Haloalkanes & Haloarenes

Syllabus Unit: Organic Chemistry

Why it matters

Chapter overview

Haloalkanes (R–X, sp3 carbon) and haloarenes (Ar–X, sp2 carbon) look similar on paper but behave very differently: the aromatic ring's resonance donates electron density into the C–X bond of a haloarene, giving it partial double-bond character that makes it far less reactive to nucleophilic substitution than a haloalkane. NEET builds heavily on the SN1/SN2 mechanism contrast (rate law, stereochemistry, substrate preference), preparation methods, and the CFC story that links this topic directly to stratospheric ozone depletion.

Chemistry Syllabus Organic Chemistry ~50% weightage

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SOCl2: alcohol → haloalkane, gaseous byproducts

Finkelstein: Cl/Br → I exchange in dry acetone

Swarts: Cl/Br → F exchange with AgF/Hg2F2

Sandmeyer: diazonium salt → haloarene

1Nomenclature & Preparation

Haloalkanes and haloarenes are named simply as halo-substituted chains/rings, but the best way to make each depends heavily on which functional group you are starting from.

Learning Objectives

Name haloalkanes and haloarenes by IUPAC rules, using halo- as a substituent prefix numbered for the lowest locants.
Describe the SOCl2 (thionyl chloride) method for converting an alcohol to a haloalkane and explain why it is preferred over HX or PCl5.
Describe the Finkelstein and Swarts reactions for halide exchange in haloalkanes.
Describe the preparation of haloarenes by direct electrophilic halogenation of benzene (Fe catalyst) and via the Sandmeyer reaction from a diazonium salt.

Key Terms

SOCl2 MethodConversion of an alcohol to a chloroalkane using thionyl chloride: R-OH + SOCl2 → R-Cl + SO2 + HCl; preferred because both byproducts are gases, giving a pure product with no separation step.

Finkelstein ReactionConversion of an alkyl chloride/bromide to the corresponding iodide using NaI in dry acetone; driven forward because NaCl/NaBr precipitate out of the acetone while NaI remains soluble.

Swarts ReactionConversion of an alkyl chloride/bromide to the corresponding fluoride using AgF, Hg2F2, CoF2 or SbF3 as the fluorinating agent.

Sandmeyer ReactionConversion of an aromatic diazonium salt to an aryl halide using CuCl, CuBr or KI, providing a route to haloarenes not accessible by direct halogenation alone.

IUPAC names haloalkanes and haloarenes with 'halo-' (fluoro, chloro, bromo, iodo) as a substituent prefix on the parent chain or ring, numbered to give the lowest locant consistent with other substituents present.

Preparing haloalkanes from alcohols

SOCl2 (thionyl chloride): R-OH + SOCl2 → R-Cl + SO2↑ + HCl↑ — the preferred lab method, since both byproducts are gases and escape, leaving a pure haloalkane without further purification.
PCl5 or PCl3: also convert -OH to -Cl, but leave POCl3/H3PO3 byproducts that require separation.
HX (with ZnCl2/Lucas reagent for reactivity testing): protonates -OH first, then X− displaces water — reactivity order 3° > 2° > 1° alcohol.

Halide exchange reactions

Finkelstein reaction: R-Cl/R-Br + NaI →(dry acetone) R-I + NaCl/NaBr↓ — driven forward as NaCl/NaBr precipitate out of acetone while NaI stays dissolved.
Swarts reaction: R-Cl/R-Br + AgF/Hg2F2/CoF2 → R-F — a route to alkyl fluorides, which are hard to make directly from HF.

Common MistakeHaloarenes cannot be made by simply replacing -OH on phenol with HX/PCl5/SOCl2 (these methods work only for alcohols) — instead they need direct electrophilic halogenation of the ring (X2/Fe, o/p-directed) or the Sandmeyer reaction from a diazonium salt.