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

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

Long Answer

Hard difficulty • Structured explanation

Hard

Question 1

Long Form

Compare and contrast the SN1 and SN2 mechanisms of nucleophilic substitution in haloalkanes with respect to kinetics, stereochemistry, effect of substrate structure, and solvent.

SN2 follows second-order kinetics (rate depends on both substrate and nucleophile concentrations) and proceeds in a single concerted step with no intermediate; the nucleophile attacks the carbon from the back, causing Walden inversion of configuration at the chiral centre.
SN1 follows first-order kinetics (rate depends only on substrate concentration) and proceeds in two steps: slow ionisation to form a planar carbocation, followed by fast nucleophilic attack from either face, giving racemisation.
Substrate structure: SN2 reactivity is CH3X > 1° > 2° > 3° (steric hindrance decreases reactivity); SN1 reactivity is 3° > 2° > 1° (carbocation stability governs rate). Allylic and benzylic halides are reactive in SN1 due to resonance-stabilised carbocations.
Polar protic solvents (water, alcohol) favour SN1 by stabilising the ion pair intermediate and the halide leaving group through solvation; polar aprotic solvents or less polar conditions favour SN2 by not solvating the nucleophile and allowing it to attack freely.
For a given alkyl group, reactivity of halide in both mechanisms follows R–I > R–Br > R–Cl >> R–F, as iodine is the best leaving group due to its large size and weak C–I bond.

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Medium

Question 2

Long Form

Discuss the methods of preparation of haloalkanes from alcohols, highlighting the reagents, conditions, and the reason for preferring thionyl chloride.

Alcohols react with HCl in the presence of anhydrous ZnCl2 (Lucas reagent) as catalyst to give alkyl chlorides; ZnCl2 is needed for primary and secondary alcohols but tertiary alcohols react readily with concentrated HCl at room temperature by shaking.
Alcohols react with phosphorus halides: with PX3 (X = Cl, Br) to give the alkyl halide and H3PO3 (3R–OH + PX3 → 3R–X + H3PO3); with PCl5 to give R–Cl + POCl3 + HCl.
Thionyl chloride (SOCl2) is the preferred reagent: R–OH + SOCl2 → R–Cl + SO2 + HCl; both by-products are gases and escape from the mixture, yielding pure alkyl chloride without any purification.
Constant-boiling HBr (48%) is used for preparing alkyl bromides; alkyl iodides are prepared by heating alcohols with NaI or KI in 95% orthophosphoric acid.
The reactivity of alcohols with haloacids follows 3° > 2° > 1°, since tertiary carbocations (formed in the mechanism) are most stable; these methods are not applicable to phenols because the C–O bond in phenols has partial double-bond character and is too strong to break.

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Long Answer

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Hard

Question 3

Long Form

Explain the four reasons why aryl halides are less reactive than alkyl halides towards nucleophilic substitution. Explain how electron-withdrawing groups at ortho/para positions increase the reactivity of haloarenes.

Resonance effect: In haloarenes, the lone pairs of the halogen are in conjugation with the ring π-electrons, giving the C–X bond partial double-bond character and making it shorter and stronger than in haloalkanes, so cleavage is more difficult.
Hybridisation difference: In haloarenes, the carbon bearing halogen is sp2-hybridised (greater s-character, more electronegative), holding the C–X bonding electrons more tightly than the sp3 carbon in haloalkanes; C–Cl bond is 169 pm in haloarene vs 177 pm in haloalkane.
Phenyl cation instability: For SN1 to operate, a phenyl cation must form; unlike a tertiary carbocation, a phenyl cation cannot be stabilised by resonance with the ring, so SN1 is ruled out.
Electron repulsion: The aromatic ring is electron-rich; an electron-rich nucleophile is repelled by the electron-dense arene, disfavouring nucleophilic attack.
Electron-withdrawing groups (e.g., –NO2) at ortho and para positions withdraw electron density from the ring, facilitating nucleophilic attack. The carbanion intermediate formed is resonance-stabilised; the negative charge is delocalised onto the oxygen atoms of the –NO2 groups at ortho/para positions, reducing activation energy and increasing reactivity significantly.

