CBSE Class 12 Chemistry (2026–27)
Chapter 7: Haloalkanes and Haloarenes
20 Important Questions and Answers
These questions cover the most important concepts prescribed in the CBSE syllabus, including classification, preparation, nucleophilic substitution reactions, SN1/SN2 mechanisms, haloarenes, named reactions, and polyhalogen compounds.
Q1. What are haloalkanes and haloarenes? Give one example of each.
Answer:
Haloalkanes and haloarenes are organic compounds containing one or more halogen atoms (F, Cl, Br, or I). In haloalkanes, the halogen atom is attached to an sp³ hybridised carbon atom of an alkyl group. For example, chloromethane (CH₃Cl) is a haloalkane. In haloarenes, the halogen atom is directly attached to an aromatic ring carbon, which is sp² hybridised. Chlorobenzene (C₆H₅Cl) is a common example of a haloarene. These compounds are important intermediates in organic synthesis and undergo various substitution and elimination reactions because of the polar nature of the carbon-halogen bond.
Q2. Explain the nature of the C–X bond in haloalkanes.
Answer:
The carbon-halogen (C–X) bond in haloalkanes is polar covalent because halogen atoms are more electronegative than carbon. As a result, the electron pair forming the bond is attracted towards the halogen atom. This creates a partial positive charge (δ⁺) on carbon and a partial negative charge (δ⁻) on the halogen atom. The polarity makes the carbon atom susceptible to attack by nucleophiles. Bond strength decreases from C–F to C–I because bond length increases down the group. Therefore, alkyl iodides are generally more reactive than alkyl fluorides in nucleophilic substitution reactions.
Q3. Differentiate between primary, secondary and tertiary alkyl halides.
Answer:
Alkyl halides are classified according to the type of carbon atom bonded to the halogen. In a primary (1°) alkyl halide, the halogen is attached to a carbon connected to only one other carbon atom. Example: CH₃CH₂Cl. In a secondary (2°) alkyl halide, the halogen-bearing carbon is attached to two carbon atoms. Example: CH₃CHClCH₃. In a tertiary (3°) alkyl halide, the halogen-bearing carbon is attached to three carbon atoms. Example: (CH₃)₃CCl. This classification is important because the reactivity of alkyl halides in SN1 and SN2 reactions depends greatly on their structure.
Q4. What is the SN1 reaction mechanism?
Answer:
SN1 stands for Substitution Nucleophilic Unimolecular reaction. It occurs in two steps. First, the carbon-halogen bond breaks to form a carbocation intermediate and a halide ion. This slow step determines the reaction rate. In the second step, the nucleophile attacks the carbocation to form the product. The reaction rate depends only on the concentration of the alkyl halide. Tertiary alkyl halides readily undergo SN1 reactions because they form stable carbocations. Since a planar carbocation intermediate is formed, the reaction may produce a racemic mixture when the reacting carbon is chiral.
Q5. Explain the SN2 reaction mechanism.
Answer:
SN2 stands for Substitution Nucleophilic Bimolecular reaction. It occurs in a single step where the nucleophile attacks the carbon atom from the side opposite to the leaving group while the carbon-halogen bond breaks simultaneously. The reaction rate depends on both the alkyl halide and nucleophile concentrations. Primary alkyl halides favour SN2 reactions because steric hindrance is minimal. A characteristic feature of SN2 reactions is inversion of configuration, known as Walden inversion. Since no carbocation intermediate is formed, rearrangements do not occur. SN2 reactions are generally faster with strong nucleophiles and polar aprotic solvents.
Q6. Distinguish between SN1 and SN2 reactions.
Answer:
SN1 reactions proceed through a carbocation intermediate and involve two steps, whereas SN2 reactions occur in a single step without any intermediate. The rate of an SN1 reaction depends only on the alkyl halide concentration, while the rate of an SN2 reaction depends on both the alkyl halide and nucleophile concentrations. Tertiary alkyl halides favour SN1 reactions because they form stable carbocations. Primary alkyl halides favour SN2 reactions due to less steric hindrance. SN1 reactions may result in racemisation, whereas SN2 reactions lead to inversion of configuration. These differences help predict the reaction pathway of haloalkanes.
