Chapter 6: Haloalkanes and Haloarenes Short Questions | chemistry Class 12 CBSE

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Chapter 6: Haloalkanes and Haloarenes Short Questions | chemistry Class 12 CBSE | ByteNotes.in

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Question 1

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Explain why haloalkanes are more reactive than haloarenes towards nucleophilic substitution reactions.

In haloalkanes, the carbon bearing the halogen is sp3-hybridised, making the C–X bond longer (177 pm for C–Cl) and weaker, and hence easier to break.
In haloarenes, the halogen lone pairs are in conjugation with the aromatic π-electrons, giving the C–X bond partial double bond character and making it shorter (169 pm) and harder to cleave.
Additionally, electron-rich arene rings repel incoming nucleophiles, while the partially positive carbon in haloalkanes readily attracts them.

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Question 2

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What is the Finkelstein reaction? State the condition that drives it to completion.

The Finkelstein reaction is the conversion of an alkyl chloride or bromide into an alkyl iodide by treating with sodium iodide (NaI) in dry acetone: R–X + NaI → R–I + NaX (X = Cl, Br).
The reaction is driven forward because NaCl or NaBr formed is insoluble in dry acetone and precipitates out of the reaction mixture.
The precipitation removes the product NaX, shifting the equilibrium to the right according to Le Chatelier's Principle, ensuring a good yield of alkyl iodide.

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Question 3

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Why is thionyl chloride (SOCl2) preferred over PCl5 or HCl for preparing alkyl chlorides from alcohols?

When an alcohol reacts with SOCl2, the products are the alkyl chloride along with gaseous SO2 and HCl, both of which escape from the reaction mixture.
Since the by-products are gases, no purification step is needed and the alkyl chloride is obtained in a pure form directly.
In contrast, reactions with PCl5 give POCl3 as a by-product which is difficult to separate, and reactions with concentrated HCl require a ZnCl2 catalyst for primary and secondary alcohols.

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Question 4

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Define optical isomerism and enantiomers. Give one example of an enantiomeric pair from this chapter.

Optical isomerism is the phenomenon in which compounds rotate the plane of plane-polarised light; the (+) and (–) forms of a compound are called optical isomers.
Enantiomers are stereoisomers that are non-superimposable mirror images of each other; they have identical physical properties except for the direction of rotation of plane-polarised light.
An example of an enantiomeric pair is (+)-butan-2-ol and (–)-butan-2-ol, both derived from butan-2-ol which contains a chiral carbon bearing four different groups (–H, –OH, –CH3, –C2H5).

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Question 5

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State Zaitsev's rule and illustrate with the dehydrohalogenation of 2-bromopentane.

Zaitsev's rule states that in dehydrohalogenation reactions, the preferred product is the alkene with the greater number of alkyl groups attached to the doubly bonded carbon atoms (i.e., the more substituted alkene).
When 2-bromopentane is treated with alcoholic KOH, β-elimination occurs; hydrogen from the β-carbon and Br from the α-carbon are removed.
The major product is pent-2-ene (81%), which is more substituted than pent-1-ene (19%), consistent with Zaitsev's rule.

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Question 6

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Distinguish between a gem-dihalide and a vic-dihalide with one example each.

Gem-dihalides (geminal dihalides) are compounds in which both halogen atoms are present on the same carbon atom; for example, 1,1-dichloroethane (CH3CHCl2), common name ethylidene chloride.
Vic-dihalides (vicinal dihalides) are compounds in which the halogen atoms are on adjacent carbon atoms; for example, 1,2-dichloroethane (ClCH2CH2Cl), common name ethylene dichloride.
In IUPAC nomenclature both are named as dihaloalkanes, but gem-dihalides are also called alkylidene dihalides and vic-dihalides are called alkylene dihalides in the common system.

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Question 7

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What are Grignard reagents? Why must their preparation be carried out in dry ether?

Grignard reagents are organomagnesium halides of formula RMgX, prepared by reacting a haloalkane or haloarene with magnesium metal in dry ether. In RMgX, the C–Mg bond is covalent but highly polar and the Mg–X bond is essentially ionic.
Grignard reagents are extremely reactive and react with any source of proton (including water, alcohols, amines) to give the corresponding hydrocarbon, destroying the reagent.
Dry ether is used to exclude even traces of moisture, which would decompose the Grignard reagent. Ether also stabilises the reagent through coordination with magnesium.

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Question 8

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Explain why the boiling point of alkyl halides follows the order RI > RBr > RCl > RF for the same alkyl group.