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Long Answer

Medium difficulty • Structured explanation

Medium

Question 4

Long Form

Describe the physical properties of haloalkanes: state of aggregation, boiling points (with comparison to hydrocarbons, effect of halogen type, and branching), solubility in water and organic solvents, and density trends.

State: Methyl chloride, methyl bromide, ethyl chloride, and some chlorofluoromethanes are gases at room temperature; higher members are liquids or solids. Alkyl halides are colourless when pure, but bromides and iodides develop colour on exposure to light; many have sweet smell.
Boiling point vs hydrocarbons: Haloalkanes are polar; stronger dipole-dipole and van der Waals forces than in parent hydrocarbons result in considerably higher boiling points despite similar or lower molecular mass.
Effect of halogen type: For the same alkyl group, boiling point order is RI > RBr > RCl > RF, because larger halogen atoms increase molecular size and polarisability, enhancing van der Waals forces.
Effect of branching: Boiling points decrease with increasing branching; for example, 1-bromobutane (375 K) > 2-bromobutane (364 K) > 2-bromo-2-methylpropane (346 K) due to decrease in surface area reducing van der Waals forces.
Solubility and density: Haloalkanes are only slightly soluble in water (new haloalkane–water attractions are weaker than existing H-bonds broken) but dissolve in organic solvents; bromo, iodo, and polychloro compounds are denser than water, and density increases with number of halogen atoms and atomic mass of halogens.

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Long Answer

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Medium

Question 5

Long Form

Discuss the classification of haloalkanes and haloarenes based on the hybridisation of the carbon bearing the halogen, giving examples for each sub-type and explaining the structural distinction.

Compounds with sp3 C–X bond include alkyl halides (RX: halogen on alkyl group, e.g., CH3CH2Br—ethyl bromide), classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of other carbon groups on the C–X carbon.
Allylic halides have halogen on sp3 carbon adjacent to C=C (e.g., CH2=CHCH2Br, allyl bromide / 3-bromopropene). Benzylic halides have halogen on sp3 carbon attached to an aromatic ring (e.g., C6H5CH2Cl, benzyl chloride).
Compounds with sp2 C–X bond include vinylic halides (halogen directly on C=C double bond carbon, e.g., CH2=CHCl, vinyl chloride / chloroethene); these are unreactive towards nucleophilic substitution.
Aryl halides have halogen directly bonded to the sp2 carbon of an aromatic ring (e.g., C6H5Cl, chlorobenzene); these are also very poorly reactive towards nucleophilic substitution due to resonance and hybridisation effects.
The distinction between allylic/benzylic halides and vinylic/aryl halides is significant: allylic and benzylic halides show enhanced SN1 reactivity due to resonance-stabilised cations, while vinylic and aryl halides are highly resistant to both SN1 and SN2.

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Long Answer

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Hard

Question 6

Long Form

Analyse the role of stereochemistry in understanding SN1 and SN2 reaction mechanisms, discussing inversion, retention, and racemisation with suitable examples.

Inversion of configuration (Walden inversion) occurs exclusively in SN2 reactions: the nucleophile attacks from the back face of the carbon bearing the leaving group, and as the leaving group departs from the front, the configuration at the asymmetric centre is completely inverted, analogous to an umbrella turning inside out.
As an example, when (–)-2-bromooctane reacts with NaOH via SN2, (+)-octan-2-ol is formed with the –OH group in the position opposite to where –Br was, confirming complete inversion.
Racemisation occurs in SN1 reactions: the carbocation intermediate is sp2-hybridised and planar (achiral), so the nucleophile has equal probability of attacking from either face, giving a 50:50 mixture of enantiomers. Hydrolysis of optically active 2-bromobutane gives (±)-butan-2-ol.
Retention of configuration occurs when no bond to the stereocentre is broken during the reaction; the spatial arrangement of groups around the chiral carbon is preserved. An example is the reaction of (–)-2-methylbutan-1-ol with HCl, where the C–OH bond at C-1 breaks but the configuration at C-2 (stereocentre) is retained in the product (+)-1-chloro-2-methylbutane.
The sign of optical rotation may change between reactant and product even with retention, because different compounds with the same configuration at the asymmetric centre can have different optical rotations; configuration and optical rotation sign are not directly related.

