From the chloroform in old medicines to the CFCs that damaged the ozone layer — the chemistry of organic halides. With interactive SN1 vs SN2 comparison, mechanism diagrams, stereochemistry, and 20+ real-life examples.
Before diving in, here's the entire chapter on one page. From classification → preparation → reactions via SN1/SN2 → haloarenes → environmental effects.
Haloalkanes (R-X) and haloarenes (Ar-X) are organic compounds with halogen atoms. The polar C-X bond makes them reactive toward nucleophiles. SN1 vs SN2 depends on substrate, nucleophile, solvent, and leaving group. Haloarenes are much less reactive to nucleophilic substitution (partial double bond character of C-X). Environmental concerns: CFCs deplete ozone; DDT is banned for bioaccumulation.
Haloalkanes (Alkyl halides, R-X): Halogen attached to sp³ hybridized carbon of an alkyl group.
Haloarenes (Aryl halides, Ar-X): Halogen attached directly to sp² hybridized carbon of an aromatic ring.
Allylic halides: Halogen on sp³ carbon adjacent to a C=C double bond.
Benzylic halides: Halogen on sp³ carbon adjacent to an aromatic ring.
Vinylic halides: Halogen on sp² carbon of a C=C double bond.
Aryl halides: Halogen on sp² carbon of an aromatic ring.
| Type | Example | Reactivity to Nucleophilic Substitution |
|---|---|---|
| Alkyl halide (primary) | CH₃-CH₂-Cl | High (SN2 favorable) |
| Alkyl halide (tertiary) | (CH₃)₃C-Cl | High (SN1 favorable) |
| Allylic halide | CH₂=CH-CH₂-Cl | Very high (allylic stabilization) |
| Benzylic halide | Ph-CH₂-Cl | Very high (benzylic stabilization) |
| Vinylic halide | CH₂=CH-Cl | Very low (partial double bond) |
| Aryl halide | Ph-Cl (chlorobenzene) | Very low (partial double bond) |
Haloalkanes = R-X (sp³ C). Haloarenes = Ar-X (sp² C). Allylic and benzylic are highly reactive; vinylic and aryl are very unreactive. Classification: 1°/2°/3° (haloalkanes), mono/di/tri/poly (by count), and by position (allylic, benzylic, vinylic).
Rule 1 — Haloalkanes (R-X):
Example: CH₃-CH₂-Cl = chloroethane (common: ethyl chloride)
Example: CH₃-CH(Cl)-CH₃ = 2-chloropropane (isopropyl chloride)
Example: CH₂Cl₂ = dichloromethane
Example: CHCl₃ = trichloromethane (chloroform)
Example: CCl₄ = tetrachloromethane (carbon tetrachloride)
Example: CH₃-CH(Br)-CH₂-Cl = 1-bromo-2-chloropropane (alphabetical: bromo before chloro)
Rule 2 — Haloarenes (Ar-X):
Example: Ph-Cl = chlorobenzene
Example: Ph-Br = bromobenzene
Example: 1,2-dichlorobenzene (or ortho-dichlorobenzene)
Example: 2-chloro-4-nitrotoluene (positional with respect to methyl)
Rule 3 — Allylic and Benzylic:
Example: CH₂=CH-CH₂-Br = 3-bromoprop-1-ene
Example: Ph-CH₂-Cl = (chloromethyl)benzene or benzyl chloride
Rule 4 — Vinylic and Aryl (halogen on sp² carbon):
Example: CH₂=CH-Cl = chloroethene (vinyl chloride)
| Common Name | IUPAC Name | Formula |
|---|---|---|
| Methyl chloride | Chloromethane | CH₃Cl |
| Ethyl chloride | Chloroethane | C₂H₅Cl |
| Isopropyl chloride | 2-Chloropropane | CH₃CHClCH₃ |
| t-Butyl chloride | 2-Chloro-2-methylpropane | (CH₃)₃CCl |
| Methylene chloride | Dichloromethane | CH₂Cl₂ |
| Chloroform | Trichloromethane | CHCl₃ |
| Carbon tetrachloride | Tetrachloromethane | CCl₄ |
| Vinyl chloride | Chloroethene | CH₂=CHCl |
| Allyl bromide | 3-Bromoprop-1-ene | CH₂=CHCH₂Br |
| Benzyl chloride | Chloromethylbenzene | C₆H₅CH₂Cl |
| Chlorobenzene | Chlorobenzene | C₆H₅Cl |
Mistake: "Halogen is always given the lowest
locant."
