Haloalkanes and Haloarenes
A halogen on carbon — the polar bond that makes alkyl halides react readily by substitution and elimination, and the resonance that makes aryl halides stubbornly inert
- How haloalkanes and haloarenes are classified, and why the C–X bond is polar.
- Preparation from alcohols (\(\ce{PCl5},\ \ce{SOCl2}\)), hydrocarbons, and by halogen exchange.
- The SN1 and SN2 mechanisms — rate laws, stereochemistry, and what favours each.
- The reactivity order of halides and the substitution-vs-elimination contest.
- E1 and E2 elimination and Saytzeff's rule.
- Why haloarenes resist nucleophilic substitution, and the uses of polyhalogen compounds.
Classification & the C–X Bond
Replacing a hydrogen of a hydrocarbon with a halogen gives a haloalkane (halogen on an \(sp^3\) carbon) or a haloarene (halogen on an aromatic \(sp^2\) carbon). Because the halogen is more electronegative than carbon, the C–X bond is polar — the carbon bears a \(\delta^+\) charge that invites attack by nucleophiles. This single feature drives nearly all the chemistry of alkyl halides.
| Class | Halogen on | Example |
|---|---|---|
| Haloalkane (1°, 2°, 3°) | \(sp^3\) carbon | \(\ce{CH3CH2Cl}\) |
| Allylic | C next to \(\ce{C=C}\) | \(\ce{CH2=CH-CH2Cl}\) |
| Vinylic | \(sp^2\) C of \(\ce{C=C}\) | \(\ce{CH2=CHCl}\) |
| Haloarene (aryl) | aromatic \(sp^2\) C | \(\ce{C6H5Cl}\) |
Preparation
Alkyl halides are most often made from alcohols, while halogen-exchange reactions convert one halide into another.
| Route | Reaction | Note |
|---|---|---|
| From alcohol (\(\ce{HX}\)) | \(\ce{R-OH + HX -> R-X + H2O}\) | order 3° > 2° > 1° |
| From alcohol (\(\ce{SOCl2}\)) | \(\ce{R-OH + SOCl2 -> R-Cl + SO2 + HCl}\) | best — by-products are gases |
| From alkene (\(\ce{HX}\)) | \(\ce{C=C + HX -> }\) haloalkane | Markovnikov |
| Finkelstein | \(\ce{R-Cl + NaI ->[acetone] R-I + NaCl}\) | makes iodides |
| Swarts | \(\ce{R-Cl + AgF -> R-F + AgCl}\) | makes fluorides |
Nucleophilic Substitution: SN2
In an SN2 (substitution, nucleophilic, bimolecular) reaction, the nucleophile attacks the carbon from the side opposite the leaving group in a single concerted step. Bond-making and bond-breaking happen together, so the rate depends on both reactants.
Because the nucleophile attacks from behind, the molecule turns inside-out like an umbrella in a gale — Walden inversion. SN2 is favoured by unhindered \(1^\circ\) halides, strong nucleophiles and polar aprotic solvents. Steric crowding slows it: \(\ce{CH3} > 1^\circ > 2^\circ > 3^\circ\).
Nucleophilic Substitution: SN1
An SN1 (substitution, nucleophilic, unimolecular) reaction goes in two steps. The leaving group departs first, forming a planar carbocation; the nucleophile then attacks. The slow, rate-determining step is the first one, so the rate depends only on the halide.
The flat carbocation can be attacked from either face, so a single enantiomer gives a near-racemic mixture. SN1 is favoured by \(3^\circ\) halides (stable cation), weak nucleophiles and polar protic solvents. Reactivity tracks carbocation stability: \(3^\circ > 2^\circ > 1^\circ\).
SN1 vs SN2 & Reactivity
The two mechanisms pull in opposite directions on substrate structure — and on stereochemistry. Knowing which dominates lets you predict both the rate and the product's configuration.
| Feature | SN1 | SN2 |
|---|---|---|
| Molecularity | unimolecular (2 steps) | bimolecular (1 step) |
| Rate law | \(k[\ce{RX}]\) | \(k[\ce{RX}][\ce{Nu}]\) |
| Substrate | \(3^\circ > 2^\circ > 1^\circ\) | \(\ce{CH3} > 1^\circ > 2^\circ > 3^\circ\) |
| Stereochemistry | racemisation | inversion |
| Favoured by | weak Nu, protic solvent | strong Nu, aprotic solvent |
Elimination Reactions
A base can remove a \(\beta\)-hydrogen and the halogen together, forming an alkene — dehydrohalogenation. Like substitution, elimination comes in bimolecular (E2) and unimolecular (E1) flavours, and when more than one alkene is possible, Saytzeff's rule predicts the more substituted one.