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Question 7

Long Form

Describe the electrophilic substitution reactions of haloarenes, explaining why halogens are ortho-para directors despite being ring deactivators.

Haloarenes undergo electrophilic substitution reactions typical of the aromatic ring: halogenation (using X2/Lewis acid), nitration (HNO3/H2SO4), sulphonation (conc. H2SO4), and Friedel-Crafts reactions (alkyl/acyl halide + AlCl3). Products are predominantly ortho- and para-substituted.
The halogen atom withdraws electrons through the inductive effect (–I effect), which is stronger and operates through σ-bonds; this reduces overall electron density in the ring and deactivates it relative to benzene, requiring more drastic conditions (higher temperature, stronger catalyst).
Through resonance, the halogen donates electron pairs to the ring, specifically increasing electron density at ortho and para positions. This is because mesomeric structures place negative charge at these positions, making them more susceptible to electrophilic attack.
Net result: inductive effect controls reactivity (deactivation), while resonance controls orientation (ortho/para direction). The ring is less reactive than benzene but electrophilic attack still preferentially occurs at ortho and para positions.
Example: Chlorobenzene + Cl2 (anhyd. FeCl3) gives 1,4-dichlorobenzene (major, para) and 1,2-dichlorobenzene (minor, ortho); Friedel-Crafts acylation similarly gives predominantly para product.

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Long Answer

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Medium

Question 8

Long Form

Discuss the properties and environmental hazards of four important polyhalogen compounds: dichloromethane, chloroform, carbon tetrachloride, and freons.

Dichloromethane (CH2Cl2): widely used as a solvent, paint remover, aerosol propellant, and in pharmaceutical manufacturing. It harms the human CNS; lower exposure causes impaired hearing and vision; higher exposure causes dizziness, nausea, tingling; direct skin contact causes burning and redness.
Chloroform (CHCl3): used as a solvent for fats, alkaloids, and iodine; major current use is production of freon refrigerant R-22. Formerly used as a general anaesthetic but replaced by safer agents. Inhalation depresses CNS; chronic exposure damages liver (metabolised to phosgene) and kidneys. On exposure to air/light, chloroform oxidises to extremely poisonous phosgene (COCl2), so stored in closed dark bottles.
Carbon tetrachloride (CCl4): formerly used as a solvent, cleaning fluid, fire extinguisher, and degreasing agent. Evidence suggests it causes liver cancer. Common effects include dizziness, nausea, vomiting, and can cause permanent nerve damage; severe cases lead to stupor, coma, or death. When released into the atmosphere, it rises and depletes the ozone layer.
Freons (CFCs, e.g., CCl2F2, Freon-12): stable, non-toxic, non-corrosive, easily liquefiable gases used as refrigerants and aerosol propellants. Prepared by Swarts reaction. Due to their stability, freons diffuse unchanged into the stratosphere where they initiate radical chain reactions that destroy ozone, increasing UV radiation reaching Earth and causing skin cancer, eye diseases, and immune system disruption.

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Long Answer

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Hard

Question 9

Long Form

Describe the preparation of haloarenes by Sandmeyer's reaction and by electrophilic substitution. Compare the two methods in terms of applicability and the halogens that can be introduced.