Correction: Halogens are treated as
substituents, not the principal functional group. If
another group (like -OH, -COOH) is present, that determines
numbering. Halogens just get whatever locants result. Example:
CH₂Cl-CH₂-OH = 2-chloroethanol (OH gets priority,
numbered 1).
IUPAC names: halo- prefix + alkane/benzene. Haloalkanes: longest chain includes C-X. Prefixes in alphabetical order. Halogens are substituents (lowest priority). Common names still widely used (ethyl chloride, chloroform).
The C-X bond is polar because the halogen is more electronegative than carbon. Electronegativity order:
F (4.0) > Cl (3.0) > Br (2.8) > I (2.5) > C (2.5)
So the carbon is δ+ (electrophilic) and halogen is δ− (nucleophilic).
This polarity makes the carbon susceptible to attack by nucleophiles → nucleophilic substitution reactions.
| Bond | BDE (kJ/mol) | Reactivity (C-X bond cleavage) |
|---|---|---|
| C-F | ~485 | Very low (strongest bond) |
| C-Cl | ~327 | Moderate (most used in reactions) |
| C-Br | ~285 | High |
| C-I | ~213 | Very high (weakest bond, best leaving group) |
Reactivity order: R-I > R-Br > R-Cl > R-F (for C-X bond cleavage)
Why? Longer, weaker bonds break more easily. The iodide ion is also a better leaving group (larger, more polarizable, more stable anion).
In aryl halides (Ar-X), the C-X bond has partial double bond character due to resonance (lone pair on X donates into the ring). This makes the C-X bond shorter and stronger than in alkyl halides.
Consequences:
I⁻ is the weakest base among halide ions (conjugate base of strongest acid HI), so it's most stable as a free anion. It's also the largest ion (most polarizable), so the C-I bond is longest and weakest. Both factors → I⁻ leaves most easily → C-I most reactive to substitution.
C-X bond is polar (C is δ+, X is δ−). Halogen more electronegative → nucleophilic attack on carbon. Reactivity: R-I > R-Br > R-Cl > R-F (weaker C-X = better leaving group). Haloarenes: C-X has partial double bond character → very unreactive.
Using HX:
R-OH + HX → R-X + H₂O
Order of reactivity: HI > HBr > HCl > HF (HCl needs ZnCl₂ catalyst for primary/secondary alcohols)
3° alcohol + HCl (room temp) → 3° alkyl chloride (no catalyst needed)
1° alcohol + HCl (anhydrous ZnCl₂) → 1° alkyl chloride (Lucas test)
Using PCl₃, PCl₅, SOCl₂:
3R-OH + PCl₃ → 3R-Cl + H₃PO₃
R-OH + PCl₅ → R-Cl + POCl₃ + HCl
R-OH + SOCl₂ → R-Cl + SO₂ + HCl (SOCl₂ = thionyl chloride — best for clean reaction, no side products)
Alkanes (free radical halogenation):
R-H + X₂ → R-X + HX (in presence of UV light or heat)
Order of reactivity of halogens: F₂ > Cl₂ > Br₂ > I₂ (I₂ reversible, so rarely used)
For alkanes with multiple types of H, order of substitution: 3° > 2° > 1° (due to radical stability)
Alkenes (addition of HX):
CH₂=CH₂ + HCl → CH₃-CH₂-Cl
Markovnikov's rule: H adds to carbon with more H, X to other carbon
Anti-Markovnikov (peroxides): HBr + alkene + peroxide → anti-Markovnikov alkyl bromide (only for HBr)
Alkynes (addition):
CH≡CH + HCl → CH₂=CH-Cl (vinyl chloride)
CH≡CH + 2HCl → CH₃-CHCl₂ (ethylidene dichloride)
Finkelstein reaction: R-X + NaI → R-I + NaX (X = Cl, Br)
In dry acetone, NaCl/NaBr precipitate out, driving forward to R-I.