Q7. What is Finkelstein reaction?
Answer:
Finkelstein reaction is a halogen exchange reaction used for the preparation of alkyl iodides from alkyl chlorides or alkyl bromides. In this reaction, an alkyl chloride or bromide is treated with sodium iodide in dry acetone. The iodide ion replaces the chlorine or bromine atom. The reaction proceeds because sodium chloride and sodium bromide are insoluble in acetone and precipitate out, shifting the equilibrium toward product formation. For example:
R–Cl + NaI → R–I + NaCl.
Finkelstein reaction is useful in organic synthesis because alkyl iodides are generally more reactive than corresponding chlorides and bromides.
Q8. What is Swarts reaction?
Answer:
Swarts reaction is used to prepare alkyl fluorides from alkyl chlorides or bromides. In this reaction, alkyl chlorides or bromides are heated with metallic fluorides such as AgF, Hg₂F₂, CoF₂, or SbF₃. The fluorine atom replaces the chlorine or bromine atom. For example:
CH₃Br + AgF → CH₃F + AgBr.
This method is important because direct fluorination is difficult to control due to the high reactivity of fluorine gas. Swarts reaction provides a convenient laboratory method for synthesizing alkyl fluorides, which are valuable compounds in pharmaceuticals and industrial chemistry.
Q9. Why are haloarenes less reactive towards nucleophilic substitution?
Answer:
Haloarenes are less reactive toward nucleophilic substitution because the lone pair of electrons on the halogen atom participates in resonance with the aromatic ring. This gives the carbon-halogen bond partial double bond character, making it shorter and stronger. Additionally, the carbon atom bonded to the halogen is sp² hybridised, resulting in a stronger bond than in haloalkanes. Formation of a phenyl carbocation is highly unstable, so the SN1 mechanism is not favored. Backside attack required for SN2 reactions is also hindered by the aromatic ring. These factors collectively reduce the reactivity of haloarenes.
Q10. What is Wurtz reaction?
Answer:
Wurtz reaction is a method for preparing higher alkanes by coupling two molecules of an alkyl halide using sodium metal in dry ether. The reaction is represented as:
2R–X + 2Na → R–R + 2NaX.
For example:
2CH₃Br + 2Na → C₂H₆ + 2NaBr.
This reaction is useful for synthesizing symmetrical alkanes containing an even number of carbon atoms. However, if two different alkyl halides are used, a mixture of products is obtained, reducing its synthetic utility. Wurtz reaction is an important named reaction in organic chemistry and frequently appears in board examinations.
Q11. What is Fittig reaction?
Answer:
Fittig reaction is used to prepare biaryl compounds by coupling aryl halides in the presence of sodium metal and dry ether. The general reaction is:
2Ar–X + 2Na → Ar–Ar + 2NaX.
For example:
2C₆H₅Cl + 2Na → C₆H₅–C₆H₅ + 2NaCl.
The product formed is biphenyl. This reaction is similar to the Wurtz reaction but involves aryl halides instead of alkyl halides. Fittig reaction is important because it provides a method for forming carbon-carbon bonds between aromatic rings. It is commonly included among the named reactions of haloarenes in CBSE examinations.
Q12. What is Wurtz-Fittig reaction?
Answer:
Wurtz-Fittig reaction is used to prepare alkyl-substituted aromatic compounds. It involves the reaction of an aryl halide and an alkyl halide with sodium metal in dry ether. The general reaction is:
Ar–X + R–X + 2Na → Ar–R + 2NaX.
For example:
C₆H₅Br + CH₃Br + 2Na → C₆H₅CH₃ + 2NaBr.
The product formed is toluene. This reaction combines features of both Wurtz and Fittig reactions and is important for synthesizing alkylbenzenes. It demonstrates the formation of carbon-carbon bonds between aromatic and aliphatic groups.
Q13. How is chlorobenzene prepared from aniline?
Answer:
Chlorobenzene is prepared from aniline through diazotisation followed by the Sandmeyer reaction. First, aniline is treated with sodium nitrite and hydrochloric acid at 273–278 K to form benzenediazonium chloride. In the second step, the diazonium salt is treated with cuprous chloride (CuCl), which replaces the diazonium group with chlorine. The reaction is:
C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂.
