Chapter 6: Haloalkanes and Haloarenes Application Questions | chemistry Class 12 CBSE

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Chapter 6: Haloalkanes and Haloarenes Application Questions | chemistry Class 12 CBSE | ByteNotes.in

Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 1

Applied Concept

A chemist wants to prepare pure 1-chloropropane from propan-1-ol. With reference to the available reagents (HCl/ZnCl2, PCl5, SOCl2), which reagent should be chosen and why? What by-products are formed in each case?

SOCl2 (thionyl chloride) should be chosen because it converts propan-1-ol to 1-chloropropane along with SO2 and HCl as by-products; both are gaseous and escape from the reaction mixture, yielding pure 1-chloropropane without any separation step.
If PCl5 were used, the by-products would include POCl3 (phosphoryl chloride), which is a liquid and must be separated from the alkyl chloride by distillation—a more complex purification step.
If HCl/ZnCl2 were used, this combination requires heating and ZnCl2 catalyst for primary alcohols; the ZnCl2 catalyst and water produced remain in the reaction mixture and must be washed away, adding purification steps compared to the clean reaction with SOCl2.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 2

Applied Concept

In a laboratory experiment, a student treats an alkyl halide of formula (CH3)3CBr with aqueous NaOH and observes formation of an alcohol. The same halide with alcoholic KOH gives a gas that decolourises bromine water. With reference to SN1/SN2 and elimination mechanisms, explain both observations.

(CH3)3CBr is a tertiary alkyl halide. With aqueous NaOH (polar protic solvent), it predominantly undergoes SN1 reaction: the C–Br bond cleaves to form a stable tertiary carbocation (CH3)3C+, which is then attacked by –OH to give the tertiary alcohol (CH3)3COH (2-methylpropan-2-ol).
With alcoholic KOH, the hydroxide acts as a strong base and the polar aprotic-like conditions favour β-elimination (E2): KOH/alcohol abstracts the β-hydrogen while Br– leaves, forming an alkene (2-methylpropene, CH2=C(CH3)2). Alkenes decolourise Br2/CCl4, confirming the gas formed is an alkene.
The difference arises because aqueous NaOH provides a dilute nucleophile in a polar protic solvent stabilising the carbocation (SN1), while alcoholic KOH provides a strong base in a less polar medium, favouring elimination over substitution; tertiary halides strongly prefer elimination due to steric hindrance blocking SN2.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 3

Applied Concept

With reference to the chapter, explain why chloroquine (used in malaria treatment) and halothane (used as an anaesthetic) are representative of the medicinal importance of organohalogen compounds.

Chloroquine is a synthetic halogen-containing compound used for the treatment of malaria; it demonstrates that haloalkanes and related compounds can be designed as therapeutic agents that interfere with parasitic biology, establishing the pharmacological utility of organohalogen compounds.
Halothane is a halogenated compound used as a general anaesthetic during surgery; it has replaced chloroform (CHCl3) which was also once used as an anaesthetic but caused too many adverse effects. This shows the evolution of safer halogen-containing anaesthetics.
These examples illustrate a broader principle from the chapter: many naturally occurring and synthetic organohalogen compounds have significant biological activity, ranging from antibiotics (chloramphenicol for typhoid) and hormones (thyroxine) to drugs, making this class of compounds central to both medicine and pharmacy.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 4

Applied Concept

A student has two compounds: benzyl chloride (C6H5CH2Cl) and chlorobenzene (C6H5Cl). When both are treated with aqueous NaOH, only one gives phenylmethanol readily. Identify which one reacts and explain the structural reason for the difference in reactivity.

Benzyl chloride (C6H5CH2Cl) reacts readily with aqueous NaOH to give benzyl alcohol (C6H5CH2OH) because the chlorine is attached to an sp3-hybridised benzylic carbon; nucleophilic substitution proceeds easily (SN2 or SN1), as the benzylic carbocation is resonance-stabilised.
Chlorobenzene (C6H5Cl) does not react under normal conditions because the chlorine is directly attached to the sp2-hybridised aromatic ring carbon; resonance delocalises the chlorine lone pairs into the ring, giving the C–Cl bond partial double-bond character and making it short (169 pm), strong, and resistant to nucleophilic attack.
For chlorobenzene to hydrolyse to phenol, extreme conditions are required (623 K, 300 atm, aqueous NaOH). The contrast demonstrates the fundamental difference between aliphatic (haloalkane-like) and aromatic (haloarene) C–X bonds towards nucleophilic substitution.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 5

Applied Concept

A textile industry uses carbon tetrachloride (CCl4) as a dry-cleaning solvent. Discuss the health risks to workers, its environmental impact, and suggest what safer alternatives might be considered based on the chapter's information.