Swarts reaction: R-X + AgF → R-F + AgX (X = Cl, Br, I)
Or: R-X + Hg₂F₂ → R-F + Hg₂X₂ (for making alkyl fluorides)
Sandmeyer reaction:
Ar-N₂⁺Cl⁻ + Cu₂Cl₂/HCl → Ar-Cl + N₂
Or with Cu₂Br₂/HBr → Ar-Br
Uses diazonium salt from aniline → various aryl halides.
Balz-Schiemann reaction (for Ar-F):
Ar-N₂⁺BF₄⁻ → Ar-F + N₂ + BF₃ (on heating)
Direct halogenation of arene:
Benzene + Cl₂ --(FeCl₃)--> chlorobenzene
Benzene + Br₂ --(FeBr₃)--> bromobenzene
Main methods: (1) Alcohol + HX/PCl₃/PCl₅/SOCl₂ → R-X. (2) Alkane/alkene/alkyne + halogen → R-X (free radical or addition). (3) Halogen exchange (Finkelstein for iodides, Swarts for fluorides). (4) Haloarenes from Sandmeyer/Balz-Schiemann reaction from diazonium salts.
Boiling points:
Melting points:
In water: Haloalkanes are only slightly soluble in water because:
However, they dissolve in organic solvents (ether, benzene, alcohols) because London dispersion forces dominate.
Boiling points: R-I > R-Br > R-Cl > R-F (mass effect). Branching decreases BP and MP. Slightly soluble in water (no H-bond), very soluble in organics. Heavier halogens → denser than water. Mostly colourless with characteristic odours.
R-X + Nu⁻ → R-Nu + X⁻
Nucleophile (Nu⁻) replaces the halogen (X⁻, leaving group).
Rate law: Rate = k[R-X][Nu⁻] (second-order kinetics)
Mechanism: ONE step — Nu⁻ attacks the carbon from the backside (opposite the leaving group), displacing X⁻ simultaneously.
Stereochemistry: Inversion of configuration (Walden inversion) — like an umbrella turning inside-out.
Favored by:
Rate law: Rate = k[R-X] (first-order kinetics)
Mechanism: TWO steps —
Stereochemistry: Racemic mixture (both d and l forms) — because carbocation is planar, Nu⁻ can attack from either face with equal probability.
Favored by:
Choose substrate, nucleophile, and solvent — predict the mechanism.
| Feature | SN1 | SN2 |
|---|---|---|
| Kinetics | Rate = k[R-X] (first-order) | Rate = k[R-X][Nu⁻] (second-order) |
| Steps | 2 (carbocation intermediate) | 1 (concerted) |
| Stereochemistry | Racemic mixture | Inversion (Walden) |
| Favored substrate | 3° > 2° > 1° (carbocation stability) | 1° > 2° > 3° (steric hindrance) |
| Solvent | Polar protic (H₂O, ROH) | Polar aprotic (DMSO, DMF, acetone) |
| Nucleophile | Weak (H₂O, ROH) | Strong (CN⁻, I⁻) |
| Rearrangement | Possible (carbocation) | No |
| Leaving group | Better = faster | Better = faster |
SN2 is like replacing a passenger in a moving car: The new passenger (Nu⁻) can only get in if they simultaneously push the old one (X⁻) out the other door — one smooth motion. Steric crowding inside (branching) makes it harder for new passenger to get in.
SN1 is like a passenger jumping out first: The old passenger (X⁻) leaves the car (slow step), then the new one (Nu⁻) gets in from any side (fast step). Faster if the car seat is roomy (stable carbocation).
SN2 = 1 step, second-order, inversion, favored by 1° + strong Nu + aprotic solvent. SN1 = 2 steps via carbocation, first-order, racemic, favored by 3° + weak Nu + protic solvent. Allylic/benzylic are reactive via both mechanisms. Aryl/vinylic are unreactive.
Elimination: Removal of HX from alkyl halide → alkene.
General: R-CH₂-CH₂-X + Base → R-CH=CH₂ + HX
Elimination competes with substitution when using strong bases (e.g., KOH in ethanol, t-BuOK).
