Haloalkanes and Haloarenes – important questions for Boards 2027
CBSE Class 12 Chemistry
Which reagent is preferred for preparing a pure alkyl chloride from an alcohol? (A) Concentrated HCl alone (B) SOCl₂ (C) NaCl (D) NaI
Answer
(B) SOCl₂ — SO₂ and HCl gases escape, leaving pure alkyl halide.
Distinguish between geminal and vicinal dihalides. How are these compounds named in the common and IUPAC systems?
Answer
1. Halogen positions — Geminal dihalides have both halogen atoms on the same carbon atom; vicinal dihalides have halogen atoms on adjacent carbon atoms. 2. Naming — In the common name system, geminal dihalides are alkylidene halides and vicinal dihalides are alkylene dihalides; in the IUPAC system, both are named as dihaloalkanes.
For each of the following, state whether it is a vinylic halide or an aryl halide, and identify the type of carbon to which the halogen is bonded: (i) CH₂=CHCl (ii) C₆H₅Cl.
Answer
1. CH₂=CHCl is a vinylic halide: chlorine is bonded to an sp²-hybridised carbon of the C=C bond. The carbon bearing chlorine is part of the carbon–carbon double bond. 2. C₆H₅Cl is an aryl halide: chlorine is directly bonded to an sp²-hybridised carbon of the aromatic ring. The bonded carbon belongs to the aromatic ring.
Describe how chlorobenzene and bromobenzene can be prepared from benzene diazonium chloride. Write the reaction equations and name the reagents used.
Answer
Chlorobenzene and bromobenzene are prepared from benzene diazonium chloride by replacing its diazonium group with chlorine or bromine, respectively. 1. Benzene diazonium chloride is prepared by treating a primary aromatic amine with sodium nitrite in cold aqueous mineral acid. 2. Mixing freshly prepared benzene diazonium chloride with cuprous chloride replaces the diazonium group with chlorine. C₆H₅N₂Cl(aq) → (CuCl) C₆H₅Cl(l) + N₂(g). 3. Mixing freshly prepared benzene diazonium chloride with cuprous bromide replaces the diazonium group with bromine. C₆H₅N₂Cl(aq) + CuBr(aq) → C₆H₅Br(l) + N₂(g) + CuCl(aq). 4. Nitrogen gas is liberated in both reactions, and the products are chlorobenzene and bromobenzene, respectively. These replacements are examples of Sandmeyer’s reaction, used to prepare aryl chlorides and bromides from diazonium salts.
Which statement best describes an SN2 substitution? (A) It occurs in two steps through a carbocation intermediate. (B) It is a concerted, single-step process in which bond formation and leaving-group departure occur together. (C) It occurs only after the leaving group has departed in a slow step. (D) It involves no bond formation with the incoming nucleophile.
Answer
(B) It is a concerted, single-step process in which bond formation and leaving-group departure occur together — both processes take place simultaneously, and no intermediate is formed.
Classify the carbon atom bearing halogen in CH₃CH₂CH(Br)CH₃ as primary, secondary or tertiary. State the rule used to make the classification.
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Which statement correctly compares the common and IUPAC naming of alkyl halides? (A) The common name gives the alkyl group followed by the halide name; the IUPAC name treats the compound as a halosubstituted hydrocarbon. (B) Both systems name the halogen first and then the alkyl group. (C) The common name uses hydrocarbon locants, while the IUPAC name uses only the halide name. (D) The common name and IUPAC name are always identical.
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Explain the preparation of chlorobenzene and bromobenzene from benzene by electrophilic substitution. Write the equations and specify the catalysts required.
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What stereochemical change occurs at the attacked carbon during an SN2 reaction?
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State the order of reactivity of primary, secondary and tertiary alcohols with a given haloacid. Which alcohols require ZnCl₂ when reacted with HCl?
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20 more probable questions in this chapter, every one with a full answer.
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