Hydrocarbons

Q1. (3 marks) Wurtz reaction: $2\,CH_3CH_2Br + 2Na \rightarrow CH_3CH_2{-}CH_2CH_3 + 2NaBr$

• Product = n-butane ($C_4H_{10}$). (2 marks correct product/eqn, 1 mark name)
• Why: Two alkyl halide fragments couple via sodium; carbon count doubles ($2\times2 = 4$).

Q2. (4 marks)

• Markovnikov's rule: In addition of a protic acid (HX) to an unsymmetrical alkene, the H atom adds to the carbon bearing more H atoms; the halide adds to the more substituted carbon (via more stable carbocation). (2 marks)
• (a) No peroxide → 2-bromopropane $CH_3CHBrCH_3$ (Markovnikov). (1 mark)
• (b) With peroxide (Kharasch effect) → 1-bromopropane $CH_3CH_2CH_2Br$ (anti-Markovnikov, radical mechanism). (1 mark)

Q3. (4 marks)

• Hückel's rule: A planar, cyclic, fully conjugated ring is aromatic if it contains $(4n+2)$ $\pi$ electrons ($n = 0,1,2,\dots$). (2 marks)
• Cyclopentadienyl anion: 5-membered ring, fully conjugated, planar. It has 6 $\pi$ electrons (4 from two double bonds + 2 from the negative-charge lone pair). (1 mark count)
• $4n+2 = 6 \Rightarrow n=1$, satisfies rule → aromatic. (1 mark)

Q4. (4 marks)

• Staggered: front and back H atoms offset by $60°$ dihedral; Eclipsed: H atoms overlap (0° dihedral). (2 marks for correct projections)
• Staggered is more stable by about 12.5 kJ/mol (torsional/eclipsing strain in eclipsed form). (2 marks)

Q5. (5 marks, ~1.7 each) (a) $CH_2{=}CH_2$ (ethene) — acid-catalysed dehydration of ethanol. (2 marks) (b) $BrCH_2{-}CH_2Br$ = 1,2-dibromoethane (anti addition). (1.5 marks) (c) $CH_3CHO$ → tautomerises to acetaldehyde (ethanal); via Markovnikov hydration of ethyne. (1.5 marks)

Q6. (4 marks)

• Baeyer's strain theory: Rings other than the "ideal" $109.5°$ tetrahedral angle suffer angle (bond) strain; deviation from $109.5°$ makes small/large rings strained and reactive. (2 marks)
• Angle strain (Baeyer's) $= \tfrac{1}{2}(109.5° - \text{ring angle}) = \tfrac{1}{2}(109.5° - 60°) = \tfrac{1}{2}(49.5°) = 24.75°$. (2 marks)

Q7. (4 marks, 1 each)

(Cl is the special case: deactivating but o/p-directing.)

Q8. (3 marks) Reductive ozonolysis of $CH_3CH{=}CHCH_3$ cleaves the C=C → two carbonyl fragments: $2\,CH_3CHO$ = two molecules of acetaldehyde (ethanal). (2 marks product, 1 mark name)

• Why: Symmetrical alkene gives identical aldehyde on each side; $Zn/H_2O$ (reductive) gives aldehyde not acid.

Q9. (4 marks)

• Terminal alkyne $\equiv$C–H: the C is $sp$-hybridised (50% s-character), holding the bonding electrons closer to nucleus → the H is more easily removed; conjugate base (acetylide) is stabilised → acidic ($pK_a \approx 25$). (2 marks)
• Reaction: $CH{\equiv}CH + NaNH_2 \rightarrow CH{\equiv}C^-Na^+ + NH_3$ (sodium acetylide). (2 marks)

Q10. (5 marks)

• Hydroboration–oxidation of propene gives propan-1-ol $CH_3CH_2CH_2OH$. (3 marks)
• Boron (electrophilic) adds to the less hindered/less substituted carbon; on oxidation OH ends up there → anti-Markovnikov, syn addition. (2 marks)

[
  {"claim":"Wurtz of 2 ethyl bromides gives C4 (butane), carbon count 4","code":"c_in=2; c_out=2*c_in; result=(c_out==4)"},
  {"claim":"Cyclopentadienyl anion has 6 pi electrons satisfying 4n+2 with n=1","code":"pi=6; n=(pi-2)/4; result=(n==1)"},
  {"claim":"Baeyer angle strain in cyclopropane = 24.75 degrees","code":"strain=Rational(1,2)*(Rational(1095,10)-60); result=(strain==Rational(495,20))"},
  {"claim":"Ozonolysis of but-2-ene (symmetric) yields 2 equivalents of a C2 aldehyde","code":"carbons_alkene=4; frag=carbons_alkene//2; result=(frag==2)"}
]