Rate law: Rate = k[R-X] (first-order)
Mechanism: 2 steps
Favored by: 3° alkyl halides, weak base, polar protic solvent
Regiochemistry: Zaitsev's rule — more substituted alkene favored (more stable)
Rate law: Rate = k[R-X][Base] (second-order)
Mechanism: 1 step — base attacks β-H while C-X bond breaks → alkene forms
Favored by: 1° and 2° alkyl halides, strong bulky base (t-BuOK), high T
Regiochemistry: Zaitsev's rule (more substituted alkene) for small bases; Hofmann product (less substituted) for bulky bases (t-BuOK)
Stereochemistry: Anti-periplanar geometry required (H and X must be on opposite sides)
| Feature | E1 | E2 |
|---|---|---|
| Kinetics | First-order | Second-order |
| Steps | 2 (carbocation) | 1 (concerted) |
| Favored substrate | 3° > 2° > 1° | 2° > 1° (3° gives mostly E2 but can be tricky) |
| Base | Weak | Strong (bulky = Hofmann) |
| Solvent | Polar protic | Often polar aprotic |
| Regiochemistry | Zaitsev (more stable alkene) | Zaitsev (small base) or Hofmann (bulky base) |
| Stereochemistry | Not strict | Anti-periplanar |
The outcome depends on:
E1 = carbocation intermediate, 2 steps, first-order, weak base. E2 = concerted, 1 step, second-order, strong base. Zaitsev's rule → more substituted alkene (small bases). Hofmann product → less substituted (bulky bases like t-BuOK). Substitution competes with elimination.
Three reasons:
Result: Haloarenes require forcing conditions for nucleophilic substitution (high T, high P, strong Nu).
Chlorobenzene + NaOH (300°C, 300 atm) → phenol
Requires extreme conditions — impossible under normal lab conditions.
However: If electron-withdrawing groups (NO₂, CN, etc.) are present on the ring (ortho or para to X), the reaction becomes much easier.
Example: 2,4,6-trinitrochlorobenzene + H₂O (room temp) → picric acid (fast — no forcing conditions!)
Reason: -NO₂ stabilizes the anionic intermediate (Meisenheimer complex) via resonance.
1. Electrophilic substitution (ring reactions):
Halogen is ortho/para directing (due to resonance donation) but deactivating (due to inductive withdrawal).
Example: chlorobenzene + Cl₂/FeCl₃ → 1,4-dichlorobenzene (para major)
2. Formation of Grignard reagent:
Ar-Br + Mg/dry ether → Ar-MgBr
Does NOT work for Ar-Cl or Ar-F (bond too strong). Works for Ar-I (very reactive) and Ar-Br (moderate).
3. Wurtz-Fittig reaction:
Ar-X + R-X + 2Na/dry ether → Ar-R + 2NaX
Forms alkylarene.
4. Fittig reaction:
2Ar-X + 2Na/dry ether → Ar-Ar + 2NaX
Forms biphenyl.
5. Reduction:
Ar-X + Zn/HCl → Ar-H + HX (rare; usually not done this way)
6. Friedel-Crafts acylation (limited):
Ar-X can be acylation substrate but reactivity is low; use AlCl₃ catalyst.
Haloarenes are unreactive to nucleophilic substitution (partial double bond, sp² C, electron-rich ring). Requires Dow process conditions (300°C, 300 atm) unless electron-withdrawing groups (NO₂, CN) are present. Reactive to electrophilic substitution (ring position). Grignard works only for Ar-I and Ar-Br.
Preparation: From ethanol/acetone + bleaching powder
2CH₃CH₂OH + 4Cl₂ + Ca(OH)₂ → 2CHCl₃ + (HCOO)₂Ca + CaCl₂ + 4H₂O (simplified)
Properties:
Uses: Solvent, precursor for chlorodifluoromethane (CFC-22), in cough syrups (trace), precursor to other chemicals.
Preparation: From methane + chlorine (free radical substitution), or from CS₂ + Cl₂
Properties:
Uses: Fire extinguishers (Pyrene), solvent, precursor for CFCs (before ban).
Freon-12 (CCl₂F₂, dichlorodifluoromethane): Common refrigerant, propellant.
Properties: Non-toxic, non-flammable, easily liquefied, chemically inert.
BUT: Extremely harmful to ozone layer — depletes ozone via radical chain reaction:
CFCl₃ --(UV)--> CFCl₂· + Cl·
Cl· + O₃ → ClO· + O₂
ClO· + O· → Cl· + O₂ (Cl is regenerated — 1 Cl destroys thousands of O₃ molecules)
Result: Montreal Protocol (1987) — worldwide ban on CFCs. Replaced by HFCs (less ozone-depleting, but greenhouse gases).