This method is important because direct chlorination of benzene may produce mixtures, whereas the diazonium route gives chlorobenzene efficiently and selectively.
Q14. What is Sandmeyer reaction?
Answer:
Sandmeyer reaction is a method used to replace the diazonium group of an aromatic diazonium salt with chlorine, bromine, or cyanide using cuprous salts. For example:
C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂.
Similarly, CuBr gives bromobenzene and CuCN gives benzonitrile. The reaction is important because it provides a convenient route for introducing various functional groups into aromatic rings. Sandmeyer reaction is widely used in organic synthesis and is a frequently asked topic in board examinations due to its synthetic applications.
Q15. What are ambident nucleophiles? Give an example.
Answer:
Ambident nucleophiles are nucleophiles that possess two different nucleophilic centres and can attack through either of them. As a result, they can produce different products depending on the reaction conditions. A common example is the cyanide ion (CN⁻). It can attack through carbon to form nitriles (R–CN) or through nitrogen to form isocyanides (R–NC). Another example is the nitrite ion (NO₂⁻), which can produce nitro compounds or alkyl nitrites. Ambident nucleophiles play an important role in substitution reactions because they demonstrate the concept of linkage and product selectivity.
Q16. What are Grignard reagents? How are they prepared?
Answer:
Grignard reagents are organomagnesium compounds having the general formula RMgX, where R is an alkyl or aryl group and X is a halogen atom. They are prepared by reacting alkyl or aryl halides with magnesium metal in dry ether. For example:
CH₃Br + Mg → CH₃MgBr.
These reagents are highly reactive because the carbon-magnesium bond is polar, giving carbon a partial negative charge. Grignard reagents are important synthetic intermediates used for preparing alcohols, hydrocarbons, and carboxylic acids. Since they react readily with water, their preparation and storage require strictly anhydrous conditions.
Q17. Explain Saytzeff’s rule.
Answer:
Saytzeff’s rule helps predict the major product formed during elimination reactions. According to this rule, when a haloalkane undergoes dehydrohalogenation, the major product is the alkene having the greater number of alkyl groups attached to the double-bonded carbon atoms. Such alkenes are more stable due to hyperconjugation and inductive effects. For example, dehydrohalogenation of 2-bromobutane produces both but-1-ene and but-2-ene, but but-2-ene is formed predominantly. Saytzeff’s rule is important for understanding elimination reactions and predicting products in organic chemistry problems.
Q18. Write a note on DDT.
Answer:
DDT (Dichloro Diphenyl Trichloroethane) is a polyhalogen compound formerly used as a powerful insecticide. It was effective against mosquitoes, flies, and agricultural pests. DDT is chemically stable and persists in the environment for long periods. Because it is non-biodegradable, it accumulates in the food chain and causes biomagnification. High concentrations of DDT can adversely affect birds, aquatic organisms, and human health. Due to these environmental concerns, its use has been restricted or banned in many countries. DDT is studied as an important example of the environmental impact of halogen-containing organic compounds.
Q19. What is chloroform? Mention its uses.
Answer:
Chloroform (CHCl₃) is a colorless, sweet-smelling liquid belonging to the class of polyhalogen compounds. It was previously used as an anesthetic, but its use for this purpose has been discontinued because of toxic side effects. Chloroform is widely used as a solvent for fats, alkaloids, resins, and various organic compounds. It is also used in the manufacture of refrigerants and fluorocarbon compounds. When exposed to air and sunlight, chloroform forms poisonous phosgene gas, so it is stored in dark bottles containing a small amount of ethanol, which prevents phosgene formation.
Q20. What is carbon tetrachloride? State its applications.
Answer:
Carbon tetrachloride (CCl₄) is a tetrahalogen compound in which four chlorine atoms are attached to a carbon atom. It is a non-flammable liquid and was widely used as a cleaning solvent for machinery and in dry-cleaning operations. It has also been used in the manufacture of refrigerants and propellants. Due to its toxicity and harmful effects on the ozone layer, its use has been significantly reduced. Carbon tetrachloride is an important example of a polyhalogen compound and helps students understand the industrial and environmental significance of haloalkanes and haloarenes.