Playbook
Haloalkanes and Haloarenes
The two substitution mechanisms and when each wins, the nucleophile and its product, elimination against substitution, and the haloarenes. Mechanism facts decide most answers.
- Questions in the bank
- 92
- q/paper in 2025–26
- 0.82
- Calculation
- 7%
- Notes pages
- 7
Strand: Reactions
When you’ll see it
A halide meets a nucleophile, a base or a metal, or a list of halides is ranked by SN1, SN2 or a physical property.
How this chapter is tested
The chapter is a reasoning chapter. A halide and a reagent are given and the work is choosing the pathway before writing the product: can the carbon form a stable cation, is it open to attack from behind, is the reagent a nucleophile or a base?
Ranking questions are common, and each ranks by one property only. SN1 follows carbocation stability, SN2 follows crowding at the carbon, nucleophilicity follows the solvent. Name the property first; most wrong orders come from mixing two of them.
The recall is small but exact: a few named reactions (Finkelstein, Swarts, Sandmeyer, Gattermann, Wurtz), a few physical trends, and the uses of polyhalogen compounds. Numeric questions are rare and are counts of products or of halides that pass a test.
The sub-skills
The distinct skills inside the chapter, in the order to learn them.
Classes and physical properties
Allylic and benzylic X sit on sp³ carbon, vinylic and aryl X on sp²; aryl C–X is shorter and less polar; b.p. rises R–F < R–Cl < R–Br < R–I and falls with branching; p-dichlorobenzene melts highest.
Preparation and named reactions
SOCl₂, PCl₅ or HX/ZnCl₂ on alcohols; Finkelstein NaI in dry acetone → RI; Swarts AgF or SbF₃ → RF; Sandmeyer CuCl, CuBr, CuCN on ArN₂⁺; phenol never gives ArX with HX.
SN1 and SN2 mechanisms
SN2: rate k[RX][Nu⁻], one step, inversion; SN1: rate k[RX], planar cation, racemisation and possible shifts; tosylation keeps the configuration.
Reactivity order in substitution
SN1 by cation stability (3°, benzylic, allylic, CH₃OCH₂Cl fast); SN2 CH₃ > 1° > 2° > 3°, neopentyl slow; vinylic, aryl and small-cage bridgehead halides do not ionise.
Nucleophiles and ambident reagents
In protic solvents I⁻ > Br⁻ > Cl⁻ > F⁻, in DMSO the order reverses; KCN → R–CN, AgCN → R–NC; KNO₂ → R–O–N=O, AgNO₂ → R–NO₂.
Elimination against substitution
Aqueous KOH substitutes, alcoholic KOH with heat eliminates; a bulky base or a 3° halide gives the alkene; Zaitsev product unless another C=C is conjugated; no β-H, no E2.
Haloarenes and metals
NO₂ ortho or para to Cl makes nucleophilic substitution easy; halogens direct electrophiles ortho-para; RX + Mg/dry ether → RMgX; Wurtz, Wurtz–Fittig and Fittig with Na.
Traps to expect
Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.
The nitrite and cyanide pairs swapped
KNO₂ gives the alkyl nitrite R–O–N=O and AgNO₂ the nitroalkane R–NO₂; KCN gives the nitrile and AgCN the isocyanide. Both silver salts bond through nitrogen.
Primary means fast SN2
Neopentyl halides are primary but react very slowly, because the tert-butyl group blocks the back of the carbon. Benzylic halides are fast by both SN1 and SN2.
A tertiary bridgehead ionises
A halogen at the bridgehead of a small cage is tertiary yet barely reacts by SN1: the cage keeps the carbon from flattening. 3-Bromocyclopropene and 7-bromocycloheptatriene ionise easily because their cations are aromatic.
Inversion read as R to S
SN2 inverts the 3-D arrangement, but the R/S letter changes only when the incoming group takes the leaving group's priority rank. Reassign priorities on the product.
Gattermann gives a cyanide
Copper powder with HCl or HBr gives ArCl or ArBr. The aryl cyanide comes from the Sandmeyer reaction with CuCN.
Learn it before you drill it
This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.
Haloalkanes and Haloarenes notesDrill every Haloalkanes and Haloarenes question
92 questions from the bank, across 7 subtopics.
Drill one subtopic at a time
The 7 subtopics, in teaching order.
- Classification, Structure and Physical PropertiesDrill Classification, Structure and Physical Properties
- Preparation of Haloalkanes and HaloarenesDrill Preparation of Haloalkanes and Haloarenes
- SN1 and SN2: Mechanism, Kinetics and StereochemistryDrill SN1 and SN2: Mechanism, Kinetics and Stereochemistry
- Reactivity Order in Nucleophilic SubstitutionDrill Reactivity Order in Nucleophilic Substitution
- Nucleophiles and Ambident ReagentsDrill Nucleophiles and Ambident Reagents
- Elimination Versus SubstitutionDrill Elimination Versus Substitution
- Haloarenes and Reactions with MetalsDrill Haloarenes and Reactions with Metals
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