Structure: Two para-chlorophenyl rings + CCl₃ group on the same carbon.
Use: Powerful insecticide — kills mosquitoes (malaria), lice (typhus), etc.
Problems:
Banned in agriculture globally (Stockholm Convention, 2001), but still used for mosquito control in some regions (WHO approved for malaria).
The Montreal Protocol is considered the most successful international environmental treaty — the ozone layer is now recovering and expected to be fully healed by 2066. HFCs (replacement for CFCs) don't deplete ozone but are potent greenhouse gases — being phased out under the Kigali Amendment (2016).
Chloroform: solvent, anaesthetic (historical), toxic (phosgene). CCl₄: solvent, fire extinguisher, ozone depleter (banned). CFCs: refrigerants, cause ozone depletion (Montreal Protocol). DDT: insecticide, bioaccumulates (banned globally for agriculture). Environmental awareness critical for organic chemistry applications.
"SN1 = 1 molecule, 1 rate, 1 step, 1° NO"
SN1 = first-order, 2 steps, racemic. SN2 = second-order, 1 step, inversion.
"1° Substrate → SN2, 3° Substrate → SN1"
Remember: "Primary Substitutes 2, Tertiary Substitutes 1" (PS2, TS1)
"I > Br > Cl > F" (I is best leaving group — weakest base, largest ion)
Remember: "Iodide Iodizes, Bromide Brominates, Chloride Chlorinates, Fluoride Fails"
"Zaitsev Zaps to Z-most" — Zaitsev (small base) → more substituted alkene (most stable).
"Bulky Bases Buck the Trend" — Hofmann (t-BuOK) → less substituted alkene.
"Finkelstein is Fine for Iodide" — R-X + NaI → R-I (dry acetone).
"Swarts uses Silver for Fluoride" — R-X + AgF → R-F.
"Cl· + O₃ → ClO· + O₂ → Cl regenerated"
One chlorine atom destroys ~100,000 ozone molecules before being removed.
"DDT — Fat-Soluble Fatale" — accumulates in fat, magnifies up food chain, kills predators.
"Double Bond, Double Trouble" — C-X partial double bond makes haloarenes unreactive to nucleophilic substitution.
"NO₂ = Now Reaction Possible" — electron-withdrawing groups (ortho/para) increase reactivity.
"H goes to H-rich carbon" — H adds to carbon with more hydrogens.
Exception: HBr with peroxide → anti-Markovnikov (only for HBr).
Use "I>Br>Cl>F" for leaving groups, "1° SN2 / 3° SN1" for mechanism choice, "Zaitsev = Z-most" for alkene regiochemistry, and "Cl· destroys thousands of O₃" for ozone depletion.
Q1. Name the following: (a) CH₃-CH(Br)-CH₂-CH₃, (b) (CH₃)₃C-Cl
4-carbon chain with Br on carbon 2 → 2-bromobutane
Central C with 3 methyls + Cl → 2-chloro-2-methylpropane (t-butyl chloride)
Q2. Why is haloarene (chlorobenzene) less reactive than haloalkane (chloroethane) toward nucleophilic substitution?
In chlorobenzene, lone pair on Cl resonates into the ring → C-Cl bond has partial double bond character → shorter, stronger, harder to break.
Carbon is sp² (more s-character) → C-Cl bond is stronger. In chloroethane, carbon is sp³ (weaker bond).
Q3. Predict the major product when 2-bromobutane reacts with KOH in aqueous vs alcoholic solution.
Nucleophilic substitution (SN2, 2° substrate with strong Nu) → 2-butanol (major product)
CH₃-CH(Br)-CH₂-CH₃ + KOH(aq) → CH₃-CH(OH)-CH₂-CH₃
Elimination (E2) → alkenes. Zaitsev product (more substituted) is major.
CH₃-CH(Br)-CH₂-CH₃ + KOH(alc) → CH₃-CH=CH-CH₃ (2-butene, major) + CH₃-CH₂-CH=CH₂ (1-butene, minor)
In aqueous solution, OH⁻ is a good Nu for substitution (H₂O solvates carbocation poorly, so SN2 dominates for 2°). In alcoholic solution, OH⁻ acts more as base → E2.
