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

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

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A forensic chemist is analysing an unknown organic compound. The compound is a colourless liquid with a sweet smell, slightly soluble in water but readily soluble in organic solvents. Its molecular formula is C2H5Cl. It reacts with aqueous KOH to give ethanol, and with alcoholic KOH to give ethene. When treated with magnesium in dry ether, a Grignard reagent is formed. The chemist notes that the C–Cl bond is polar due to the higher electronegativity of chlorine compared to carbon, making the carbon atom partially positive and hence susceptible to nucleophilic attack. The compound belongs to the primary alkyl halide class.

Question 1: What is the IUPAC name of the compound C2H5Cl, and how is it classified?

The IUPAC name is Chloroethane.
It is classified as a primary (1°) alkyl halide since chlorine is attached to a primary carbon atom.

Question 2: Why does the reaction of C2H5Cl with aqueous KOH give ethanol while alcoholic KOH gives ethene?

Aqueous KOH acts as a nucleophile (OH⁻), promoting nucleophilic substitution (SN2) to give ethanol.
Alcoholic KOH acts as a strong base in a non-aqueous medium, promoting elimination (E2) by abstracting a β-hydrogen to give ethene.

Question 3: Describe the formation and a key property of the Grignard reagent formed from C2H5Cl. Why must the reaction be performed in dry ether?

C2H5Cl reacts with magnesium in dry ether to form ethylmagnesium chloride (C2H5MgCl), called a Grignard reagent.
The C–Mg bond is covalent but highly polar with carbon bearing a partial negative charge, making it a strong nucleophile/base.
The reaction must be done in dry ether because Grignard reagents react readily with water (even traces of moisture) to give a hydrocarbon (ethane), destroying the reagent.
Dry ether acts as an inert, anhydrous solvent that stabilises the Grignard reagent through coordination.

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In an organic chemistry laboratory, students are studying the SN1 and SN2 mechanisms. They observe that tert-butyl bromide in aqueous ethanol reacts by a first-order mechanism, producing tert-butanol. The rate depends only on the concentration of tert-butyl bromide. In contrast, methyl bromide reacts with NaOH by a second-order mechanism. The students also note that when optically active (–)-2-bromooctane reacts with NaOH under SN2 conditions, the product is (+)-octan-2-ol with inverted configuration. Under SN1 conditions with an optically active substrate, the product is a racemic mixture.

Question 1: State the order of reactivity of alkyl halides towards SN2 reactions and explain why tert-butyl bromide is least reactive in SN2.

Order for SN2: Primary > Secondary > Tertiary.
Tert-butyl bromide is least reactive in SN2 because the three bulky methyl groups around the carbon bearing the leaving group create steric hindrance, preventing the nucleophile from approaching.

Question 2: Why does SN1 reaction of an optically active alkyl halide lead to a racemic mixture?

In SN1, the slow step forms a carbocation intermediate that is sp²-hybridised and planar (achiral).
The nucleophile can attack from either face of the planar carbocation with equal probability, giving equal amounts of both configurations (racemic mixture).

Question 3: Explain the mechanism of the SN2 reaction of (–)-2-bromooctane with NaOH and what stereochemical outcome is expected. Name this phenomenon.

In SN2, the hydroxide nucleophile attacks the carbon bearing the bromine from the back side (180° to the leaving group) in a single concerted step.
As the C–OH bond forms, the C–Br bond weakens simultaneously; the three groups on carbon flip like an inverted umbrella.
This results in inversion of configuration at the chiral centre — the product (+)-octan-2-ol has the opposite spatial arrangement to the reactant.
This phenomenon is called Walden inversion or inversion of configuration.

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Chirality plays an important role in drug action and organic chemistry. A student is studying butan-2-ol and propan-2-ol. She draws their structures and their mirror images. For butan-2-ol, the mirror image is non-superimposable on the original molecule. For propan-2-ol, the mirror image can be superimposed. This difference arises because butan-2-ol has a carbon atom attached to four different groups (–H, –OH, –CH3, –C2H5), making it a stereocentre or asymmetric carbon. Such molecules are described as chiral and are optically active. Enantiomers of a chiral compound rotate plane polarised light equally but in opposite directions.

