Halogen Derivatives
Easy Overview
Organic compounds containing halogen atoms (F, Cl, Br, I) are everywhere — from Teflon frying pans and PVC pipes to fire extinguishers and the anaesthetic that put you to sleep during surgery. These halogen derivatives are versatile intermediates in organic synthesis and have enormous practical applications. They are classified as alkyl halides (halogen bonded to sp³ carbon), vinyl halides (halogen bonded to sp² carbon of alkene), and aryl halides (halogen bonded directly to a benzene ring). The chemistry of each is very different because of how the carbon-halogen bond behaves. The carbon-halogen bond polarity decreases from C-F (most polar) to C-I (least polar). But bond strength decreases from C-F (very strong, 485 kJ/mol) to C-I (weak, 240 kJ/mol). This makes alkyl iodides the most reactive in substitution reactions (weakest bond) and alkyl fluorides the least reactive. The C-X bond strength is the key to understanding why certain reactions happen — the bond must break, and weaker bonds break more easily. Alkyl halides are the workhorses of organic chemistry because the C-X bond can be broken in many ways, leading to substitution products (via SN1 or SN2), elimination products (alkenes via E1 or E2), or reaction with metals (Grignard reagents). Aryl halides are much less reactive — the C-X bond in chlorobenzene has partial double-bond character due to resonance with the aromatic ring. This makes nucleophilic substitution very difficult unless extreme conditions or electron-withdrawing groups are present. Polyhalogen compounds like chloroform (CHCl₃), iodoform (CHI₃), DDT, and freons have specific uses — some useful and some harmful. The use of CFCs taught us a hard lesson: chemicals we release into the environment can have consequences we never imagined. The Montreal Protocol's success in phasing out CFCs shows that international cooperation can solve global environmental problems. Today, safer alternatives like HFCs (hydrofluorocarbons) are used as refrigerants.
Classification and nomenclature
Alkyl halides (R-X): halogen attached to an sp³ carbon. Classified as primary (1°), secondary (2°), or tertiary (3°) depending on the carbon bearing the halogen. Allylic halides: X bonded to a carbon adjacent to a C=C bond (CH₂=CH-CH₂Cl, allyl chloride). Benzylic halides: X bonded to a carbon adjacent to benzene ring (C₆H₅-CH₂Br, benzyl bromide). Vinylic halides: X bonded to a C=C carbon (CH₂=CHCl, vinyl chloride). Aryl halides: X bonded directly to benzene ring (C₆H₅Br, bromobenzene). IUPAC names: fluoro, chloro, bromo, iodo as prefixes. Common names: methyl chloride, ethyl bromide, isopropyl iodide, n-butyl fluoride. Di- and trihalogen compounds: methylene chloride (CH₂Cl₂, dichloromethane), chloroform (CHCl₃, trichloromethane), carbon tetrachloride (CCl₄, tetrachloromethane).
Physical properties of alkyl halides
Alkyl halides have higher boiling points than corresponding alkanes due to stronger dipole-dipole interactions (C-X bond is polar). For isomeric alkyl halides, boiling point decreases with branching (more branching = more spherical shape = smaller surface area = weaker van der Waals forces). Boiling point increases with halogen: RI > RBr > RCl > RF (due to increasing molecular mass and polarizability). Alkyl halides are immiscible with water (they cannot form hydrogen bonds significantly) but are soluble in organic solvents like ether, alcohol, and benzene. Density increases with number and atomic mass of halogens. CHCl₃, CH₂I₂, and CCl₄ are denser than water. Alkyl fluorides and chlorides are generally less dense than water; alkyl bromides and iodides are more dense. Many alkyl halides have characteristic sweetish smells.