Q4. (a) What is Finkelstein reaction? (b) What is Swarts reaction? Give one example each.
R-X + NaI (dry acetone) → R-I + NaX (X = Cl, Br)
NaCl/NaBr precipitate out in dry acetone, driving the reaction forward. Used to make alkyl iodides.
R-X + AgF → R-F + AgX (or use Hg₂F₂, CoF₃)
Used to make alkyl fluorides (Finkelstein doesn't work for F because NaF is soluble in acetone).
Finkelstein: alkyl iodides for radiolabeling. Swarts: fluorocarbons for refrigerants.
Q5. Compare SN1 and SN2 mechanisms with respect to: kinetics, stereochemistry, effect of substrate, solvent, and nucleophile.
SN1: Rate = k[R-X] (first-order). SN2: Rate = k[R-X][Nu⁻] (second-order).
SN1: Racemic (carbocation planar, attacked from either face). SN2: Inversion (Walden inversion, backside attack).
SN1: 3° > 2° > 1° (carbocation stability). SN2: 1° > 2° > 3° (steric hindrance).
SN1: Polar protic (H₂O, ROH) — stabilizes carbocation. SN2: Polar aprotic (DMSO, DMF, acetone) — leaves Nu⁻ free.
SN1: Weak (H₂O, ROH) — doesn't participate in rate-determining step. SN2: Strong (CN⁻, I⁻) — directly attacks in the single step.
Q6. (a) What are CFCs? (b) How do they deplete the ozone layer? (c) What is the Montreal Protocol?
Chlorofluorocarbons — compounds with both Cl and F on the same carbon. Example: CFC-12 (CCl₂F₂, Freon-12). Used as refrigerants, propellants, blowing agents.
In the stratosphere, UV light breaks CFC:
CCl₂F₂ --(UV)--> CClF₂· + Cl·
Chlorine radical destroys ozone:
Cl· + O₃ → ClO· + O₂
ClO· + O· → Cl· + O₂ (Cl regenerated!)
One Cl atom can destroy ~10⁵ ozone molecules before being removed (by CH₄ or other species).
International treaty signed in 1987 to phase out CFCs and other ozone-depleting substances. Ratified by all 197 UN members. Phase-out completed by 2010 (developed countries) and 2016 (India). Success: ozone layer is recovering — expected to heal by 2066.
Q7. Predict the products: (a) CH₃CH₂CH₂Cl + KOH (alc), (b) (CH₃)₃CBr + H₂O, (c) Ph-Cl + NaOH (300°C, 300 atm)
1° alkyl halide + strong base → E2 elimination. Product: propene (CH₃-CH=CH₂). Only one alkene possible.
3° alkyl halide + weak nucleophile (H₂O) → SN1. Carbocation forms (t-Bu⁺ is very stable), then H₂O attacks → t-butanol (2-methyl-2-propanol).
Dow process — nucleophilic aromatic substitution under forcing conditions. Product: sodium phenoxide, then acidification gives phenol. Requires extreme T and P because of partial double bond character.
Selected exercise questions from NCERT Class 12 Chemistry Chapter 6. Click each to reveal the solution.
(a) 2-chloro-3-methylpentane: 5-carbon chain (pentane). Cl on C2, methyl on C3.
CH₃-CH(Cl)-CH(CH₃)-CH₂-CH₃
(b) 1-chloro-4-ethylcyclohexane: Cyclohexane ring, Cl on C1, ethyl on C4. Para-arrangement.
(c) 4-tert-butyl-3-iodoheptane: 7-carbon chain. I on C3, t-butyl on C4.
CH₃-CH₂-CH(I)-CH(C(CH₃)₃)-CH₂-CH₂-CH₃
(d) 1,4-dibromobut-2-ene: 4-carbon chain with double bond at C2, Br on C1 and C4.
Br-CH₂-CH=CH-CH₂-Br
Answer: H₂SO₄ oxidizes the iodide ions to I₂:
2KI + H₂SO₄ → 2HI + K₂SO₄
Then HI (or KI) is oxidized by H₂SO₄:
2HI + H₂SO₄ → I₂ + SO₂ + 2H₂O
So instead of getting R-I (which requires KI as nucleophile), we get I₂. The alcohol is wasted. Instead, use H₃PO₄ or dry HCl for iodination via Finkelstein-type reaction.