Question 1: Define chirality and state the condition for a carbon to be called an asymmetric carbon.

Chirality is the property of a molecule whose mirror image is non-superimposable on itself.
A carbon atom is called asymmetric (stereocentre) when all four groups attached to it are different from each other.

Question 2: What is a racemic mixture and what is the process of its formation called? What is its optical rotation?

A racemic mixture (or racemic modification) is an equimolar mixture of two enantiomers of a chiral compound.
The process of converting an enantiomer into a racemic mixture is called racemisation.
A racemic mixture has zero optical rotation because the rotations of the two enantiomers cancel each other out.

Question 3: What are enantiomers? How do they differ from each other in terms of physical and chemical properties? Give two examples of chiral molecules from the chapter.

Enantiomers are stereoisomers that are non-superimposable mirror images of each other.
They have identical physical properties — same melting point, boiling point, refractive index, solubility, etc.
They differ only in the direction of rotation of plane-polarised light: one is dextrorotatory (+), the other is laevorotatory (–).
Examples: 2-chlorobutane and 2-bromopropanoic acid (or bromochloro-iodomethane).

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Haloarenes are generally less reactive towards nucleophilic substitution reactions compared to haloalkanes. A chemistry teacher explains this to students using chlorobenzene as an example. The C–Cl bond in chlorobenzene has partial double bond character due to resonance between the lone pairs of chlorine and the π-electrons of the benzene ring. The C–Cl bond length in haloarene (169 pm) is shorter than in haloalkane (177 pm). When electron-withdrawing groups like –NO2 are placed at ortho or para positions relative to –Cl, the reactivity towards nucleophilic substitution increases significantly, as demonstrated by the conversion of 2,4-dinitrochlorobenzene to corresponding phenol at much lower temperature.

Question 1: Why is chlorobenzene less reactive than chloroethane towards nucleophilic substitution?

In chlorobenzene, lone pairs on Cl are delocalised into the benzene ring by resonance, giving C–Cl partial double bond character.
The C–Cl bond becomes shorter and stronger (169 pm vs 177 pm), making it harder to break.
Also, the sp² carbon in chlorobenzene is more electronegative than the sp³ carbon in chloroethane, holding the bonding electrons more tightly.

Question 2: Why does the –NO2 group at ortho/para positions increase the reactivity of chlorobenzene towards nucleophilic substitution, but not at the meta position?

The –NO2 group at ortho/para positions withdraws electrons from positions ortho and para to it by resonance, stabilising the carbanion intermediate formed during nucleophilic substitution.
When –Cl is at ortho/para to –NO2, the negative charge in the intermediate is directly stabilised by the –NO2 group.
At meta position, resonance structures do not place the negative charge on the carbon bearing –NO2, so it cannot stabilise the intermediate and has no activating effect.

Question 3: Explain the electrophilic substitution reactions of haloarenes. Why does halogen act as an ortho/para director despite being a deactivator?

Haloarenes undergo electrophilic substitution (halogenation, nitration, sulphonation, Friedel-Crafts) but more slowly than benzene due to –I effect of halogen deactivating the ring.
Halogen has two opposing effects: inductive effect (–I) which withdraws electrons and deactivates the ring; resonance effect (+R) which donates electrons by conjugation and stabilises carbocation at ortho/para positions.
The –I effect is stronger, so overall the ring is deactivated (reactivity controlled by inductive effect).
But resonance stabilisation of the intermediate carbocation is more effective at ortho/para positions, so orientation is controlled by resonance, making halogen an ortho/para director.

