrolysis, or alcohol in alcoholysis). It typically proceeds via SN1 with tertiary halides in polar protic solvents.
QHow many MCQs from Alkyl Halides in MDCAT?
Alkyl Halides typically contributes 2–3 MCQs. High-yield because reaction mechanisms (SN1, SN2, E1, E2) form the foundation for later organic chemistry chapters.
QWhat is the effect of temperature on substitution vs elimination?
High temperature favors elimination because elimination produces more molecules (higher entropy), so the TΔS term of the free energy equation dominates at high T.
QWhy is neopentyl chloride slow in SN2 despite being primary?
Even though neopentyl is 1°, the bulky tert-butyl group sterically blocks the backside attack required for SN2, making the reaction extremely slow.
QWhat is a Grignard reagent and how is it prepared?
A Grignard reagent (R–MgX) is prepared from an alkyl halide + Mg in dry ether. Water must be avoided because RMgX reacts with water to form alkane (R–H).
QWhat is stereospecific vs stereoselective?
Stereospecific: the mechanism gives a specific stereoisomer from a specific stereoisomer of reactant (SN2 gives inversion). Stereoselective: one stereoisomer is favored over others (SN1 gives racemization with slight inversion excess).
QWhat is the best way to prepare alkyl chlorides from alcohols?
SOCl₂ in pyridine is preferred — by-products (SO₂ and HCl) are gases, so the product is very pure. PCl₅ and PCl₃ also work but leave side products. HCl/ZnCl₂ (Lucas reagent) works best for 3° alcohols.
📋Syllabus & Learning Outcomes
According to PMDC/PMC MDCAT syllabus, Alkyl Halides cover:
- Name alkyl halides using the IUPAC system.
- Discuss the structure and reactivity of RX.
- Describe the mechanism and types of nucleophilic substitution reactions (SN1 and SN2).
- Describe the mechanism and types of elimination reactions (E1 and E2).
📝Full Notes — Alkyl Halides
1. IUPAC Nomenclature of Alkyl Halides
- General formula: R–X (X = F, Cl, Br, I).
- Prefix: fluoro-, chloro-, bromo-, iodo- (alphabetical order).
- Number the longest chain to give the halogen the lowest possible locant.
- Examples: CH₃CH₂Br → bromoethane; (CH₃)₂CHCl → 2-chloropropane; CH₂=CHCH₂Br → 3-bromoprop-1-ene.
- Cyclic halides: chlorocyclohexane.
2. Structure and Reactivity of RX
- C–X bond polarity: Carbon δ+, halogen δ− (electrophilic carbon).
- Bond strength: C–F > C–Cl > C–Br > C–I (F strongest, weakest leaving group).
- Leaving group ability: I⁻ > Br⁻ > Cl⁻ > F⁻ (weak bases are good leaving groups).
- Reactivity toward nucleophilic substitution/elimination: RI > RBr > RCl > RF.
- Alkyl halides are versatile synthetic intermediates — undergo substitution or elimination.
3. Nucleophilic Substitution — SN1 & SN2
| Feature | SN2 | SN1 |
| Kinetics | Second order (rate = k[RX][Nu]) | First order (rate = k[RX]) |
| Steps | One concerted (no intermediate) | Two: carbocation formation then Nu attack |
| Stereochemistry | Inversion of configuration (Walden inversion) | Racemisation (planar carbocation, attack both faces) |
| Substrate preference | CH₃X > 1° > 2° > 3° | 3° > 2° > 1° > CH₃X |
| Nucleophile | Strong nucleophile required | Weak nucleophile sufficient |
| Solvent | Polar aprotic (DMSO, acetone) | Polar protic (H₂O, ROH) |
Key: Tertiary alkyl halides favor SN1; primary alkyl halides favor SN2. Allylic/benzylic substrates can do both (stabilised carbocation).
4. Elimination — E1 & E2
| Feature | E2 | E1 |
| Kinetics | Second order (rate = k[RX][Base]) | First order (rate = k[RX]) |
| Steps | One concerted | Two: carbocation then H⁺ loss |
| Substrate | 3° > 2° > 1° (stable alkene) | 3° > 2° (carbocation stability) |
| Base | Strong base required (OH⁻, OR⁻) | Weak base often sufficient |
| Stereochemistry | Anti-periplanar required | No special orientation |
Zaitsev's rule: Major product is the more substituted alkene. Bulky bases (t-BuO⁻) give Hofmann product (less substituted).
Competition: SN2 competes with E2; SN1 competes with E1. Strong bases + heat favor elimination. Aqueous KOH → substitution; alcoholic KOH → elimination.
High-Yield Points & Common Traps
- IUPAC: halogen as prefix; alphabetical order; lowest number to halogen.
- Reactivity: RI > RBr > RCl > RF (C–X bond strength & leaving group ability).
- SN2: inversion, one step, 1° substrates, strong Nu, aprotic solvent.
- SN1: racemisation, two steps, 3° substrates, weak Nu, protic solvent.
- E2: anti-periplanar geometry; strong base; Zaitsev alkene.
