Why does HBr give anti-Markovnikov addition with peroxides but HCl and HI do not?

Energetics: HBr's homolysis (H-Br BDE 366 kJ/mol) and Br radical addition to the alkene are both favourable. For HCl, the propagation step is too endothermic; for HI, the I-H BDE is too low and I radical addition is reversible.

Why does benzene undergo substitution rather than addition?

Addition would destroy the aromatic 6-pi-electron system worth ~150 kJ/mol of resonance stabilisation. Substitution preserves aromaticity, restoring it after the electrophile departs (usually as H+).

Why are halogens ortho/para directing despite being deactivating?

Their -I effect deactivates the ring (slowing reaction), but their lone pairs donate +M into ortho and para positions, stabilising the sigma-complex at those sites.

What is the difference between hydroboration-oxidation and acid-catalysed hydration?

H2SO4/H2O gives Markovnikov alcohols (via carbocation, can rearrange). BH3 then H2O2/OH- gives anti-Markovnikov alcohols (concerted, no rearrangement, syn addition).

Why does Markovnikov's rule work?

The proton adds to the alkene to form the more stable carbocation; tertiary > secondary > primary by hyperconjugation and induction. Br- then attacks this stabler cation.

HBr adds to propene in the presence of peroxide to give predominantly:

1-bromopropane. Peroxides switch the mechanism to free-radical, giving anti-Markovnikov addition; Br adds to the terminal (less substituted) carbon.

Ozonolysis of 2-methylbut-2-ene followed by Zn/H2O gives:

Acetone and acetaldehyde. (CH3)2C=CHCH3 cleaves at the C=C: (CH3)2C=O (acetone) and CH3CHO (acetaldehyde).

Which catalyst gives cis-alkenes from alkynes?

Lindlar's catalyst. Lindlar (Pd/CaCO3 poisoned with Pb) does syn addition of H2 to give cis-alkene; Na/NH3 gives trans.

In Friedel-Crafts alkylation of benzene with 1-chloropropane and AlCl3, the major product is:

Isopropylbenzene (cumene). The primary propyl cation rearranges via 1,2-H shift to the more stable secondary isopropyl cation before attack on benzene.

Which reagent distinguishes a terminal alkyne from an internal alkyne?

Tollens' reagent (AgNO3/NH3). Terminal alkynes (RC≡CH) form a white silver acetylide precipitate with Tollens' reagent; internal alkynes have no acidic H and do not react.

Hydrocarbons MDCAT 2026 — Notes, Key Concepts & MCQs (Chemistry)

Hydrocarbons

Hydrocarbons averages 3 MCQs per MDCAT paper, dominated by Markovnikov addition, ozonolysis, Friedel-Crafts, and free-radical halogenation.

Hydrocarbons is a Chemistry chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 3 MCQs to the 45-MCQ Chemistry section. Mastering the core concepts below typically secures the full chapter weightage.

Alkanes — saturated, sp3, and sluggish

Alkanes (C_nH_2n+2) react only by free-radical substitution and combustion. Halogenation: CH₄ + Cl₂ → CH₃Cl + HCl proceeds by initiation (Cl₂ → 2Cl• under UV), propagation (Cl• + CH₄ → HCl + CH₃•; CH₃• + Cl₂ → CH₃Cl + Cl•), and termination. Selectivity follows radical stability 3° > 2° > 1°, but Cl is less selective than Br because of its highly exothermic abstraction step (Hammond postulate, Clayden Chapter 39). Synthesis: Wurtz reaction (2RX + 2Na → R-R + 2NaX) couples two alkyl halides; Kolbe's electrolysis of carboxylate salts gives R-R at the anode.

Alkenes — electrophilic addition central

The π-bond is electron-rich and attacks electrophiles. HBr addition to propene gives 2-bromopropane via the more stable secondary carbocation (Markovnikov rule: H goes to the carbon with more H's, Br to the more substituted carbon). With peroxides (ROOR), the regiochemistry inverts: HBr adds anti-Markovnikov via a free-radical mechanism, giving 1-bromopropane. Hydration with dilute H₂SO₄ gives Markovnikov alcohols; oxymercuration-demercuration achieves the same without rearrangement; hydroboration-oxidation (BH₃ then H₂O₂/OH⁻) gives anti-Markovnikov alcohols. Bromine adds via a bromonium ion to give anti dibromides. Ozonolysis (O₃ then Zn/H₂O) cleaves C=C to two carbonyls — used to locate double-bond positions. KMnO₄ (cold, dilute) gives a syn-diol (Baeyer's test); hot KMnO₄ cleaves to acids/ketones.

Alkynes — terminal acidity and triple-bond chemistry

Terminal alkynes (RC≡CH) are weakly acidic (pK_a ≈ 25); NaNH₂ deprotonates them to acetylides RC≡C⁻, powerful nucleophiles for SN2 alkylation. Hydration (HgSO₄/H₂SO₄) gives an enol that tautomerises to a methyl ketone (Markovnikov), except for ethyne which gives acetaldehyde. Hydroboration-oxidation gives the anti-Markovnikov aldehyde. Lindlar's catalyst (Pd/CaCO₃ + Pb) gives cis-alkenes; Na/liq NH₃ gives trans-alkenes. Cu₂Cl₂/NH₄Cl gives a red Cu acetylide, AgNO₃/NH₃ gives a white Ag acetylide — classic identification tests for terminal alkynes.

Aromatics and Friedel-Crafts

Benzene's six π electrons form a delocalised system stabilised by 150 kJ/mol of resonance energy. It undergoes electrophilic aromatic substitution (EAS) rather than addition, preserving aromaticity. The five named EAS reactions: nitration (HNO₃/H₂SO₄, electrophile NO₂⁺), sulphonation (oleum, SO₃), halogenation (X₂/FeX₃), Friedel-Crafts alkylation (RCl/AlCl₃, electrophile R⁺ — beware rearrangements!), Friedel-Crafts acylation (RCOCl/AlCl₃, electrophile RC=O⁺). Activating, ortho/para-directing groups: -OH, -NH₂, -OR, -R. Deactivating, meta-directing: -NO₂, -CN, -COOH, -CHO. The halogens are deactivating but ortho/para-directing — the classic exception.

Mechanism summary and trap-spotters

Markovnikov vs anti-Markovnikov hinges on whether the intermediate is a carbocation (Markovnikov, polar HX) or a radical (anti-Markovnikov, HBr + ROOR). Carbocations rearrange (1,2-H or 1,2-CH₃ shifts) — this is why Friedel-Crafts alkylation of benzene with 1-chloropropane gives mostly isopropylbenzene. Aldol, Cannizzaro, and other carbonyl reactions belong to the next chapter; for hydrocarbons, fixate on ozonolysis products, Lindlar/Na-NH₃ stereochemistry, and EAS directing effects. Morrison & Boyd Chapters 6-12 and FSc Chemistry XII Chapters 9-10 cover these in MDCAT depth.

Key Concepts

Worked MCQs

Frequently Asked Questions

How Hydrocarbons Is Tested

MDCAT questions on Hydrocarbons are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.