Nucleophilic substitution reactions — SN1 and SN2
Nucleophilic substitution: a nucleophile (Nu⁻) replaces the halogen. SN2 (bimolecular): One step. Nu⁻ attacks the carbon from the back, C-X bond breaks simultaneously, product forms with inversion of configuration (Walden inversion). Rate = k[RX][Nu⁻] — depends on both. Favored by: primary R-X (least steric hindrance), strong nucleophiles (OH⁻, CN⁻, OR⁻), polar aprotic solvents (DMSO, acetone). SN1 (unimolecular): Two steps. (1) Slow R-X → R⁺ + X⁻ (rate-determining — carbocation formation). (2) Fast R⁺ + Nu⁻ → R-Nu. Rate = k[RX] — depends only on alkyl halide. Favored by: tertiary R-X (stable carbocation), weak nucleophiles (H₂O, ROH), polar protic solvents (water, alcohol). Carbocations rearrange (hydride shift, methyl shift) to form more stable carbocations → rearranged products. Important distinction: SN2 gives inversion; SN1 gives racemization (if chiral center involved). Since carbocation is planar, Nu⁻ can attack from either side → 50:50 mixture of enantiomers → racemic mixture (optically inactive).
Elimination reactions — E1 and E2
Elimination reactions produce alkenes by removing HX from adjacent carbons. E2 (bimolecular elimination): One step — base abstracts β-hydrogen while C-X bond breaks and π-bond forms (concerted). Requires anti-periplanar arrangement (H and X must be 180° apart — anti-position). Favored by: strong bulky base (t-BuOK, KOH/ethanol), primary R-X, high temperature. Rate = k[RX][base]. Zaitsev's rule: is more substituted alkene (C=C with more alkyl groups) is the major product and more stable. E1 (unimolecular elimination): Two steps — (1) R-X → R⁺ + X⁻ (rate-determining, carbocation formation). (2) Base removes β-H → alkene. Rate = k[RX] — independent of base concentration. Favored by: tertiary R-X, weak base, polar protic solvents. Carbocations can rearrange before elimination. When both substitution and elimination are possible, lower temperature favors substitution, higher temperature favors elimination. For primary R-X with strong base → SN2 dominates (except with bulky base → E2). For tertiary R-X → elimination dominates (E1 or E2 depending on base).
Reactions with metals — Grignard reagents
Alkyl halides react with Mg in dry ether to form Grignard reagents (R-Mg-X, a carbon-metal bond). R-X + Mg → RMgX (organomagnesium halide). This is one of the most important reactions in organic synthesis because the carbon attached to Mg acts as a carbanion (R⁻ δ⁻) — a strong nucleophile and base. Grignard reagents react with: (1) H₂O (or any proton source like alcohol, acid) → alkane (RH). This is why Grignard reactions must be absolutely dry — any moisture destroys the reagent. (2) Formaldehyde → 1° alcohol. (3) Other aldehydes → 2° alcohol. (4) Ketones → 3° alcohol. (5) CO₂ → carboxylic acid (after hydrolysis). (6) Nitriles (R'CN) → ketones (after hydrolysis). The reaction with CO₂ is the classic laboratory synthesis of carboxylic acids with one more carbon: RMgX + CO₂ → RCOOMgX → (H₃O⁺) RCOOH. Vinyl and aryl halides also form Grignard reagents (vinylmagnesium bromide, phenylmagnesium bromide). The carbon-magnesium bond is highly polar (C- Mg+), making the carbon strongly nucleophilic — this is the opposite polarity (umpolung) of the usual electrophilic carbon in alkyl halides.
Aryl halides — low reactivity explained
Aryl halides (C₆H₅X) are much less reactive toward nucleophilic substitution than alkyl halides. Reasons: (1) Resonance — the lone pair on halogen participates in resonance with the aromatic ring, giving the C-X bond partial double-bond character (C₆H₅-X ↔ =C₆H₅=X⁺ resonance forms). This strengthens the bond and makes it harder to break. (2) The carbon attached to halogen is sp² hybridized (more s-character = shorter, stronger bond) vs sp³ in alkyl halides. (3) The aromatic ring is electron-rich, making it difficult for a nucleophile to attack (repulsion by π-electrons). (4) SN2 is impossible because the aromatic ring physically blocks backside attack (the 180° approach is sterically hindered by the ring). Extreme conditions are needed: high temperature (~350°C), high pressure, strong nucleophiles, often with catalysts. Bimolecular displacement mechanism (addition-elimination) occurs via a Meisenheimer complex (a negatively charged intermediate), not a simple SN2. If electron-withdrawing groups (NO₂, CN) are present at ortho and para positions, the reactivity increases dramatically — the negative charge of the intermediate Meisenheimer complex is stabilized by resonance with the NO₂ group.