- E1: carbocation intermediate; competes with SN1; Zaitsev rule.
- Bulky bases (t-BuO⁻) → Hofmann product (less substituted).
- Heat favours elimination over substitution.
- Traps: "SN2 occurs with tertiary alkyl halides" → False (SN1). "E2 requires a carbocation" → False (concerted). "Fluorine is a good leaving group" → False. "SN2 has retention of configuration" → False (inversion).
❔Frequently Asked Questions
What are alkyl halides?
Alkyl halides (haloalkanes) are organic compounds with formula R–X where X is F, Cl, Br, or I bonded to an sp³ hybridized carbon. They are versatile synthetic intermediates undergoing nucleophilic substitution (SN1/SN2) and elimination (E1/E2).
What is the difference between SN1 and SN2?
SN1: two-step via carbocation, first-order, tertiary substrates, polar protic solvent, racemization. SN2: one-step backside attack, second-order, methyl/primary substrates, strong nucleophile, polar aprotic solvent, inversion of configuration.
What is the difference between E1 and E2?
E1: two-step via carbocation, first-order, tertiary + heat. E2: one-step anti-periplanar, second-order, strong base + 2°/3° substrate. Both follow Zaitsev's rule unless a bulky base is used (Hofmann product).
Why is the C–I bond most reactive?
The C–I bond is the weakest (lowest bond energy), and I⁻ is the best leaving group (largest, most polarizable, weakest conjugate base). Despite C–F being more polar, the strong C–F bond and the poor leaving ability of F⁻ make fluoroalkanes least reactive.
What is Zaitsev's rule?
Zaitsev's rule: in an elimination reaction, the major product is the more substituted (more stable) alkene. For example, 2-bromobutane + alcoholic KOH gives mostly 2-butene (not 1-butene).
What is the Hofmann rule?
The Hofmann rule states that when a bulky base (e.g., tert-butoxide) is used, the major product is the less substituted alkene, because the bulky base cannot reach the more hindered β-hydrogen.
What is Walden inversion?
Walden inversion is the inversion of stereochemical configuration at a chiral carbon during SN2 substitution. The nucleophile attacks from the opposite side of the leaving group, "flipping" the geometry.
Which mechanism does tertiary alkyl halide follow?
Tertiary alkyl halides predominantly follow SN1 (with weak nucleophiles) or E1 (with heat) because the tertiary carbocation intermediate is highly stable. With strong bases they shift to E2.
What is the Finkelstein reaction?
Finkelstein reaction prepares alkyl iodides: R–Cl + NaI (in dry acetone) → R–I + NaCl↓. NaCl/NaBr precipitate, driving the reaction forward.
What is the Wurtz reaction?
Wurtz reaction: 2R–X + 2Na (dry ether) → R–R + 2NaX. Couples two alkyl halides into a higher alkane. Works best with primary halides; tertiary halides undergo elimination.
What is the Swarts reaction?
Swarts reaction: R–Cl + AgF (or Hg₂F₂) → R–F + AgCl. Prepares alkyl fluorides.
Why does aqueous KOH give substitution but alcoholic KOH gives elimination?
In aqueous KOH, OH⁻ is a good nucleophile, so substitution dominates. In alcoholic KOH, ethoxide is a stronger base and the non-aqueous environment favors elimination.
Why are vinyl halides unreactive?
Vinyl halides are unreactive because (1) C–X bond has partial double bond character via resonance, (2) sp² carbon is more electronegative and holds electrons tighter, (3) the π-cloud blocks backside attack.
What is solvolysis?
Solvolysis is a substitution where the solvent itself acts as the nucleophile (e.g., water in hydrolysis, or alcohol in alcoholysis). It typically proceeds via SN1 with tertiary halides in polar protic solvents.
How many MCQs from Alkyl Halides in MDCAT?
Alkyl Halides typically contributes 2–3 MCQs. High-yield because reaction mechanisms (SN1, SN2, E1, E2) form the foundation for later organic chemistry chapters.
What is the effect of temperature on substitution vs elimination?
High temperature favors elimination because elimination produces more molecules (higher entropy), so the TΔS term dominates at high T.
Why is neopentyl chloride slow in SN2 despite being primary?
Even though neopentyl is 1°, the bulky tert-butyl group sterically blocks the backside attack required for SN2, making the reaction extremely slow.
What is a Grignard reagent and how is it prepared?
A Grignard reagent (R–MgX) is prepared from an alkyl halide + Mg in dry ether. Water must be avoided because RMgX reacts with water to form alkane (R–H).
What is stereospecific vs stereoselective?
Stereospecific: mechanism gives a specific stereoisomer from a specific stereoisomer of reactant (SN2 gives inversion). Stereoselective: one stereoisomer is favored over others (SN1 gives racemization with slight inversion excess).
What is the best way to prepare alkyl chlorides from alcohols?
SOCl₂ in pyridine is preferred — by-products (SO₂ and HCl) are gases, so the product is very pure. PCl₅ and PCl₃ also work but leave side products. HCl/ZnCl₂ (Lucas reagent) works best for 3° alcohols.