NCERT MCQ Solutions for Class 12 Chemistry Chapter 6 Haloalkanes and Haloarenes

Haloalkanes as containing halogen atom(s) attached to the sp³ hybridised carbon atom of an alkyl group.

Allylic halides as compounds where halogen is bonded to an sp³ hybridised carbon atom adjacent to a C=C double bond (the allylic carbon).

Aryl halides as compounds where the halogen atom is directly bonded to the sp² hybridised carbon atom of an aromatic ring.

In the IUPAC system, alkyl halides are named as halosubstituted hydrocarbons (e.g., 1-Bromopropane).

Dihalo-compounds with both halogens on the same carbon atom as geminal halides or gem-dihalides.

Halogen atoms are more electronegative than carbon, leading to a polarized C–X bond with a partial positive charge on carbon (δ⁺) and a partial negative charge on halogen (δ⁻).

Thionyl chloride is preferred because the other products, SO₂ and HCl, are gases and escape, yielding pure alkyl halide.

This specific halogen exchange reaction (R–X + NaI → R–I + NaX) is identified as the Finkelstein reaction.

The Swarts reaction as the heating of alkyl chloride/bromide in the presence of a metallic fluoride (like AgF, Hg₂F₂) to produce alkyl fluorides.

Sandmeyer’s reaction involves forming a diazonium salt from a primary aromatic amine, which is then reacted with cuprous chloride or bromide to yield the aryl halide.

Boiling points decrease in the order RI > RBr > RCl > RF, because the magnitude of van der Waals forces increases with the size and mass of the halogen atom.

Boiling points of isomeric haloalkanes decrease with increase in branching, likely due to decreased surface area affecting van der Waals forces.

Low solubility by stating that the energy needed to overcome haloalkane attractions and break water H-bonds is not sufficiently compensated by the energy released when weaker new attractions form between haloalkanes and water.

The SN2 mechanism as a bimolecular, single-step process where the nucleophile attacks as the leaving group departs, proceeding through a transition state.

SN2 reactivity decreases with increasing steric hindrance around the carbon atom, leading to the order:
Methyl > Primary > Secondary > Tertiary.

The SN1 reaction proceeds in two steps: slow formation of a carbocation (rate-determining step), followed by rapid attack by the nucleophile.

SN1 reactions are accompanied by racemisation because the intermediate planar carbocation can be attacked by the nucleophile from either side.

This reaction, where a hydrogen from the β-carbon and halogen from the α-carbon are removed to form an alkene, as β-elimination.

Zaitsev’s rule is summarised in the chapter as favouring the formation of the more substituted alkene (having more alkyl groups on the double bond carbons).

The resonance effect which imparts partial double bond character to the C–X bond, making bond cleavage more difficult.