Haloalkanes General methods of preparation, properties, and reactions; Nature of C-X bond; Mechanisms of substitution reactions.: NEET MCQs with Answers

Explanation: An S_N2 reaction proceeds by backside attack of the nucleophile on the carbon bearing the leaving group, through a crowded five-coordinate transition state. Anything that blocks the back face slows the reaction, so the rate is controlled by steric hindrance. All four substrates are primary bromides, so the difference lies in the bulk of the group next to the reacting carbon. CH₃CH₂Br has only a small methyl group adjacent and is the least hindered. CH₃CH₂CH₂Br has an ethyl group adjacent, (CH₃)₂CHCH₂Br a bulky isopropyl group, and (CH₃)₃CCH₂Br a very bulky tert-butyl group; such neopentyl halides are notoriously unreactive in S_N2. Bromoethane is therefore the fastest.

Explanation: Racemisation can only be observed if the molecule has a chiral centre at the reacting carbon; otherwise there is no configuration to scramble. (i) Benzyl chloride, C₆H₅CH₂Cl: the reacting carbon carries two hydrogens, so it is achiral. (ii) 1-Chloropropane, CH₃CH₂CH₂Cl: the reacting carbon carries two hydrogens, so it is achiral. (iii) Isobutyl chloride, (CH₃)₂CHCH₂Cl: the reacting carbon carries two hydrogens, so it is achiral. (iv) sec-Butyl chloride, CH₃CH(Cl)CH₂CH₃: the reacting carbon carries CH₃, C₂H₅, Cl and H, four different groups, so it is a genuine chiral centre. Only sec-butyl chloride can racemise, via the planar carbocation of the S_N1 pathway.

Explanation: An S_N1 reaction requires the formation of a reasonably stable carbocation in the slow step. C₆H₅CH₂CH₂Cl (2-phenylethyl chloride) would give a primary carbocation with the ring one carbon too far away to help by resonance. Worse, the phenyl group exerts a −I effect that actively destabilises the positive charge. This carbocation is too unstable to form, so no S_N1 occurs. The other three all give stabilised cations: allyl chloride gives a resonance-stabilised allyl carbocation; tert-butyl chloride gives a tertiary carbocation with nine hyperconjugative C—H bonds; and benzyl chloride gives a resonance-stabilised benzyl carbocation.

Explanation: Racemisation needs a chiral carbon that reacts through a planar carbocation. Only in (iv) does the carbon bearing chlorine hold four different groups (H, CH₃, Cl and vinyl), and its allylic cation is stabilised, so attack from both faces gives a racemic product. In the other three the reacting carbon is not a stereocentre.

Explanation: The Wurtz reaction couples two identical alkyl groups, so it can only give alkanes with an EVEN number of carbon atoms. n-Heptane has seven carbons and would need two different halides, which produces a mixture in poor yield.

Explanation: Lactic acid has a single chiral centre, so its two stereoisomers are non-superimposable mirror images of each other, i.e. a pair of enantiomers.

Explanation: Although the carbocation is planar, the departing halide ion shields one face for a short time, so attack from the opposite side is slightly favoured. The result is partial racemisation with a small excess of the inverted product.

Explanation: Optical activity requires a chiral centre, a carbon with four different groups. In 2-chlorobutane C-2 carries H, Cl, CH₃ and C₂H₅; butan-1-ol, propan-1-ol and heptan-4-ol have no such carbon, the C-4 of heptan-4-ol carrying two identical propyl groups.

Explanation: Reaction of an alcohol with HBr begins with protonation of –OH, and for tertiary alcohols loss of water then gives a carbocation. 2-Methylpropan-2-ol is the only tertiary alcohol here and forms the stable tert-butyl cation, so it reacts fastest (reactivity 3° > 2° > 1°).

Explanation: The outcome depends sharply on temperature. At ordinary temperature chlorine adds across the double bond of propene to give 1,2-dichloropropane. At about 400 °C, however, chlorine dissociates into chlorine atoms and the reaction switches to a free-radical pathway. The radical abstracts a hydrogen from the allylic position — the sp³ carbon next to the double bond — because the resulting allyl radical is stabilised by resonance. The double bond survives intact: CH₃—CH=CH₂ + Cl₂ —400 °C→ ClCH₂—CH=CH₂ + HCl The product is allyl chloride, and the process is called allylic substitution.

Explanation: A Grignard reagent is made by inserting magnesium metal into the carbon–halogen bond of an alkyl (or aryl) halide in dry ether: R—X + Mg —dry ether→ R—Mg—X Dry ether is essential for two reasons: its oxygen lone pairs coordinate to the magnesium and stabilise the reagent, and it excludes moisture. Even a trace of water destroys the reagent, since R—MgX is strongly basic and is instantly protonated to the alkane. Magnesium does not react with alkanes or with aromatic hydrocarbons, since there is no polar bond for it to insert into; the zinc analogue exists but is a different class of reagent (organozinc, used in the Reformatsky reaction).

Explanation: The separation of a racemic modification into its individual d and l enantiomers is called resolution. Since the two enantiomers are identical in all ordinary physical properties, resolution is usually achieved by converting them into a pair of diastereomers using an optically pure chiral reagent, separating those by fractional crystallisation, and then recovering the pure enantiomers. Other standard approaches are chromatography on a chiral stationary phase and biochemical resolution, in which an enzyme destroys or transforms only one enantiomer. Dehydration and dehydrohalogenation are elimination reactions and have nothing to do with separation.

Explanation: Enantiomers have identical physical properties — the same melting point, boiling point and solubility — so they cannot be separated by ordinary crystallisation or distillation. The chemical solution is to react the racemic mixture with a single pure enantiomer of some other chiral substance, called a resolving agent. A racemic acid, for instance, is treated with an optically pure base to give a pair of diastereomeric salts. Diastereomers are not mirror images, so they do have different physical properties and can be separated by fractional crystallisation. Regenerating the original compound from each separated salt then yields the individual enantiomers.

Explanation: A chiral compound needs a carbon carrying four different groups. In 3-methylhexane, C-3 carries H, CH₃, C₂H₅ and n-C₃H₇, all different, whereas in 2-methylhexane, neopentane and isopentane no carbon has four different groups.

Explanation: The conversion is R—Cl + NaOH(aq) → R—OH + NaCl. The hydroxide ion, acting as a nucleophile, replaces the halide ion at the saturated carbon. The carbon skeleton is unchanged and no new π-bond is created — one group has simply been exchanged for another, which is nucleophilic substitution. Dehydrohalogenation and elimination are the same process under different names, and both would remove HX to give an alkene rather than an alcohol. Addition would require an existing multiple bond, which a saturated alkyl halide does not have.