Haloalkanes And Haloarenes – NEET UG ChemistryPractice Questions & PYQs

Free-radical chlorination of $n$-butane generates a planar, $sp^2$-hybridized radical intermediate at the secondary carbon atom. Because this intermediate is flat and symmetrical with respect to the two faces, a chlorine radical can attack from either the top or bottom with equal statistical probability. This non-stereoselective attack forms equal quantities of the dextrorotatory and laevorotatory enantiomers of 2-chlorobutane. Consequently, the resulting product exists entirely as an optically inactive racemic mixture.

Molecules with the formula $\mathrm{C}_{7}\mathrm{H}_{16}$ exhibit optical isomerism only when they possess a chiral center, defined as a carbon atom bonded to four entirely distinct groups. Structural analysis reveals that 2,3-dimethylpentane contains a carbon atom bonded to a methyl group, an ethyl group, a hydrogen atom, and an isopropyl group simultaneously. Because this specific carbon satisfies all criteria for asymmetry without any compensating elements of symmetry, the molecule becomes chiral and capable of optical rotation.

Halogenated aromatic rings react with magnesium metal in dry ether to form organomagnesium halides, commonly known as Grignard reagents. These reagents possess a strongly nucleophilic carbon center that readily abstracts acidic protons from compounds containing hydroxyl groups. When phenylmagnesium chloride interacts with ethanol, the active hydrogen of the alcohol protonates the aromatic ring, regenerating benzene and producing a magnesium ethoxy chloride byproduct.

The bimolecular nucleophilic substitution mechanism proceeds via a concerted transition state where steric hindrance around the electrophilic carbon heavily dictates the reaction rate. Because the incoming nucleophile must perform a backside attack, bulky substituent groups severely impede this approach and slow down the process. Among the given choices, ethyl bromide possesses the least steric crowding compared to the branched or longer-chain alternatives, thereby offering the lowest activation energy barrier and the highest relative rate.

Bimolecular nucleophilic substitution reactions heavily favor primary substrates due to minimal steric hindrance around the reaction center. The reactant $\left(\mathrm{CH}_{3}\right)_{2}\mathrm{CH}-\mathrm{CH}_{2}\mathrm{Br}$ is a primary alkyl halide, where the formation of an unstable primary carbocation is energetically unfavorable for an $\mathrm{S}_{\mathrm{N}}1$ pathway. In the first reaction, ethanol acts as a solvolysis medium forcing a substitution on the primary carbon, while the second reaction employs a strong ethoxide nucleophile that directly attacks in a concerted step. Consequently, both transformations proceed exclusively via the $\mathrm{S}_{\mathrm{N}}2$ mechanism.

At elevated temperatures, unsaturated hydrocarbons undergo free-radical substitution at the allylic position rather than electrophilic addition across the double bond. Passing chlorine gas through propene at 400 °C initiates homolytic cleavage of chlorine molecules to generate chlorine radicals. These radicals abstract an allylic hydrogen atom from the methyl group of propene, leading to the formation of 3-chloropropene, commonly known as allyl chloride.

The synthesis of a Grignard reagent involves the insertion of elemental magnesium metal directly into the polar carbon-halogen bond of an alkyl halide. This reaction must be performed under strictly anhydrous conditions using dry ether to stabilize the newly formed organometallic complex through coordination. The resulting compound features a strongly nucleophilic carbon bonded directly to magnesium halide, creating a versatile intermediate for organic synthesis.