Electrophilic substitution: aryl chlorides and bromides are prepared by treating arenes with Cl2 or Br2 in the presence of Lewis acid catalysts (Fe or FeCl3/AlCl3). The reaction gives ortho and para isomers; para can be separated due to large difference in melting points.
Electrophilic iodination is reversible and requires an oxidising agent (HNO3 or HIO4) to oxidise the HI produced, driving the equilibrium forward. Fluorine is too reactive for controlled electrophilic substitution by this method.
Sandmeyer's reaction: a primary aromatic amine is diazotised with NaNO2 + HX at 273–278 K to form a diazonium salt. The diazonium salt then reacts with CuCl or CuBr to give aryl chloride or aryl bromide (with N2 evolution). Aryl iodides are obtained by treating the diazonium salt with KI (no cuprous halide needed).
For fluorides, the Balz-Schiemann reaction (diazonium fluoroborate decomposition) can be used—not the Sandmeyer route; this is an extension beyond the Sandmeyer method.
Comparison: electrophilic substitution is simpler and directly applicable to halogenation of arenes but cannot introduce iodine or fluorine easily. Sandmeyer's reaction can introduce Cl, Br, and I via a diazonium intermediate from primary aromatic amines; it is more versatile for specific positional isomers and for compounds where direct substitution fails.

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Long Answer

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Hard

Question 10

Long Form

Explain the mechanism of the SN2 reaction between CH3Cl and OH– with a diagram description, and discuss the factors that influence SN2 reaction rates.

In the SN2 mechanism, the hydroxide ion (nucleophile) attacks the electrophilic carbon of CH3Cl from the back side, i.e., at 180° to the C–Cl bond. The reaction is a single concerted step with no intermediate; as the OH– approaches, the C–Cl bond weakens and the C–OH bond starts forming simultaneously.
In the transition state, the carbon is simultaneously partially bonded to both OH (incoming) and Cl (outgoing) and is bonded to five atoms; the three H atoms are in a plane perpendicular to the O–C–Cl axis.
As OH– bonds fully and Cl– leaves, the three C–H bonds invert their positions (umbrella inversion), giving complete inversion of configuration at the carbon centre (Walden inversion).
Factor 1 – Steric hindrance: the rate of SN2 decreases dramatically with increasing substitution around the reaction centre: CH3X (30 relative) > 1° (1) > 2° (0.02) > 3° (≈0). Bulky groups block the approaching nucleophile.
Factor 2 – Nature of nucleophile and leaving group: stronger nucleophiles (I–, CN–, OH–) react faster; better leaving groups (I–, Br–) give faster SN2. Reactivity of R–X: R–I > R–Br > R–Cl >> R–F for both SN1 and SN2.

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Chapter 6: Haloalkanes and Haloarenes | Long Questions

Compare and contrast the SN1 and SN2 mechanisms of nucleophilic substitution in haloalkanes with respect to kinetics, stereochemistry, effect of substrate structure, and solvent.

Discuss the methods of preparation of haloalkanes from alcohols, highlighting the reagents, conditions, and the reason for preferring thionyl chloride.

Explain the four reasons why aryl halides are less reactive than alkyl halides towards nucleophilic substitution. Explain how electron-withdrawing groups at ortho/para positions increase the reactivity of haloarenes.

Describe the physical properties of haloalkanes: state of aggregation, boiling points (with comparison to hydrocarbons, effect of halogen type, and branching), solubility in water and organic solvents, and density trends.

Discuss the classification of haloalkanes and haloarenes based on the hybridisation of the carbon bearing the halogen, giving examples for each sub-type and explaining the structural distinction.

Analyse the role of stereochemistry in understanding SN1 and SN2 reaction mechanisms, discussing inversion, retention, and racemisation with suitable examples.

Describe the electrophilic substitution reactions of haloarenes, explaining why halogens are ortho-para directors despite being ring deactivators.

Discuss the properties and environmental hazards of four important polyhalogen compounds: dichloromethane, chloroform, carbon tetrachloride, and freons.

Describe the preparation of haloarenes by Sandmeyer's reaction and by electrophilic substitution. Compare the two methods in terms of applicability and the halogens that can be introduced.

Explain the mechanism of the SN2 reaction between CH3Cl and OH– with a diagram description, and discuss the factors that influence SN2 reaction rates.

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