Preparation & Conversions

CHEClassification & Preparation

Haloalkanes are made from alcohols, from alkanes (free-radical halogenation), and from alkenes (addition of HX or halogen); haloarenes come from arenes or via diazonium salts. "Convert X to Y" questions test whether you can pick the right reagent for each transformation.

Alcohol to halide with

SOCl_2

R\text{-}OH + SOCl_2 \rightarrow R\text{-}Cl + SO_2\!\uparrow + HCl\!\uparrow

Darzens method; gaseous by-products leave a pure halide

Finkelstein reaction

R\text{-}Cl + NaI \xrightarrow{\text{dry acetone}} R\text{-}I + NaCl\!\downarrow

NaCl

is insoluble in acetone and precipitates, driving the equilibrium right

Swarts reaction

R\text{-}Cl \;(\text{or } R\text{-}Br) + AgF \rightarrow R\text{-}F + AgCl

Hg_2F_2

CoF_2

SbF_3

may replace

AgF

to make fluoroalkanes

Markovnikov vs peroxide addition

CH_3CH{=}CH_2 + HBr \rightarrow CH_3CHBrCH_3 \quad ; \quad \xrightarrow{\text{peroxide}} CH_3CH_2CH_2Br

Markovnikov gives

2^\circ

halide; peroxide (anti-Markovnikov) gives

1^\circ

halide, only with

HBr

From alcohols: $R\text{-}OH$ with $HX$ , $PCl_5$ , $PCl_3$ or $SOCl_2$ . $SOCl_2$ is best — by-products $SO_2$ and $HCl$ escape as gases, leaving a pure halide.
Reactivity of HX with a given alcohol: $HI > HBr > HCl$ ; reactivity of alcohols with a given HX: $3^\circ > 2^\circ > 1^\circ$ .
Addition of HX to alkenes follows Markovnikov's rule; anti-Markovnikov (peroxide / Kharasch effect) occurs only with HBr.
Finkelstein reaction: $R\text{-}Cl + NaI \xrightarrow{\text{dry acetone}} R\text{-}I + NaCl\!\downarrow$ — NaCl precipitates and drives the equilibrium forward.
Swarts reaction: $R\text{-}Cl/Br + AgF \;(\text{or } Hg_2F_2) \rightarrow R\text{-}F$ , used to make fluoroalkanes.
Free-radical halogenation of alkanes gives mixtures; chlorination is less selective than bromination.
Aryl halides are made via diazonium salts (Sandmeyer / Gattermann), since direct halogenation of benzene needs a Lewis-acid carrier such as $FeCl_3$ .
From alkenes, adding $X_2$ ( $Cl_2$ or $Br_2$ ) gives a vicinal dihalide; decolourisation of bromine water is the classic alkene test.
Concentrated $HCl$ with $1^\circ$ / $2^\circ$ alcohols needs anhydrous $ZnCl_2$ (Lucas reagent) as catalyst; $3^\circ$ alcohols react with conc. HCl alone.
Sandmeyer: $C_6H_5N_2^+Cl^- \xrightarrow{CuCl/HCl} C_6H_5Cl$ (and $CuBr/HBr$ for bromobenzene); Gattermann uses $Cu$ powder with $HX$ .
Iodobenzene is made directly from the diazonium salt with $KI$ (no copper catalyst needed); fluorobenzene via the Balz–Schiemann route ( $HBF_4$ , then heat).
Free-radical halogenation: bromination is more selective for the more substituted carbon ( $3^\circ > 2^\circ > 1^\circ$ H abstraction) because the bromine radical is less reactive and more selective.

\mathrm{C_2H_5OH} + \mathrm{SOCl_2} \xrightarrow{\text{pyridine}} \mathrm{C_2H_5Cl} + \mathrm{SO_2} + \mathrm{HCl}

ConditionsThionyl chloride with pyridine, warmed

The examiner's favourite 'why is this the best reagent' answer: both by-products,

\mathrm{SO_2}

and

\mathrm{HCl}

, are gases that escape, so the alkyl halide is left pure and no separation is needed.