(a) 1-bromopentane: 1° substrate → SN2 → 1-methoxypentane (CH₃-CH₂-CH₂-CH₂-CH₂-OCH₃)
(b) 2-bromopentane: 2° substrate, strong Nu → SN2 and E2 compete. Major = substitution (2-methoxypentane) + elimination (2-pentene and 1-pentene). Both possible.
(c) 2-bromo-2-methylbutane: 3° substrate with CH₃O⁻ (strong base + good Nu) → E2 dominant. Major = 2-methyl-2-butene (Zaitsev). Minor: 2-methyl-1-butene. Substitution minor.
Analysis:
• C₅H₁₀ has 1 double bond (or 1 ring) — degree of unsaturation = 1.
• No reaction with Cl₂ in dark → alkene (would react with Cl₂ even in dark) — so it must be a cycloalkane!
• Single monochloro compound in sunlight (free radical) → all H atoms equivalent → high symmetry.
Candidate: Cyclopentane (C₅H₁₀). All 10 H atoms are equivalent → only one monochloro product: chlorocyclopentane.
Answer: Cyclopentane
(a) Ph-CH₂Cl: chloromethylbenzene (benzyl chloride)
(b) Ph-CHCl-CH₃: (1-chloroethyl)benzene
(c) CH₂=CH-CH₂-Br: 3-bromoprop-1-ene (allyl bromide)
(d) CH₂=CH-CHCl-CH₂Br: 1-bromo-3-chlorobut-3-ene — wait, numbering from end closer to double bond. Actually CH₂=CH-CHCl-CH₂Br — numbering starts at CH₂= (C1), CH (C2), CHCl (C3), CH₂Br (C4). So: 1-bromo-3-chlorobut-3-ene → no, this is wrong. Let's renumber: right side gives lower locant for halogens? Chain: CH₂=CH-CHCl-CH₂Br. Numbering: C1 = CH₂ (double bond end, gives C2 to CH), C3 = CHCl, C4 = CH₂Br → 1-bromo-3-chlorobut-3-ene? No — double bond gets priority: C1=C2 must be double bond. So: 1-bromo-3-chlorobut-3-ene.
Actually correct answer: numbering from left so double bond gets position 1: CH₂=CH-CHCl-CH₂Br → 1-bromo-3-chlorobut-3-ene is wrong (double bond is at 1-2, not 3). Correct: 1-bromo-3-chlorobut-3-ene → 1-bromo-3-chlorobut-3-ene is not right. Let me redo: C1=CH₂, C2=CH, C3=CHCl, C4=CH₂Br. Double bond is C1=C2. So locants: 1-bromo... no wait, Br is on C4. From C1 numbering: 4-bromo-3-chlorobut-1-ene. But numbering from C4: 1-bromo-3-chlorobut-3-ene? No — the double bond would then be at C3-C4 which is C3=C4. Hmm.
Better: number from CH₂Br end: C1=CH₂Br, C2=CHCl, C3=CH, C4=CH₂. Double bond = C3-C4. Locants: 1-bromo-2-chlorobut-3-ene. Now double bond has lower locant (3) than other numbering (1). So first option (double bond at C1) is preferred.
Correct answer: 4-bromo-3-chlorobut-1-ene
(a) SN1 order: 3° > 2° > 1° (carbocation stability: 3° most stable due to hyperconjugation and inductive effects)
(a) SN2 order: 1° > 2° > 3° (steric hindrance: 1° least crowded → fastest SN2)
(b) SN1 and SN2 order (based on leaving group): 1-iodobutane > 1-bromobutane > 1-chlorobutane (I⁻ is better leaving group than Br⁻ than Cl⁻)
(a) In chlorobenzene, the lone pair on Cl participates in resonance with the aromatic ring → C-Cl bond has partial double bond character → the C-Cl bond is shorter. But more importantly, the resonance shifts electron density into the ring → the net dipole moment is lower. In cyclohexyl chloride, no such resonance → full dipole moment of C-Cl bond.
Dipole moment: cyclohexyl chloride (~2.2 D) > chlorobenzene (~1.7 D).
(b) Grignard reagents (R-MgX) are extremely strong bases/nucleophiles. Water protonates them:
R-MgX + H₂O → R-H + Mg(OH)X
So they get destroyed by moisture. Must be prepared in dry ether (anhydrous solvent, excluding moisture). Also use dry glassware, dry atmosphere.
