Mixed & Previous-Year-Style Long Answers

CHEExam Practice

These high-mark questions stitch the whole chapter together — reaction roadmaps, "distinguish between", reasoning, and named-reaction recall. Treat the set as a synthesis of every other subtopic rather than a topic of its own.

Substitution vs elimination switch

R\text{-}X \xrightarrow{\text{aq. } KOH} R\text{-}OH \qquad R\text{-}X \xrightarrow{\text{alc. } KOH} \text{alkene}

aqueous

KOH

gives nucleophilic substitution; alcoholic

KOH

gives

\beta

-elimination

Chain-lengthening via nitrile

R\text{-}X \xrightarrow{\text{alc. } KCN} R\text{-}CN \xrightarrow{H_3O^+} R\text{-}COOH

KCN

adds one carbon as

-CN

; acid hydrolysis gives the carboxylic acid

C–X bond cleavage order

C\text{-}I < C\text{-}Br < C\text{-}Cl < C\text{-}F \;\;(\text{bond strength})

weakest bond (

C\text{-}I

) breaks fastest, so

R\text{-}I

is most reactive in substitution

Most long answers chain reactions: know the reagent that flips each step — alcoholic KOH gives elimination, aqueous KOH gives substitution.
"Identify A, B, C" roadmaps usually pass through a Grignard, a nitrile, or a diazonium intermediate — track how the carbon count changes at each step.
Reactivity toward nucleophilic substitution: $R\text{-}I > R\text{-}Br > R\text{-}Cl > R\text{-}F$ (the weakest C–X bond breaks first).
Haloarenes are far less reactive than haloalkanes toward nucleophilic substitution — resonance, $sp^2$ carbon, and partial double-bond character all strengthen the C–X bond.
Common test reagent: $AgNO_3$ in ethanol — alkyl halides give a silver-halide precipitate; aryl and vinyl halides do not under mild conditions.
Always state conditions (temperature, solvent, catalyst) explicitly — examiners award those marks separately from the product.
Dipole-moment reasoning recurs: chlorobenzene ( $\approx 1.69\ \text{D}$ ) has a smaller dipole moment than cyclohexyl chloride ( $\approx 2.13\ \text{D}$ ) mainly because the chlorine lone pair is delocalised into the ring (resonance) and the $sp^2$ carbon makes the $C\text{-}Cl$ bond shorter and less polar, so less charge is displaced.
Ambident-nucleophile pairs are a favourite: $KCN$ gives the nitrile $R\text{-}CN$ , $AgCN$ gives the isocyanide $R\text{-}NC$ ; $KNO_2$ gives the alkyl nitrite, $AgNO_2$ gives the nitroalkane.
"Optically active $C_4H_9Br$ giving but-1-ene and but-2-ene with alc. KOH" identifies the substrate as 2-bromobutane (the only chiral $C_4H_9Br$ ); aq. KOH gives optically active butan-2-ol.
Diazonium roadmaps: aniline $\rightarrow$ benzenediazonium chloride $\rightarrow$ chlorobenzene/bromobenzene (Sandmeyer) or iodobenzene (with KI), letting you place halogens on the ring precisely.
Wurtz-type cross steps appear in roadmaps: $C_6H_5Cl + CH_3Cl + Na$ in dry ether is a Wurtz–Fittig reaction giving toluene, $C_6H_5CH_3$ .
Boiling-point reasoning: alkyl halides boil higher than the parent alkane because of greater molecular mass and stronger dipole–dipole and van der Waals forces; branching lowers the boiling point by reducing surface area of contact.

Attacking a reaction-roadmap question

1Count carbons at every stage first. A jump of one carbon means $\mathrm{KCN}$ or $\mathrm{CO_2}$ with a Grignard; a doubling means Wurtz.
2Read the reagent, not the arrow: aqueous $\mathrm{KOH}$ substitutes, alcoholic $\mathrm{KOH}$ eliminates, and that single word decides the whole roadmap.
3Track the functional group through each step and name the intermediate before moving on — most marks are for the intermediates, not the final answer.
4For an unknown identified by a test, work backwards from the test: a white precipitate with $\mathrm{AgNO_3}$ in ethanol means an easily-ionised C–X bond, so the compound is an alkyl, allylic or benzylic halide, never an aryl or vinylic one.

Where the marks go

Swapping the conditions — writing alcoholic KOH for an alcohol product or aqueous KOH for an alkene; the medium decides substitution vs elimination.
Forgetting that $AgCN$ and $KCN$ (or $AgNO_2$ and $KNO_2$ ) give different products — silver salts are covalent and attack through nitrogen.
In hydrolysis of an aryl halide, quoting mild conditions; chlorobenzene needs 623 K and 300 atm (Dow process), not ordinary aqueous NaOH.
Mislabelling the chiral $C_4H_9Br$ : only 2-bromobutane is optically active, not 1-bromobutane, isobutyl bromide, or tert-butyl bromide.
Omitting reaction conditions (dry ether, temperature, catalyst) and losing the separately awarded condition marks.

How the board asks it

Identify / classifygrignard, nitrile or diazonium intermediates and carbon-count tracking
An organic compound $A$ ( $C_2H_5Br$ ) on treatment with alcoholic $KCN$ gives $B$ , which on reduction with $Na/C_2H_5OH$ gives $C$ . Identify $A$ , $B$ and $C$ and write the equations involved.
Conversionalcoholic vs aqueous $KOH$ and chain-lengthening through a nitrile
How will you carry out the following conversions: (i) ethyl bromide to propan-1-amine; (ii) $1$ -bromopropane to propene; (iii) chlorobenzene and methyl chloride to toluene? Give the reagents and conditions in each case.
Give reasonsresonance, $sp^2$ carbon and dipole-moment reasoning for haloarenes
Account for the following: (i) chlorobenzene has a lower dipole moment ( $\approx 1.69\ \text{D}$ ) than cyclohexyl chloride ( $\approx 2.13\ \text{D}$ ); (ii) haloarenes are less reactive than haloalkanes towards nucleophilic substitution.
Distinguish $AgNO_3$ in ethanol test and ambident-nucleophile products
How will you distinguish between (i) chlorobenzene and benzyl chloride using $AgNO_3$ solution; and (ii) the products formed when ethyl bromide reacts with $KCN$ and with $AgCN$ ?
Identify / classifychirality of $C_4H_9Br$ with Saytzeff elimination vs substitution
An optically active alkyl halide $X$ ( $C_4H_9Br$ ) on reaction with alcoholic $KOH$ gives a mixture of but-1-ene and but-2-ene, while with aqueous $KOH$ it gives an optically active alcohol. Identify $X$ and explain your reasoning.
Conversionsandmeyer and wurtz-fittig recall
Name the reaction and write the equation for: (i) conversion of benzenediazonium chloride to chlorobenzene using $CuCl/HCl$ ; (ii) treatment of chlorobenzene with methyl chloride and sodium in dry ether.

PreviousPolyhalogen Compounds

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.