Formula sheet: Haloalkanes and Haloarenes
Haloalkanes and Haloarenes14 min readLesson 1 of 8
1.1 Bond data you must remember
The carbon halogen bond is polar because the halogen is more electronegative than carbon. The carbon end carries a partial positive charge, which is the site a nucleophile attacks. Bond length grows from the fluoride to the iodide, bond strength falls in the same direction, and the dipole moment stays high but does not follow the bond-strength order.
| Bond | Bond length (pm) | Bond energy (kJ mol-1) | Dipole moment (D) |
|---|---|---|---|
| CH3F | 139 | 452 | 1.847 |
| CH3Cl | 178 | 351 | 1.860 |
| CH3Br | 193 | 293 | 1.830 |
| CH3I | 214 | 234 | 1.636 |
Two trends follow from this table. Longer and weaker bonds break more easily, so iodide is the best leaving group among the halides. Stronger bonds hold the halogen more tightly, so the fluoride is the poorest leaving group and the least reactive in substitution.
1.2 Rate laws
The two substitution mechanisms differ in what the rate depends on. That difference is the quickest way to tell them apart in an exam answer.
1.3 Reactivity orders
Leaving ability with one alkyl group fixed increases from fluoride to iodide: R-F < R-Cl < R-Br < R-I. The same rank holds in both mechanisms. For a given halide, steric crowding decides the SN2 order and carbocation stability decides the SN1 order.
| Question | Order |
|---|---|
| Speed of SN2 with one halide fixed | methyl fastest, then primary, secondary, tertiary slowest |
| Speed of SN1 with one halide fixed | tertiary fastest, then secondary, primary, methyl slowest |
| Boiling point with one alkyl group fixed | iodide highest, bromide next, chloride lower, fluoride lowest |
| Boiling point of isomers | unbranched highest, then lightly branched, heavily branched lowest |
1.4 Named reactions and conditions
| Reaction | Reagents and conditions | Product |
|---|---|---|
| From alcohol with HX | conc. HCl with ZnCl2 for 1 degree and 2 degree alcohols, conc. HCl alone for 3 degree; NaBr with H2SO4 for bromides | Alkyl halide |
| From alcohol with SOCl2 | Thionyl chloride, warm | Alkyl chloride, with SO2 and HCl escaping |
| From alcohol with PX3 | PBr3 or PI3 made in situ from red phosphorus and Br2 or I2 | Alkyl bromide or iodide |
| Finkelstein | Alkyl chloride or bromide with NaI in dry acetone | Alkyl iodide |
| Swarts | Alkyl chloride or bromide with AgF, Hg2F2, CoF2 or SbF3, heat | Alkyl fluoride |
| Sandmeyer | Diazonium salt with CuCl or CuBr | Aryl chloride or bromide |
| Wurtz | Two molecules of alkyl halide with Na in dry ether | Symmetrical alkane |
| Wurtz-Fittig | Alkyl halide plus aryl halide with Na in dry ether | Alkylbenzene |
| Fittig | Aryl halide with Na in dry ether | Biphenyl |
| Grignard formation | Alkyl or aryl halide with Mg in dry ether | RMgX |
1.5 Product rules
The addition of HBr to propene gives 2-bromopropane as the major product by Markovnikov addition. Dehydrohalogenation with alcoholic KOH follows the Zaitsev rule, so the alkene with more alkyl groups on the double bond wins. Chlorobenzene needs 623 K and about 300 atmospheres with aqueous NaOH before the chlorine is replaced by OH, and a nitro group at the ortho or para position makes that substitution much easier.
Exam
Write the mechanism as words, not as arrows. Credit comes from naming the slow step, the intermediate and the stereochemical result. SN2: one step, backside attack, inversion. SN1: two steps, planar carbocation, racemisation.