The more substituted (more stable) alkene — but-2-ene here — dominates over the terminal alkene. E2 is concerted and favoured by strong bases; E1 goes via a carbocation like SN1. Strong, bulky bases and high temperature push toward elimination over substitution.
Why Haloarenes Are Inert
Aryl halides barely react with nucleophiles under ordinary conditions — a stark contrast to alkyl halides. Several reinforcing factors explain this stubbornness.
| Factor | Effect |
|---|---|
| Resonance | C–X gains partial double-bond character → shorter, stronger |
| \(sp^2\) carbon | more electronegative, holds the C–X electrons tighter |
| Unstable phenyl cation | SN1 route is blocked |
| Ring electron density | repels the incoming nucleophile |
Reactions of Haloarenes
Haloarenes do react — but only under forcing conditions, or when an electron-withdrawing group activates the ring toward nucleophilic attack.
Only high temperature and pressure displace the halogen of plain chlorobenzene. But an ortho/para nitro group makes substitution far easier (addition–elimination): \(p\)-nitrochlorobenzene reacts under much milder conditions because the \(\ce{-NO2}\) stabilises the intermediate. Haloarenes also undergo normal electrophilic substitution, with halogen as an \(o/p\)-director.
Polyhalogen Compounds
Compounds with several halogens have wide industrial and historical use — though many are now restricted for safety or environmental reasons.
| Compound | Formula | Use / note |
|---|---|---|
| Dichloromethane | \(\ce{CH2Cl2}\) | solvent, paint remover |
| Chloroform | \(\ce{CHCl3}\) | solvent; oxidises in air to toxic phosgene \(\ce{COCl2}\) |
| Iodoform | \(\ce{CHI3}\) | former antiseptic |
| Carbon tetrachloride | \(\ce{CCl4}\) | solvent, once a fire extinguisher |
| Freons | CFCs | refrigerants; deplete ozone |
| DDT | — | insecticide; persistent, largely banned |
Putting It to Work
Problem. Will \(\ce{(CH3)3C-Br}\) hydrolyse mainly by SN1 or SN2? Explain.
Solution. A \(3^\circ\) halide forms a stable cation and is too hindered for backside attack:
Problem. Arrange \(\ce{CH3Cl},\ \ce{CH3Br},\ \ce{CH3I}\) by reactivity toward nucleophilic substitution.
Solution. The weaker the C–X bond, the better the leaving group:
Problem. An optically active \(2^\circ\) halide reacts by SN2. What happens to its configuration?
Solution. Backside attack flips the configuration:
Problem. Give two reasons chlorobenzene does not undergo nucleophilic substitution easily.
Solution. Bond strengthening plus a blocked mechanism:
Problem. Write the Finkelstein conversion of bromoethane to iodoethane.
Solution. \(\ce{NaI}\) in acetone exchanges the halogen (NaBr precipitates):
Problem. Give the major alkene from dehydrohalogenation of 2-bromobutane with alc. KOH.
Solution. Saytzeff favours the more substituted alkene:
Chapter Summary
Polar bond, \(\delta^+\) carbon invites nucleophiles; sp³ (alkyl) reactive, sp² (aryl) inert.
From alcohols (\(\ce{SOCl2}\) cleanest), alkenes, and halogen exchange (Finkelstein, Swarts).
One step, \(k[\ce{RX}][\ce{Nu}]\), inversion; favours 1°, strong Nu, aprotic solvent.
Two steps via carbocation, \(k[\ce{RX}]\), racemisation; favours 3°, protic solvent.
E1/E2 dehydrohalogenation; Saytzeff gives the more substituted alkene.
Inert (resonance, sp² C, unstable phenyl cation); react only under force or with \(-\)M groups.
Problems
For each item, first decide whether it concerns preparation, a substitution mechanism, elimination, or haloarene reactivity — then apply the relevant rule. Difficulty rises down the list.
- Why is the C–X bond polar, and how does this make alkyl halides reactive?
- Write the preparation of chloroethane from ethanol using \(\ce{SOCl2}\), and say why this reagent is preferred.
- Compare the SN1 and SN2 mechanisms in molecularity, rate law and stereochemistry.
- Arrange \(\ce{CH3Cl},\ \ce{CH3Br},\ \ce{CH3I}\) by reactivity toward substitution and explain.
- Why does a 3° halide prefer SN1 while a 1° halide prefers SN2?
- What is Walden inversion, and which mechanism shows it?
- Write the Finkelstein and Swarts reactions.
- State Saytzeff's rule and give the major product from 2-bromobutane with alc. KOH.
- Give three reasons haloarenes resist nucleophilic substitution.
- Under what conditions does chlorobenzene react with \(\ce{NaOH}\), and what activates the ring toward milder substitution?
- Why is chloroform stored in dark, filled bottles with a little ethanol?
- Explain why halogens are deactivating yet ortho/para-directing on a benzene ring.