Health risks: CCl4 exposure causes dizziness, light-headedness, nausea, and vomiting; chronic exposure may cause permanent nerve cell damage. In severe cases, effects include stupor, coma, or death. It is a suspected cause of liver cancer in humans, and irregular heartbeat may result from exposure; direct eye contact causes irritation.
Environmental impact: when released into the atmosphere, CCl4 rises to the stratosphere and depletes the ozone layer by participating in radical chain reactions. Ozone depletion allows increased UV radiation to reach Earth's surface, causing increased incidence of skin cancer, eye diseases, and immune system disruption.
Safer alternatives mentioned in the chapter include dichloromethane (methylene chloride) for certain solvent applications; although it also has CNS effects, it does not significantly deplete ozone. Modern industry has largely shifted to biodegradable, non-chlorinated solvents. The chapter also mentions that many uses of CCl4 have been discontinued since the mid-1960s due to its toxicity.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 6

Applied Concept

With reference to the chapter, explain why KCN and AgCN give different products when reacted with an alkyl halide, and predict the products of CH3Br reacting with each.

KCN is predominantly ionic and provides free CN– ions in solution. Although both the carbon and nitrogen atoms of CN– can donate electron pairs, the attack occurs mainly through carbon because the C–C bond formed is more stable than the C–N bond; CH3Br + KCN → CH3CN (acetonitrile, a nitrile) as the main product.
AgCN is covalent in nature; in the AgCN molecule, nitrogen is free to donate its electron pair to the carbon of the alkyl halide. Therefore, AgCN reacts through nitrogen to give CH3Br + AgCN → CH3NC (methyl isocyanide, an isonitrile) as the main product.
This difference arises because CN– is an ambident nucleophile with two potential bonding sites (carbon and nitrogen); the nature of the counter-cation (K+ ionic vs Ag+ covalent) determines which site is available for bonding, illustrating how the reagent controls regioselectivity in nucleophilic substitution.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 7

Applied Concept

In refrigeration technology, freons (CFCs) were replaced by newer refrigerants after the 1970s. With reference to the chapter, explain the chemical properties of freons that made them initially desirable and the environmental reason they were phased out.

Freons (e.g., Freon-12, CCl2F2) were initially regarded as ideal refrigerants because they are extremely stable, chemically unreactive, non-toxic, non-corrosive, and easily liquefiable gases; these properties made them safe for household use without risk of poisoning or corrosion of equipment.
By 1974, world freon production was about 2 billion pounds annually. However, it was discovered that their extreme stability meant they did not degrade in the lower atmosphere; instead, they diffused unchanged into the stratosphere.
In the stratosphere, freon molecules absorb UV radiation and release chlorine radicals, which initiate radical chain reactions that destroy ozone (O3) molecules, upsetting the natural ozone balance. Depletion of the ozone layer allows harmful UV-B radiation to reach Earth's surface, causing skin cancer, eye diseases, and immune system disruption in humans and other organisms, leading to international agreements to phase out CFCs.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 8

Applied Concept

A student observes that 2-bromopentane on treatment with alcoholic KOH gives predominantly pent-2-ene rather than pent-1-ene. With reference to the relevant rule from the chapter, explain this observation with the percentages of products formed.

This observation is explained by Zaitsev's rule (formulated by Alexander Zaitsev in 1875), which states that in dehydrohalogenation reactions, the preferred product is the alkene with the greater number of alkyl groups attached to the doubly bonded carbon atoms (the more substituted alkene).
In 2-bromopentane, there are β-hydrogens on both C-1 and C-3. Removal of a β-hydrogen from C-3 gives pent-2-ene (CH3CH=CHCH2CH3), a disubstituted (more substituted) alkene. Removal of a β-hydrogen from C-1 gives pent-1-ene (CH2=CHCH2CH2CH3), a monosubstituted alkene.
Pent-2-ene is the major product (81%) because it is more thermodynamically stable (more substituted alkene), and pent-1-ene is the minor product (19%), consistent with Zaitsev's rule; the more stable alkene is formed preferentially.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 9

Applied Concept

In an organic synthesis lab, a student needs to convert bromobenzene to phenyl magnesium bromide (a Grignard reagent) for a further reaction. What conditions are needed and what would happen if the procedure is done without properly drying the apparatus?

