NEET Properties Of Haloalkanes and Haloarenes Important Questions by PW

Why does one haloalkane form a carbocation while another undergoes direct nucleophilic attack? The answer depends on the structure of the carbon attached to the leaving group, steric hindrance, nucleophile strength and reaction conditions. These differences also explain why haloarenes behave differently from haloalkanes.

Physics Wallah’s Chemistry Important Questions can help you revisit important reaction mechanisms, compare similar reactions and practise product-based questions. Use the following questions to revise the concepts and reactions commonly tested in NEET.

Important Questions

Q1. What is the difference between SN1 and SN2 reactions?

Answer: Both are nucleophilic substitution reactions, but they follow different mechanisms.

Q2. Why are tertiary haloalkanes more favourable for SN1 reactions?

Answer: SN1 reactions begin with the formation of a carbocation. Tertiary carbocations are stabilised by the electron-releasing effects of alkyl groups and hyperconjugation.

The general carbocation stability order is:

3° > 2° > 1° > methyl

Resonance can change this order. For example, benzylic carbocations are stabilised by resonance.

Remember: Compare the carbocation formed in the initial step, and check for possible rearrangements when required.

Q3. Why do primary haloalkanes generally favour SN2 reactions?

Answer: SN2 reactions involve a nucleophile attacking the carbon attached to the leaving group from the opposite side. Less hindered carbon atoms allow easier backside attack.

The usual order is:

Methyl > 1° > 2° > 3°

Tertiary haloalkanes are strongly hindered and generally do not undergo ordinary SN2 reactions.

Q4. What is the stereochemical outcome of SN1 and SN2 reactions?

Answer:

• SN1: The carbocation is approximately planar, allowing attack from either side. Partial racemisation is common, although the exact product ratio depends on the reaction.

• SN2: Backside attack produces inversion of configuration, known as Walden inversion.

Racemisation is relevant only when the product can exist as enantiomers.

Q5. How do solvents affect SN1 and SN2 reactions?

Answer:

• Polar protic solvents, such as water and alcohols, stabilise ions and generally favour SN1 reactions.

• Polar aprotic solvents, such as DMSO, DMF and acetone, can increase the reactivity of many anionic nucleophiles and favour SN2 reactions.

The effect of a solvent also depends on the nucleophile and substrate involved.

Q6. What is the order of leaving-group ability in haloalkanes?

Answer: The general order is:

I⁻ > Br⁻ > Cl⁻ > F⁻

For the same alkyl group, alkyl iodides generally undergo substitution more readily than the corresponding alkyl bromides, chlorides or fluorides because the carbon–iodine bond is weaker and iodide is a stable leaving group.

Q7. How do alcohols react with hydrogen halides to form haloalkanes?

Answer: Alcohols react with hydrogen halides to produce haloalkanes and water.

ROH + HX → RX + H₂O

The hydroxyl group is first protonated, allowing water to leave more readily.

• Primary alcohols generally favour an SN2 pathway.

• Tertiary alcohols generally favour an SN1 pathway.

• Secondary alcohols may follow either pathway, depending on the conditions.

Carbocation rearrangements should be considered when an SN1 intermediate forms.

Q8. Which reagents are used to convert alcohols into alkyl chlorides?

Answer: Common reagents include phosphorus trichloride (PCl₃), phosphorus pentachloride (PCl₅) and thionyl chloride (SOCl₂).

Thionyl chloride is particularly useful because its gaseous by-products, SO₂ and HCl, escape from the reaction mixture.

The reaction with SOCl₂ may proceed by different pathways depending on the solvent and conditions, so the stereochemical outcome should not be assumed without the specified conditions.

Q9. What are the Finkelstein and Swarts reactions?

Answer: Both reactions are used to prepare alkyl halides.

• Finkelstein reaction: Converts an alkyl chloride or bromide into an alkyl iodide, commonly using NaI in acetone.

• Swarts reaction: Converts an alkyl chloride or bromide into an alkyl fluoride using suitable fluorinating agents, such as certain metal fluorides.

Memory tip: Finkelstein gives iodides; Swarts gives fluorides.

Q10. How does ether cleavage occur with HI or HBr?

Answer: Ether cleavage begins with protonation of the oxygen atom. The carbon–oxygen bond then breaks through a pathway determined by the ether’s structure and the reaction conditions.

For a simple unsymmetrical ether, the halide ion often attacks the less hindered alkyl group by SN2. If a stable carbocation can form, an SN1 pathway may be favoured.

In an aryl–alkyl ether, cleavage generally occurs at the alkyl–oxygen bond, not the aryl–oxygen bond. This is because the aryl–oxygen bond has partial double-bond character due to resonance.

Q11. Why do haloarenes resist ordinary SN1 and SN2 reactions?

Answer: Haloarenes are less reactive towards ordinary nucleophilic substitution because:

• The carbon–halogen bond has partial double-bond character due to resonance.

• The carbon attached to halogen is sp²-hybridised.

• Formation of a phenyl carbocation is highly unfavourable.

• Backside attack at the aromatic carbon is difficult.

However, haloarenes can undergo nucleophilic substitution under suitable conditions through other mechanisms.

Q12. How does a nitro group activate a haloarene towards nucleophilic substitution?

Answer: A nitro group at the ortho or para position to the halogen can stabilise the negatively charged intermediate formed during nucleophilic attack.

The reaction proceeds through nucleophilic addition followed by the departure of the halide ion. The intermediate is called a Meisenheimer complex.

A nitro group at the meta position does not provide the same resonance stabilisation to the intermediate.

Q13. What are the Dow process and the benzyne mechanism?

Answer:

• Dow process: Chlorobenzene reacts with aqueous NaOH at high temperature and pressure to form sodium phenoxide, which is acidified to produce phenol.

• Benzyne mechanism: Under strong conditions, such as NaNH₂ in liquid ammonia, a haloarene can undergo elimination followed by addition through a highly reactive benzyne intermediate.

For example, chlorobenzene can form aniline under suitable benzyne-forming conditions.

Quick Revision

• SN1: Carbocation intermediate; first-order kinetics.

• SN2: One-step mechanism; backside attack and inversion.

• Finkelstein: Preparation of alkyl iodides.

• Swarts: Preparation of alkyl fluorides.

• Ether cleavage: Usually breaks the alkyl–oxygen bond in aryl–alkyl ethers.

• Nitro-activated haloarenes: Ortho and para nitro groups stabilise the substitution intermediate.

• Benzyne mechanism: Elimination followed by addition.

For NEET revision, focus on recognising the reaction mechanism from the substrate, reagent and solvent. Practise predicting products and distinguishing SN1, SN2 and aromatic nucleophilic substitution rather than memorising reaction names alone. Physics Wallah’s Chemistry Important Questions can help you reinforce these concepts and prepare for mechanism-based questions.

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