NCERT Solutions for class 12th Chemistry Chapter 10 Haloalkanes and Haloarenes


Question 1. Write structures of the following compounds:

(i) 2-Chloro-3-methylpentane
(ii) 1-Chloro-4-ethylcyclohexane
(iii) 4-tert-Butyl-3-iodoheptane
(iv) 1,4-Dibromobut-2-ene
(v) 1-Bromo-4-sec- butyl-2-methylbenzene.

Question 2. Why is sulphuric acid not used during the reaction of alcohols with KI?

Sol. H2SO4 is an oxidising agent. It oxidises Hl produced during the reaction to I2 and thus prevents the reaction between an alcohol and Hl to form alkyl iodide.

Question 3. Write structures of different dihalogen derivatives of propane.

Question 4. Among the isomeric alkanes of molecular formula C5H12, identify the one that on photochemical chlorination yields

(i) A single monochloride.
(ii) Three isomeric monochlorides.
(iii) Four isomeric monochlorides.

Question 5. Draw the structures of major monohalo products in each of the following reactions:

Question 6. Arrange each set of compounds in order of increasing boiling points.

(i) Bromomethane, Bromoform, Chloromethane, Dibromomethane.
(ii) 1-Chloropropane,Isopropyl chloride, 1-Chlorobutane.

Question 7. Which alkyl halide from the following pairs would you expect to react more rapidly by an SN2 mechanism? Explain your answer.

Note: Edward Davies Hughes and Sir C. Ingold in 1937 proposed a mechanism for an SN2 reaction.

Question 8. In the following pairs of halogen compounds, which compound undergoes faster SN1 reaction?

Question 9. Identify A, B, C, D, E, Rand R’ in the following:


Question 1. Name the following halides according to IUPAC system and classify them as alkyl, allyl, benzyl (primary, secondary, tertiary), vinyl or aryl halides:

Question 2. Give the IUPAC names of the following compounds:

Question 3. Write the structures of the following organic halogen compounds:

Question 4. Which one of the following has the highest dipole moment?

Question 5. A hydrocarbon C5H10 does not react with chlorine in dark but gives a single monochloro compound C5H9CI in bright sunlight. Identify the hydrocarbon.

Question 6. Write the isomers of the compound having formula C4H9Br.

Question 7. Write the equations for the preparation of l-iodobutane from

(i) 1-butanol (ii) 1-chlorobutane
(iii) but-1-ene.

Question 8. What are ambident nucleophiles? Explain with an example.

Question 9. Which compound in each of the following pairs will react faster in SN2 reaction with -OH?

Question 10. Predict all the alkenes that would be formed by dehydrohalogenation of the following halides with sodium ethoxide in ethanol and identify the major alkene.

(i) 1-Bromo-1-methylcyclohexane
(ii) 2-Chloro-2-methylbutane
(iii) 2, 2, 3-Trimethyl-3-bromopentane.

Note: Base abstracts H-atom from Beta -carbon and halogen atom is eliminated from a-carbon.

Question 11. How will you bring about the following conversions?

(i) Ethanol to but-1-yne (ii) Ethane to bromoethene
(iii) Propene to 1-nitropropane (iv) Toluene to benzyl alcohol
(v) Propene to propyne (vi) Ethanol to ethyl fluoride
(vii) Bromomethane to propanone (viii) But-1-ene to but-2-ene
(ix) 1-Chlorobutane ton-octane (x) Benzene to biphenyl

Question 12. Explain why

  1. The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride?
  2. Alkyl halides, though polar, are immiscible with water?
  3. Grignard reagents should be prepared under anhydrous conditions?

Sol. (i) sp2– Hybrid carbon in chlorobenzene is more electronegative than a sp3– hybrid carbon in cyclohexylchloride, due to greater s-character. Thus, C atom of chlorobenzene has less tendency to release electrons to Cl than carbon atom of cyclohexylchloride. As a result, C – Cl bond in chlorobenzene is less polar than in cyclohexylchloride. Further, due to delocalization of lone pairs of electrons of the Cl atom over the benzene ring, C-Cl bond in chlorobenzene acquires some double bond character while the C – Cl in cyclohexyl chloride is a pure single bond. In other words, C-Cl bond in chlorobenzene is shorter than in cyclohexyl chloride.