Polyhalogen compounds — chloroform, iodoform, DDT, freons
Chloroform (CHCl₃): prepared by chlorination of methane or the haloform reaction (ethanol/acetone + NaOCl → CHCl₃). Used as a solvent and historically as an anaesthetic. It is toxic, suspected carcinogen, and stored in dark bottles (decomposes in light to phosgene — carbonyl chloride COCl₂, extremely toxic!). To prevent this, 1% ethanol is added to commercial chloroform to destroy phosgene: COCl₂ + 2C₂H₅OH → (C₂H₅)₂CO₃ + 2HCl. Iodoform (CHI₃): yellow solid with a characteristic antiseptic smell. The iodoform test (methyl ketones or ethanol with NaOI/I₂ → yellow precipitate of CHI₃) is used to identify CH₃CO- group or CH₃CHOH- group. DDT (dichlorodiphenyltrichloroethane): first synthetic pesticide, discovered by Paul Müller (Nobel 1948). Highly effective against malaria mosquitoes — saved millions of lives. But DDT is persistent in the environment (non-biodegradable) and accumulates in food chains (biomagnification), causing bird egg-shell thinning and other ecological damage. Banned in many countries but still used in some for malaria control. Chlorofluorocarbons (CFCs or freons): CCl₂F₂, CCl₃F, etc. — stable, non-toxic, non-flammable refrigerants and propellants. They destroy the stratospheric ozone layer by releasing Cl atoms that catalytically break down ozone. Banned under the Montreal Protocol (1987). HFCs (hydrofluorocarbons) and HCFCs are safer alternatives that do not contain chlorine.
Key Points
- •Alkyl halides: R-X (sp³ C); Aryl halides: Ar-X (sp² C, aromatic ring)
- •C-X bond strength: C-F > C-Cl > C-Br > C-I; reactivity increases as bond strength decreases
- •SN2: one step, inversion, 1° RX favored, rate = k[RX][Nu⁻], polar aprotic solvent
- •SN1: two steps, racemization, 3° RX favored, rate = k[RX], polar protic solvent, carbocation intermediate
- •E2: concerted, anti-periplanar, Zaitsev product (more substituted alkene)
- •E1: carbocation intermediate, rearrangements possible
- •Grignard: R-X + Mg → RMgX (in dry ether); reacts with H₂O, aldehydes, ketones, CO₂
- •Aryl halides: less reactive due to resonance, sp² character, steric hindrance
- •Meisenheimer complex: intermediate in nucleophilic substitution of activated aryl halides
- •Chloroform: stored dark with 1% ethanol to prevent phosgene formation
- •Iodoform test: CH₃CO- or CH₃CHOH- → yellow CHI₃ precipitate
- •DDT: persistent organic pollutant, biomagnification in food chains
- •CFCs: ozone depleters, banned by Montreal Protocol (1987)
Practice Questions
- Distinguish between SN1 and SN2 reactions with one example each.
- Explain factors affecting the reactivity of aryl halides toward nucleophilic substitution. Why is chlorobenzene less reactive than ethyl chloride?
- What are Grignard reagents? How are they prepared? Write reactions with (a) H₂O (b) CO₂ (c) acetone.
- Explain E1 and E2 elimination reactions. State Zaitsev's rule with an example.
- What is the iodoform test? Write the reaction for the formation of iodoform from ethanol.
- Why is DDT considered an environmental hazard? What steps have been taken to regulate CFCs?
- Explain the following with the reason: (a) Chloroform is stored in dark bottles. (b) Alkyl halides are immiscible with water but soluble in organic solvents.
- Write the structures of freons used as refrigerants. Why were they banned under the Montreal Protocol?