10 questions on classification of Haloalkanes and Haloarenes (Class 12):
1. Basic Conceptual Questions
Q1. On what basis are haloalkanes classified into primary, secondary and tertiary? Give an example of each.
- Basis: Number of carbon atoms attached to the carbon carrying the halogen.
- Primary (1°): Halogen-bearing C attached to 1 other C. Example: CH₃CH₂Cl (ethyl chloride).
- Secondary (2°): Halogen-bearing C attached to 2 other C atoms. Example: CH₃CHClCH₃ (isopropyl chloride).
- Tertiary (3°): Halogen-bearing C attached to 3 other C atoms. Example: (CH₃)₃CCl (tert-butyl chloride).
Q2. How are mono-, di- and polyhalogenated hydrocarbons different? Give one example of each.
- Monohalogen compounds: Contain one halogen atom. Example: CH₃Cl.
- Dihalogen compounds: Contain two halogen atoms. Example: CH₂Cl₂.
- Polyhalogen compounds: Contain more than two halogen atoms. Example: CCl₄.
Q3. What is the difference between geminal dihalides and vicinal dihalides? Give one example of each.
- Geminal dihalides: Both halogen atoms on the same carbon. Example: CH₃–CCl₂–H (1,1-dichloroethane).
- Vicinal dihalides: Halogen atoms on adjacent carbons. Example: CH₂Cl–CH₂Cl (1,2-dichloroethane).
Q4. Why are allylic and benzylic halides more reactive towards nucleophilic substitution than simple alkyl halides?
- Because their carbocations formed during the reaction are stabilized by resonance with the double bond (allylic) or benzene ring (benzylic).
2. Haloarenes Classification Questions
Q5. How are aryl halides (haloarenes) different from alkyl halides in terms of structure and reactivity?
- Structure: In aryl halides, halogen is directly bonded to an aromatic ring (e.g., chlorobenzene). In alkyl halides, halogen is bonded to a saturated carbon chain (e.g., CH₃Cl).
- Reactivity: Aryl halides are less reactive towards nucleophilic substitution due to resonance stabilization and partial double bond character of C–X bond.
Q6. Classify chlorobenzene, o-dichlorobenzene and benzyl chloride on the basis of haloalkane/haloarene classification.
- Chlorobenzene: Aryl halide (haloarene).
- o-Dichlorobenzene: Dihaloarene.
- Benzyl chloride: Primary benzylic halide (haloalkane).
Q7. Why is benzyl chloride more reactive than chlorobenzene in nucleophilic substitution reactions?
- In benzyl chloride, the carbocation formed is resonance stabilized by the benzene ring.
- In chlorobenzene, the C–Cl bond has partial double bond character due to resonance, making it strong and difficult to break.
3. Mixed/Competency-Based Questions
Q8. Classify the following as primary, secondary, tertiary or aryl halides:
- 1-bromopropane → Primary haloalkane.
- Isopropyl chloride → Secondary haloalkane.
- tert-butyl bromide → Tertiary haloalkane.
- Chlorobenzene → Aryl halide.
Q9. Out of CCl₄ and CH₃Cl, which one is called a polyhalogen compound? Why?
- CCl₄ is a polyhalogen compound because it contains more than one halogen atom (four Cl atoms).
- CH₃Cl is a monohalogen compound.
Q10. Differentiate between alkyl halides, allylic halides, benzylic halides, vinylic halides and aryl halides with one example each.
| Type | Definition | Example |
|---|---|---|
| Alkyl halide | Halogen attached to saturated sp³ carbon | CH₃CH₂Cl (ethyl chloride) |
| Allylic halide | Halogen attached to sp³ carbon adjacent to C=C | CH₂=CH–CH₂Cl (allyl chloride) |
| Benzylic halide | Halogen attached to sp³ carbon next to benzene | C₆H₅CH₂Cl (benzyl chloride) |
| Vinylic halide | Halogen attached directly to C=C carbon | CH₂=CHCl (vinyl chloride) |
| Aryl halide | Halogen directly bonded to aromatic ring | C₆H₅Cl (chlorobenzene) |
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