Since dipole moment is a product of charge and distance, therefore, chlorobenzene has lower dipole moment than cyclohexylchloride due to lower magnitude of negative charge on the Cl atom and shorter C- Cl distance.

(ii) Alkyl halides are polar molecules, therefore, their molecules are held together by dipole-dipole attraction. The molecules of H2O are held together by H-bonds. Since the new forces of attraction between water and alkyl halide molecules are weaker than the forces of attraction already existing between alkyl halide- alkyl halide molecules and water-water molecules, therefore, alkyl halides are immiscible (not soluble) in water. Alkyl halides are neither able to form H- bonds with water nor are able to break the H-bonding network of water.

(iii) Grignard reagents are very reactive. They react with moisture present in the apparatus to form alkanes.

Question 13. Give the uses of freon-12, DDT, carbon tetrachloride and iodoform.

Sol. Iodoform: It was earlier used as an antiseptic but the antiseptic properties are due to the liberation of free iodine and not due to iodoform itself. Due to its objectionable smell, it has been replaced by other formulations containing iodine.

Carbon tetrachloride:

  1. As an industrial solvent for oils, fats, resins etc. and also in dry cleaning.
  2. CCl4 vapours are highly non-inflammable, thus CCl4 is used as a fire extinguisher under the name of pyrene.
  3. Used in the manufacture of refrigerants and propellants for aerosol cans.

Freons: Freon-12 (CCl2F2) is most common freon in industrial use.
Uses: For aerosol propellants, refrigeration and air conditioning purposes.

DDT (p-p’ – Dichlorodiphenyl trichloroethane):

The use of DDT increased enormously on a world wide basis after World War II, primarily because of its effectiveness against the mosquitoes that spreads malaria and other insects which damage crops. However, problems related to extensive use of DDT began to appear in the late 1940s. Many species of insects developed resistance to DDT, it was also discovered to have a high toxicity towards fishes. DDT is not metabolised very rapidly by animals, instead it is deposited and stored in the fatty tissues. If the ingestion continues at a steady rate, DDT builds up within the animals overtime.

Question 14. Write the structure of the major organic product in each of the following reactions:

Question 15. Write the mechanism of the following reaction:

Question 16. Arrange the compounds of each set in order of reactivity towards SN2 displacement:

  1. 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane.
  2. 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-2- methylbutane.
  3. 1-Bromobutane, l-Bromo-2, 2-dimethylpropane, 1-Bromo-2- methylbutane, 1-Bromo-3-methyl butane.

Question 17. Out of C6H5CH2Cl and C6H5CHCIC6H5 which is more easily hydrolysed by aqueous KOH.

Question 18. p-Dichlorobenzene has higher m.p. and lower solubility than those of o­ and m-isomers. Discuss.

Sol. The p-isomer being more symmetrical fits closely in the crystal lattice and thus has stronger inter- molecular forces of attraction than o- and m-isomers. Since during melting or dissolution, the crystal lattice breaks, therefore, a large amount of energy is needed to melt or dissolve the p-isomer than the corresponding a-and m-isomers. In other words, the melting point of the p­ isomer is higher and its solubility lower than the corresponding o- and m­ isomers.

Note: A highly symmetrical structure has a higher melting point.

Question 19. How the following conversions can be carried out:

Question 20. The treatment of alkyl chlorides with aqueous KOH leads to the formation of alcohols but in the presence of alcoholic KOH, alkenes are major products. Explain.

Sol. In aqueous solution, KOH is almost completely ionised to give OH- ions which being a strong nucleophile brings about a substitution reaction on alkyl halides to form alcohols. Further in the aqueous solution, OH ions are highly solvated (hydrated). This solvation reduces the basic character of OH ions which, therefore, fails to abstract a hydrogen from the -carbon of the alkyl chloride to form alkenes. In contrast, an alcoholic solution of KOH contains alkoxide (RO) ion which being a much stronger base than OH­ ions preferentially eliminates a molecule of HCl from an alkyl chloride to form alkenes.

Question 21. Primary alkyl halide C4H9Br (a) is reacted with alcoholic KOH to give compound (b). Compound (b) is reacted with HBr to give (c) which is an isomer of (a). When (a) is reacted with sodium metal it gives compound (d) C8H18 which is different from the compound formed when n-butyl bromide is reacted with sodium. Give the structural formula of (a) and write the equations for all the reactions.

Question 22. What happens when

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