Substitution Reactions & Reagents

CHEReactions of Haloalkanes

The polarised C–X bond makes haloalkanes undergo a whole family of nucleophilic substitutions — each nucleophile converts $R\text{-}X$ into a different functional class. Memorising the reagent → product map is the fastest way to score on this subtopic.

Ambident nucleophile

CN^-

R\text{-}X \xrightarrow{KCN} R\text{-}CN \qquad R\text{-}X \xrightarrow{AgCN} R\text{-}NC

KCN

is ionic and attacks through C (nitrile);

AgCN

is covalent and attacks through N (isocyanide)

Ambident nucleophile

NO_2^-

R\text{-}X \xrightarrow{KNO_2} R\text{-}O\text{-}N{=}O \qquad R\text{-}X \xrightarrow{AgNO_2} R\text{-}NO_2

KNO_2

gives alkyl nitrite (O-attack);

AgNO_2

gives nitroalkane (N-attack)

Williamson ether synthesis

R\text{-}X + Na^+\,O R'^- \rightarrow R\text{-}O\text{-}R' + NaX

use a

1^\circ

halide with the alkoxide to avoid elimination

$R\text{-}X$ + aqueous $KOH$ → alcohol; + $NaOR'$ → ether (Williamson synthesis).
+ $KCN$ → nitrile $R\text{-}CN$ (one carbon added); + $AgCN$ → isocyanide $R\text{-}NC$ .
+ $KNO_2$ → alkyl nitrite $R\text{-}O\text{-}N{=}O$ ; + $AgNO_2$ → nitroalkane $R\text{-}NO_2$ .
+ $NH_3$ → amines (a mixture of $1^\circ, 2^\circ, 3^\circ$ plus the quaternary salt); + $R'COOAg$ → ester.
Ambident nucleophiles ( $CN^-$ , $NO_2^-$ ): the K-salt is ionic and attacks through carbon (or O in $NO_2^-$ ); the Ag-salt is covalent, leaving the other atom (N) free to bond.
+ $LiAlH_4$ → alkane (reduction); + $Na$ in dry ether → alkane (Wurtz reaction).
Ammonolysis ( $R\text{-}X + NH_3$ ) is stepwise; an excess of ammonia favours the primary amine, while excess halide drives it on to secondary, tertiary amine and finally the quaternary ammonium salt.
Nitrile hydrolysis ( $R\text{-}CN + H_3O^+$ ) gives the carboxylic acid $R\text{-}COOH$ , and reduction ( $H_2/\text{Ni}$ or $LiAlH_4$ ) gives the amine $R\text{-}CH_2NH_2$ — so $KCN$ is a two-step gateway to acids and amines with one extra carbon.
With $KCN$ the cyanide ion is free (ionic salt) and attacks through carbon because the C–C bond is stronger, giving the nitrile; with $AgCN$ the Ag–C bond is covalent, so only nitrogen's lone pair is free and the isocyanide forms.
$R\text{-}X$ + $AgNO_3$ in ethanol gives a silver-halide precipitate ( $AgCl$ white, $AgBr$ pale yellow, $AgI$ yellow) — used to confirm a reactive (alkyl) halide.
Williamson synthesis of an unsymmetrical ether works best from a $1^\circ$ halide and the alkoxide; a $3^\circ$ halide gives mainly alkene because the strongly basic alkoxide promotes elimination.
The reaction with carboxylate silver salts ( $R'COOAg$ ) gives esters $R'COO\text{-}R$ , a useful esterification route from halides.

\mathrm{R\text{–}Cl} + \mathrm{NaI} \xrightarrow{\text{dry acetone}} \mathrm{R\text{–}I} + \mathrm{NaCl}

ConditionsFinkelstein reaction — dry acetone is essential

It works only because

\mathrm{NaCl}

is insoluble in acetone and precipitates out, dragging the equilibrium to the right.

\mathrm{C_6H_5ONa} + \mathrm{C_2H_5Cl} \longrightarrow \mathrm{C_6H_5\text{–}O\text{–}C_2H_5} + \mathrm{NaCl}

ConditionsWilliamson synthesis — sodium alkoxide or phenoxide with a

1^{\circ}

halide

Use a primary halide. With a

3^{\circ}

halide the alkoxide acts as a base instead and you get elimination, not the ether.

Reagent	Attacking atom	Product	Name
$\mathrm{KCN}$ (ionic)	Carbon	$\mathrm{R\text{–}CN}$	Alkyl cyanide (nitrile)
$\mathrm{AgCN}$ (covalent)	Nitrogen	$\mathrm{R\text{–}NC}$	Alkyl isocyanide
$\mathrm{KNO_2}$ (ionic)	Oxygen	$\mathrm{R\text{–}ONO}$	Alkyl nitrite
$\mathrm{AgNO_2}$ (covalent)	Nitrogen	$\mathrm{R\text{–}NO_2}$	Nitroalkane

Ambident nucleophiles: the same anion gives two different products depending on which atom attacks, and the metal decides. The rule is ionic salt → attack through the more electronegative atom's partner; covalent silver salt → attack through the less electronegative atom.

Where the marks go

Treating $CN^-$ and $NO_2^-$ as ordinary nucleophiles and ignoring the K-salt vs Ag-salt difference in the product.
Writing the isocyanide as $R\text{-}CN$ — $AgCN$ gives $R\text{-}NC$ (carbon bonded to nitrogen of the group).
Attempting Williamson synthesis with a tertiary halide and expecting the ether; it gives the alkene by elimination.
Forgetting that ammonolysis gives a mixture; you must state excess $NH_3$ to favour the primary amine.
Confusing alkyl nitrite ( $R\text{-}O\text{-}N{=}O$ , an ester of nitrous acid) with nitroalkane ( $R\text{-}NO_2$ , C–N bonded).

How the board asks it

Conversionthe reagent → product map3 mkAsked 2025 · 2026
How will you convert ethyl bromide ( $C_2H_5Br$ ) into (i) ethyl alcohol, (ii) propanenitrile, and (iii) diethyl ether? Give the reagents and conditions in each case.
Predict the productK-salt vs Ag-salt of ambident nucleophiles1 mkAsked 2026
Name the reagent and write the product formed when bromoethane reacts with (i) $KCN$ , (ii) $AgCN$ , (iii) $KNO_2$ , and (iv) $AgNO_2$ .
Distinguish $AgNO_3$ silver-halide test
How will you distinguish between chloroethane and iodoethane by a chemical test using $AgNO_3$ ? State the observation in each case.
Give reasonsambident nucleophiles ( $CN^-$ ); ionic vs covalent metal salt1 mkAsked 2026
Account for the fact that $R\text{-}X$ with $KCN$ gives mainly the nitrile $R\text{-}CN$ , whereas with $AgCN$ it gives mainly the isocyanide $R\text{-}NC$ .
Conversion $KCN$ gateway via nitrile hydrolysis (one extra carbon)
How will you obtain propanoic acid from bromoethane? Give the two steps with the reagents used in each.
Give reasonsWilliamson synthesis: substitution on $1^\circ$ vs elimination on $3^\circ$ halide3 mkAsked 2026
Give reasons why Williamson synthesis is best carried out with a primary halide, while a tertiary halide such as $(CH_3)_3CBr$ gives mainly an alkene instead of the ether.

Practise this topic

PreviousPreparation & Conversions NextMechanisms (S_N1, S_N2, stereochemistry)

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.