To prepare phenylmagnesium bromide, bromobenzene is treated with magnesium metal in dry ether: C6H5Br + Mg (dry ether) → C6H5MgBr. The apparatus must be scrupulously dried (oven-dried glassware, dry ether solvent) to exclude all moisture.
If the apparatus is not properly dried and traces of water are present, the water would react with the Grignard reagent immediately upon its formation: C6H5MgBr + H2O → C6H6 (benzene) + Mg(OH)Br. This destroys the Grignard reagent and the synthesis fails.
Even alcohols and amines are sufficiently acidic to decompose Grignard reagents by proton transfer (RMgX + ROH → RH + Mg(OR)X). This is why the reaction must be conducted under strictly anhydrous conditions in dry ether, which also coordinates to Mg and stabilises the Grignard reagent through complexation.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 10

Applied Concept

With reference to the chapter, explain why iodination of benzene ring requires the presence of an oxidising agent while chlorination and bromination do not, and why fluorination by direct electrophilic substitution is not used.

Chlorination and bromination of arenes are straightforward electrophilic substitutions in the presence of Lewis acid catalysts (Fe or FeCl3); the reactions are effectively irreversible under normal conditions because HCl or HBr produced does not reverse the reaction significantly.
Iodination is reversible in nature; the HI produced in the reaction can react with the aryl iodide and reverse the halogenation. An oxidising agent (HNO3 or HIO4) must be present to oxidise HI to I2 as it forms, preventing the reverse reaction and driving the equilibrium forward.
Fluorination by direct electrophilic substitution is not performed because fluorine (F2) is extremely reactive and the reaction is too vigorous and exothermic to control, leading to degradation rather than selective substitution. Aryl fluorides are instead prepared via the Balz-Schiemann reaction (from diazonium fluoroborates) or similar methods.

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Chapter 6: Haloalkanes and Haloarenes | Application Questions

A chemist wants to prepare pure 1-chloropropane from propan-1-ol. With reference to the available reagents (HCl/ZnCl2, PCl5, SOCl2), which reagent should be chosen and why? What by-products are formed in each case?

In a laboratory experiment, a student treats an alkyl halide of formula (CH3)3CBr with aqueous NaOH and observes formation of an alcohol. The same halide with alcoholic KOH gives a gas that decolourises bromine water. With reference to SN1/SN2 and elimination mechanisms, explain both observations.

With reference to the chapter, explain why chloroquine (used in malaria treatment) and halothane (used as an anaesthetic) are representative of the medicinal importance of organohalogen compounds.

A student has two compounds: benzyl chloride (C6H5CH2Cl) and chlorobenzene (C6H5Cl). When both are treated with aqueous NaOH, only one gives phenylmethanol readily. Identify which one reacts and explain the structural reason for the difference in reactivity.

A textile industry uses carbon tetrachloride (CCl4) as a dry-cleaning solvent. Discuss the health risks to workers, its environmental impact, and suggest what safer alternatives might be considered based on the chapter's information.

With reference to the chapter, explain why KCN and AgCN give different products when reacted with an alkyl halide, and predict the products of CH3Br reacting with each.

In refrigeration technology, freons (CFCs) were replaced by newer refrigerants after the 1970s. With reference to the chapter, explain the chemical properties of freons that made them initially desirable and the environmental reason they were phased out.

A student observes that 2-bromopentane on treatment with alcoholic KOH gives predominantly pent-2-ene rather than pent-1-ene. With reference to the relevant rule from the chapter, explain this observation with the percentages of products formed.

In an organic synthesis lab, a student needs to convert bromobenzene to phenyl magnesium bromide (a Grignard reagent) for a further reaction. What conditions are needed and what would happen if the procedure is done without properly drying the apparatus?

With reference to the chapter, explain why iodination of benzene ring requires the presence of an oxidising agent while chlorination and bromination do not, and why fluorination by direct electrophilic substitution is not used.

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