\mathrm{C_6H_5N_2^{+}Cl^{-}} \xrightarrow{\mathrm{CuCl}/\mathrm{HCl}} \mathrm{C_6H_5Cl} + \mathrm{N_2}

ConditionsSandmeyer reaction — diazonium salt with cuprous halide

Aryl halides cannot be made by direct substitution on phenol, so the diazonium route is the only way in. With

\mathrm{KI}

no copper salt is needed.

2\,\mathrm{CHI_3} + 6\,\mathrm{Ag} \xrightarrow{\Delta} \mathrm{C_2H_2} + 6\,\mathrm{AgI}

ConditionsIodoform heated with silver powder

Asked as a conversion in both 2024 and 2026 — worth knowing as a one-liner.

\mathrm{C_6H_5Cl} + 2\,\mathrm{NH_3} \xrightarrow[\text{high pressure}]{\mathrm{Cu_2O},\ 475\ \mathrm{K}} \mathrm{C_6H_5NH_2} + \mathrm{NH_4Cl}

ConditionsAqueous ammonia,

\mathrm{Cu_2O}

, high temperature and pressure

The harsh conditions are the point: a haloarene will not give up its halogen under ordinary nucleophilic conditions.

Where the marks go

Applying the peroxide (anti-Markovnikov) effect to $HCl$ or $HI$ — it works only with $HBr$ .
Forgetting "dry acetone" in Finkelstein or "dry ether" elsewhere; moisture ruins these reactions and forfeits marks.
Trying to make aryl halides by direct nucleophilic substitution on benzene; you need a Lewis-acid carrier ( $FeCl_3$ ) for electrophilic halogenation, or a diazonium route.
Mixing up the two reactivity orders for HX/alcohol: HI is most reactive among acids, while $3^\circ$ is most reactive among alcohols.
Quoting $AgF$ but writing the product as an iodide or chloride; Swarts specifically installs fluorine.

How the board asks it

Conversionreagents for each preparation route2 mkAsked 2024 · 2026
How will you convert $(i)$ propan- $2$ -ol into $2$ -iodopropane, $(ii)$ ethanol into chloroethane using $SOCl_2$ , and $(iii)$ but- $1$ -ene into $1$ -bromobutane?
Give reasons $SOCl_2$ as best reagent; reactivity order of $HX$
Account for the following: $(i)$ $SOCl_2$ is preferred over $PCl_5$ for preparing pure alkyl chlorides from alcohols, and $(ii)$ among $HCl$ , $HBr$ and $HI$ , $HI$ reacts fastest with a given alcohol.
Predict the productnamed reactions: finkelstein, swarts, sandmeyer
What happens when (give balanced equations): $(i)$ chloroethane is heated with $NaI$ in dry acetone, $(ii)$ chloromethane is treated with $AgF$ , and $(iii)$ benzene diazonium chloride is warmed with $CuCl/HCl$ ?
Conversiondiazonium route to aryl halides2 mkAsked 2026
Starting from aniline, give the reagents and conditions required to prepare $(i)$ chlorobenzene, $(ii)$ iodobenzene and $(iii)$ fluorobenzene.
Give reasonsmarkovnikov vs peroxide (anti-markovnikov) addition
Name the major product formed when propene reacts with $HBr$ $(i)$ in the absence and $(ii)$ in the presence of organic peroxide, and give reasons for the difference.
Conversionpicking reagents from a fixed list2 mk
Given only alcoholic and aqueous $\mathrm{KOH}$ , $\mathrm{Cl_2}$ , $\mathrm{HBr}$ , acidified $\mathrm{K_2Cr_2O_7}$ , $\mathrm{NaOH}$ and soda lime, carry out a stated pair of conversions.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.