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In industry, several methods are used to prepare haloalkanes. An industrial chemist is preparing different alkyl halides for use as synthetic intermediates. She reacts alcohols with thionyl chloride (SOCl2) to get alkyl chlorides, uses Finkelstein reaction to convert alkyl chloride to alkyl iodide using NaI in dry acetone, and prepares alkyl fluorides using Swarts reaction with AgF. She notes that thionyl chloride is preferred over PCl5 for preparing alkyl chlorides because the by-products SO2 and HCl are gases and escape from the reaction mixture, giving pure product. She also prepares Grignard reagent from the alkyl iodide.

Question 1: Write the reaction for the preparation of n-propyl chloride from n-propanol using thionyl chloride. Why is SOCl2 the preferred reagent?

CH3CH2CH2OH + SOCl2 → CH3CH2CH2Cl + SO2 + HCl
SOCl2 is preferred because both by-products (SO2 and HCl) are gases that escape the reaction mixture, leaving behind pure alkyl chloride without need for further purification.

Question 2: What is the Finkelstein reaction? Write the equation and explain why NaI is used in dry acetone.

Finkelstein reaction: alkyl chloride or bromide is treated with NaI in dry acetone to yield alkyl iodide. R–X + NaI (dry acetone) → R–I + NaX (X = Cl, Br).
NaCl or NaBr formed is insoluble in dry acetone and precipitates out, shifting equilibrium forward (Le Chatelier's principle), thus giving a good yield of alkyl iodide.

Question 3: Describe the Swarts reaction with an example. Also compare the order of reactivity of alcohols with haloacids and explain.

Swarts reaction: an alkyl chloride or bromide is heated with a metallic fluoride (AgF, Hg2F2, CoF2, or SbF3) to give the corresponding alkyl fluoride. Example: CH3Br + AgF → CH3F + AgBr.
The order of reactivity of alcohols with a given haloacid is: 3° > 2° > 1°.
Tertiary alcohols are most reactive because they form the most stable carbocation intermediates, facilitating ionisation in SN1-type mechanism.
Primary alcohols require a catalyst (ZnCl2 with HCl) while tertiary alcohols react easily just by shaking with concentrated HCl at room temperature.

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Chapter 6: Haloalkanes and Haloarenes | Case Study Questions

Passage

Question 1: What is the IUPAC name of the compound C2H5Cl, and how is it classified?

Question 2: Why does the reaction of C2H5Cl with aqueous KOH give ethanol while alcoholic KOH gives ethene?

Question 3: Describe the formation and a key property of the Grignard reagent formed from C2H5Cl. Why must the reaction be performed in dry ether?

Passage

Question 1: State the order of reactivity of alkyl halides towards SN2 reactions and explain why tert-butyl bromide is least reactive in SN2.

Question 2: Why does SN1 reaction of an optically active alkyl halide lead to a racemic mixture?

Question 3: Explain the mechanism of the SN2 reaction of (–)-2-bromooctane with NaOH and what stereochemical outcome is expected. Name this phenomenon.

Passage

Question 1: Define chirality and state the condition for a carbon to be called an asymmetric carbon.

Question 2: What is a racemic mixture and what is the process of its formation called? What is its optical rotation?

Question 3: What are enantiomers? How do they differ from each other in terms of physical and chemical properties? Give two examples of chiral molecules from the chapter.

Passage

Question 1: Why is chlorobenzene less reactive than chloroethane towards nucleophilic substitution?

Question 2: Why does the –NO2 group at ortho/para positions increase the reactivity of chlorobenzene towards nucleophilic substitution, but not at the meta position?

Question 3: Explain the electrophilic substitution reactions of haloarenes. Why does halogen act as an ortho/para director despite being a deactivator?

Passage

Question 1: Write the reaction for the preparation of n-propyl chloride from n-propanol using thionyl chloride. Why is SOCl2 the preferred reagent?

Question 2: What is the Finkelstein reaction? Write the equation and explain why NaI is used in dry acetone.

Question 3: Describe the Swarts reaction with an example. Also compare the order of reactivity of alcohols with haloacids and explain.

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