The boiling point of alkyl halides depends on the strength of intermolecular van der Waals forces, which in turn depend on the size and polarisability of the molecules.
As the size and atomic mass of the halogen atom increases from F to I, the molecular size and number of electrons in the molecule increase, enhancing van der Waals forces.
Stronger van der Waals forces require more energy to overcome during boiling; therefore, RI has the highest boiling point and RF has the lowest for the same alkyl group.

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Question 9

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What is a racemic mixture? How is it formed in SN1 reactions of optically active haloalkanes?

A racemic mixture (dl or ± mixture) is a mixture containing two enantiomers in equal (50:50) proportions; it has zero optical rotation because the rotations of the two enantiomers cancel each other.
In the SN1 mechanism, the rate-determining step produces a planar, sp2-hybridised carbocation intermediate that is achiral.
The nucleophile can attack this planar carbocation from either face with equal probability, giving a 50:50 mixture of both enantiomers (racemisation).

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Question 10

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What are ambident nucleophiles? Give the products formed when KCN and AgCN react separately with an alkyl halide.

Ambident nucleophiles are species that possess two nucleophilic centres and can attack through either atom; examples are cyanide (CN–) and nitrite (NO2–) ions.
KCN is predominantly ionic and provides free CN– ions, which attack mainly through carbon (as C–C bond is more stable than C–N bond), giving alkyl cyanides (nitriles, R–CN) as the main product.
AgCN is covalent in nature, making nitrogen available to donate its electron pair; it therefore gives isocyanides (isonitriles, R–NC) as the main product.

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Question 11

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Explain the Sandmeyer's reaction with a relevant equation.

Sandmeyer's reaction converts a primary aromatic amine into an aryl halide via a diazonium salt intermediate. The amine is first treated with NaNO2 and HX at 273–278 K to form the benzene diazonium halide.
The freshly prepared diazonium salt solution is then mixed with cuprous chloride (CuCl) or cuprous bromide (CuBr), which replaces the diazonium group (–N2+) with –Cl or –Br respectively, releasing N2 gas.
For introduction of iodine, no cuprous halide is needed; the diazonium salt is simply shaken with potassium iodide (KI) to give the aryl iodide.

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Question 12

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Why do haloalkanes have higher boiling points than hydrocarbons of comparable molecular mass?

Molecules of organic halogen compounds are polar due to the electronegativity difference between carbon and the halogen, resulting in significant dipole-dipole interactions between molecules.
In addition, the larger molecular mass compared to the parent hydrocarbon increases the magnitude of van der Waals (London dispersion) forces.
Both dipole-dipole and van der Waals forces are stronger in haloalkanes than in hydrocarbons of comparable molecular mass, so more energy is required to overcome these attractions during boiling.

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Question 13

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Give the IUPAC name and classify the type of each of the following: (i) CH2=CHCl (ii) C6H5CH2Cl.

CH2=CHCl has the IUPAC name chloroethene (common name: vinyl chloride); it is classified as a vinylic halide because the chlorine is bonded to an sp2-hybridised carbon of a C=C double bond.
C6H5CH2Cl has the IUPAC name chlorophenylmethane (common name: benzyl chloride); it is classified as a primary benzylic halide because the chlorine is bonded to an sp3-hybridised carbon directly attached to the aromatic ring.
Vinylic halides are very unreactive towards nucleophilic substitution, while benzylic halides are highly reactive towards both SN1 and SN2 reactions due to resonance stabilisation of the intermediate.

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Chapter 6: Haloalkanes and Haloarenes | Short Questions

Explain why haloalkanes are more reactive than haloarenes towards nucleophilic substitution reactions.

What is the Finkelstein reaction? State the condition that drives it to completion.

Why is thionyl chloride (SOCl2) preferred over PCl5 or HCl for preparing alkyl chlorides from alcohols?

Define optical isomerism and enantiomers. Give one example of an enantiomeric pair from this chapter.

State Zaitsev's rule and illustrate with the dehydrohalogenation of 2-bromopentane.

Distinguish between a gem-dihalide and a vic-dihalide with one example each.

What are Grignard reagents? Why must their preparation be carried out in dry ether?

Explain why the boiling point of alkyl halides follows the order RI > RBr > RCl > RF for the same alkyl group.

What is a racemic mixture? How is it formed in SN1 reactions of optically active haloalkanes?

What are ambident nucleophiles? Give the products formed when KCN and AgCN react separately with an alkyl halide.

Explain the Sandmeyer's reaction with a relevant equation.

Why do haloalkanes have higher boiling points than hydrocarbons of comparable molecular mass?

Give the IUPAC name and classify the type of each of the following: (i) CH2=CHCl (ii) C6H5CH2Cl.

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