Answer: This is an SN2 reaction (primary substrate, strong nucleophile, aqueous solution):
Step 1 (slow, rate-determining): OH⁻ attacks the backside of C attached to Br.
OH⁻ + Bu-Br → [HO···C(Bu)···Br]‡ (transition state)
Step 2 (fast): Br⁻ leaves as the C-O bond forms simultaneously.
Transition state → Bu-OH + Br⁻
Key features: One-step, second-order kinetics (Rate = k[BuBr][OH⁻]), inversion of configuration (if chiral substrate), favored by strong Nu and primary substrate.
Answer: Alcoholic KOH is a strong base → E2 elimination.
2-bromopentane: CH₃-CH(Br)-CH₂-CH₂-CH₃
Two β-hydrogens available (on C1 and C3):
• Remove H from C1 → 1-pentene (CH₂=CH-CH₂-CH₂-CH₃) — Hofmann product (less substituted)
• Remove H from C3 → 2-pentene (CH₃-CH=CH-CH₂-CH₃) — Zaitsev product (more substituted)
Major product: 2-pentene (Zaitsev) — more stable alkene (more alkyl substituents on C=C).
Also trans-2-pentene is more stable than cis-2-pentene (less steric strain).
DDT:
• Bioaccumulates in fat tissue (not metabolized)
• Biomagnifies up the food chain (higher in predators)
• Causes eggshell thinning in birds (nearly wiped out bald eagles)
• Very persistent in the environment
• Banned for agricultural use by Stockholm Convention (2001)
CFCs:
• Deplete stratospheric ozone layer via Cl radical chain reaction
• One Cl atom destroys ~100,000 O₃ molecules
• Banned by Montreal Protocol (1987)
• Replaced by HFCs (which don't deplete ozone but are greenhouse gases)
Control: International treaties (Stockholm, Montreal, Kigali), phase-out of harmful compounds, use of safer alternatives (HFCs for CFCs, biopesticides for DDT).
(a) Chloroform (CHCl₃): Solvent for fats, oils, rubber, resins, alkaloids. Historically as anesthetic (now banned). Precursor to CFC-22.
(b) Iodoform (CHI₃): Antiseptic (yellow powder — old medical use). Now used as a laboratory reagent (test for methyl ketones).
(c) Carbon tetrachloride (CCl₄): Fire extinguishers (Pyrene), solvent for fats/oils/resins. Banned due to ozone depletion.
(d) Freons (CFCs): Refrigerants, propellants in aerosol sprays, blowing agents for foams. Banned by Montreal Protocol due to ozone depletion.
(e) DDT: Powerful insecticide — kills mosquitoes (malaria), lice (typhus). Banned for agriculture due to bioaccumulation but still WHO-approved for malaria control in some regions.
| Leaving group | I⁻ > Br⁻ > Cl⁻ > F⁻ | I⁻ weakest base, best leaving |
| SN2 substrate | 1° > 2° > 3° | Less steric hindrance |
| SN1 substrate | 3° > 2° > 1° | Carbocation stability |
| E2 substrate | 3° > 2° > 1° | Elimination favored |
| C-X bond strength | C-F > C-Cl > C-Br > C-I | F strongest, I weakest |
| Haloalkane boiling point | R-I > R-Br > R-Cl > R-F | Heavier halogen = higher BP |
| Reactivity to Nu sub | Allyl/Benzyl > 1° > 2° > 3° > vinyl/aryl | Resonance effects |
| HX reactivity with alcohol | HI > HBr > HCl > HF | Strongest acid reacts fastest |
| R-X | Alkyl halide (haloalkane) |
| Ar-X | Aryl halide (haloarene) |
| Nu⁻ | Nucleophile |
| SN1 | Substitution, unimolecular |
| SN2 | Substitution, bimolecular |
| E1 | Elimination, unimolecular |
| E2 | Elimination, bimolecular |
| CFC | Chlorofluorocarbon |
| HFC | Hydrofluorocarbon (CFC replacement) |
| DDT | Dichlorodiphenyltrichloroethane |
| t-BuOK | Potassium tert-butoxide (bulky base) |
(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
All key reactions & facts at one place — perfect for quick revision before exams.