CUET UG Chemistry Booster Test -3 Classification of Halo Compounds

Among the given organohalogen compounds, density is influenced by molecular mass and halogen content. Iodine-containing compounds generally have higher density because iodine is very heavy. Carbon tetrachloride and chloroform are also dense due to multiple chlorine atoms. The decreasing order is: n-C₃H₇I > CCl₄ > CHCl₃ > n-C₃H₇Cl So the correct sequence is: (A), (B), (C), (D)

The C—F bond has the highest bond enthalpy because fluorine is small and forms a strong bond with carbon. Bond length increases from C—F to C—I because halogen atomic size increases down the group: F < Cl < Br < I So: C—F < C—Cl < C—Br < C—I Although fluorine is most electronegative, the C—Cl bond has slightly higher dipole moment than C—F because dipole moment depends on both charge separation and bond length. Statement D is incorrect because C—I does not have the highest dipole moment.

In an SN2 reaction, the nucleophile attacks the carbon from the side opposite to the leaving group. This causes inversion of configuration, also called Walden inversion. Here, hydroxide ion replaces bromide in optically active 2-bromooctane, forming octan-2-ol with inverted configuration.

Dipole moment represents the extent of charge separation in a polar bond. In haloalkanes, the C—X bond is polar due to the higher electronegativity of halogens. In NCERT tables, dipole moment values are commonly expressed in Debye (D).

Retention of configuration means the spatial arrangement around the chiral carbon remains unchanged. Inversion of configuration is typical of SN2 reactions because of backside attack. Racemisation is commonly seen in SN1 reactions because the planar carbocation can be attacked from both sides. A chiral molecule is one that is non-superimposable on its mirror image. Thus: 1-a, 2-b, 3-c, 4-d

Alkenes decolourise bromine in CCl₄ because bromine adds across the carbon-carbon double bond. Example: CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br The two bromine atoms are attached to adjacent carbon atoms. Such compounds are called vicinal dibromides.

Chloroform is slowly oxidized by air in the presence of light to form phosgene, which is highly poisonous. To prevent this oxidation, chloroform is stored in dark coloured bottles filled completely to keep air out. Chronic exposure to chloroform can cause liver damage because it may be metabolised to toxic phosgene. Statement C is incorrect because antiseptic action due to free iodine is associated with iodoform, not chloroform.

Freon-12, CCl₂F₂, is a chlorofluorocarbon. It is extremely stable, non-toxic, non-corrosive, and easily liquefiable. It is manufactured from carbon tetrachloride using hydrogen fluoride in the presence of antimony pentachloride, known as Swarts reaction.

Primary alkyl halides have the halogen attached to a carbon bonded to only one alkyl group. This makes the carbon less crowded and more accessible to nucleophilic backside attack. Therefore, primary alkyl halides generally undergo SN2 reactions readily. They do not favour SN1 because primary carbocations are highly unstable.

Secondary alkyl halides can react through different pathways depending on conditions. With suitable nucleophiles, they can undergo SN2 reactions. In polar protic solvents, they may form secondary carbocations and undergo SN1 reactions. With strong bases, β-elimination can occur to form alkenes. Therefore, secondary alkyl halides can undergo SN1, SN2, or elimination.

The structure (CH₃)₃CBr contains a central carbon attached to three methyl groups and one bromine atom. The longest chain contains three carbon atoms, so the parent hydrocarbon is propane. Both bromine and methyl substituents are present on carbon-2. Therefore, the correct IUPAC name is: 2-Bromo-2-methylpropane

Tertiary alkyl halides are sterically crowded around the carbon attached to the halogen. Bulky nucleophiles or bases find it difficult to perform backside attack required for SN2 substitution. Instead, they abstract a β-hydrogen, leading to dehydrohalogenation and formation of an alkene. Thus, elimination becomes the dominant pathway.

Reactivity of alcohols with haloacids follows: Tertiary > Secondary > Primary because tertiary alcohols form more stable carbocation-like intermediates. SN2 reactivity follows: Primary > Secondary > Tertiary because steric hindrance increases from primary to tertiary. SN1 reactivity follows: 3° > 2° > 1° because carbocation stability increases in the same order. Bond enthalpy order is: C—F > C—Cl > C—Br > C—I because bond strength decreases as halogen size increases. Therefore: 1-a, 2-b, 3-c, 4-d

In SN1 reactions, the rate depends on the stability of the carbocation intermediate. Allylic halides form allylic carbocations after loss of halide ion. These carbocations are stabilised by resonance with the adjacent carbon-carbon double bond. Example: CH₂=CH–CH₂⁺ ↔ CH₂⁺–CH=CH₂ This resonance stabilization makes allylic halides highly reactive towards SN1 reactions.

Benzylic halides are reactive in SN1 because the carbocation formed is resonance stabilised by the aromatic ring. Carbocation stability order here is: C₆H₅C(CH₃)(C₆H₅)⁺ > C₆H₅CH(C₆H₅)⁺ > C₆H₅CH(CH₃)⁺ > C₆H₅CH₂⁺ The first carbocation is most stable because it is stabilised by two phenyl groups and one methyl group. The last is least stabilized among the given set. Therefore, decreasing SN1 reactivity is: (A), (B), (C), (D) Ambident nucleophiles can attack through two different atoms. Cyanide ion can attack through carbon or nitrogen. KCN is ionic and provides CN⁻ freely, so attack usually occurs through carbon, forming alkyl cyanides: R–X + KCN → R–CN AgCN is more covalent, so attack occurs mainly through nitrogen, producing isocyanides: R–X + AgCN → R–NC Statement D is incorrect because nitrite ion is also an ambident nucleophile and can attack through nitrogen or oxygen.

CH₂=CHCl is a two-carbon alkene with chlorine attached to one carbon of the double bond. The parent compound is ethene, and the substituent is chloro. Therefore, the systematic IUPAC name is: Chloroethene

During dehydrohalogenation of alkyl halides, a hydrogen atom and a halogen atom are removed from adjacent carbon atoms to form an alkene. According to Zaitsev's rule, the major product is the more substituted alkene, meaning the alkene with more alkyl groups attached to the double-bonded carbon atoms.

Wurtz-Fittig reaction involves treatment of a mixture of alkyl halide and aryl halide with sodium in dry ether to form alkylbenzene. Fittig reaction involves coupling of two aryl halide molecules using sodium in dry ether. Haloarenes undergo electrophilic substitution at ortho and para positions because halogens donate electron density through resonance. Haloarenes are much less reactive toward nucleophilic substitution because the C–X bond has partial double bond character due to resonance.

In aryl halides, the lone pair on halogen participates in resonance with the π-electrons of the benzene ring. This resonance gives the C—Cl bond partial double bond character. As a result, the bond becomes shorter and stronger than the C—Cl bond in alkyl halides. Because bond cleavage becomes difficult, aryl halides are less reactive towards nucleophilic substitution.

Ambident nucleophiles can attack through two different atoms. Cyanide ion can attack through carbon or nitrogen.

KCN is ionic and provides CN⁻ freely, so attack usually occurs through carbon, forming alkyl cyanides:

R–X + KCN → R–CN

AgCN is more covalent, so attack occurs mainly through nitrogen, producing isocyanides:

R–X + AgCN → R–NC

Statement D is incorrect because nitrite ion is also an ambident nucleophile and can attack through nitrogen or oxygen.