Hydrocarbon flash cards

1. 1.What is the general formula and hybridization of carbon in alkanes?

   General formula $C_nH_{2n+2}$; each carbon is $sp^3$ hybridized with bond angle $109.5^\circ$ (tetrahedral). All C–C and C–H are single ($\sigma$) bonds.

   Hint: Saturated, single bonds only.

2. 2.State the Wurtz reaction and its purpose.

   $2\,R\text{-X} + 2\,Na \xrightarrow{\text{dry ether}} R\text{-}R + 2\,NaX$. It couples two alkyl halides to make a symmetrical alkane with double the carbons.

   Hint: Alkyl halide + Na in dry ether.

3. 3.Why is Wurtz reaction unsuitable for preparing alkanes with an odd number of carbons or unsymmetrical alkanes?

   Two different halides give a mixture of three products (R–R, R'–R', R–R') that are hard to separate. It works cleanly only for symmetrical alkanes.

   Hint: Cross vs self coupling.

4. 4.Describe Kolbe's electrolytic method for alkanes.

   Electrolysis of aqueous sodium/potassium salt of a carboxylic acid: $2\,RCOO^- \xrightarrow{\text{electrolysis}} R\text{-}R + 2CO_2 + 2e^-$ at the anode. Gives symmetrical alkane.

   Hint: Anode decarboxylation of carboxylate.

5. 5.What is decarboxylation and its reagent for making alkanes?

   Sodium salt of a carboxylic acid heated with soda lime ($NaOH + CaO$) loses $CO_2$: $RCOONa + NaOH \xrightarrow{CaO,\Delta} R\text{-}H + Na_2CO_3$. Product has one fewer carbon.

   Hint: Soda lime, loses CO2.

6. 6.How is methane prepared by decarboxylation? Write the reaction.

   $CH_3COONa + NaOH \xrightarrow{CaO,\Delta} CH_4 + Na_2CO_3$. Sodium acetate + soda lime gives methane.

   Hint: Sodium acetate.

7. 7.What does hydrogenation of alkenes/alkynes give, and what is the catalyst?

   Alkene/alkyne $+ H_2 \xrightarrow{Ni/Pt/Pd}$ alkane (Sabatier–Senderens reaction with Ni). Unsaturation is removed by syn addition of $H_2$.

   Hint: H2 over Ni/Pt/Pd.

8. 8.What alkane forms when a Grignard reagent reacts with water?

   $RMgX + H_2O \rightarrow R\text{-}H + Mg(OH)X$. The alkane has the same number of carbons as R. Any source of active H ($H_2O$, alcohols, $NH_3$) works.

   Hint: Grignard + active hydrogen.

9. 9.Compare the two staggered conformations of ethane in terms of stability.

   Ethane has staggered (dihedral $60^\circ$) and eclipsed ($0^\circ$) forms. Staggered is more stable by about $12.5\ \text{kJ/mol}$ due to minimum torsional strain.

   Hint: Torsional strain minimized when H's are apart.

10. 10.Rank the four conformations of n-butane (about C2–C3) by stability.

    anti ($180^\circ$) > gauche ($60^\circ$) > eclipsed (partial, $120^\circ$) > fully eclipsed / syn ($0^\circ$). Anti is most stable (methyls farthest apart).

    Hint: Anti = both methyls opposite.

11. 11.Why is the gauche conformation of n-butane less stable than anti?

    In gauche the two methyl groups are $60^\circ$ apart, causing steric (van der Waals) strain. In anti they are $180^\circ$ apart, minimizing steric repulsion.

    Hint: Steric crowding of bulky groups.

12. 12.Do conformations of ethane/butane represent different compounds? Why can't they be isolated?

    No — they are non-separable spatial arrangements interconvertible by rotation about the C–C $\sigma$ bond. The energy barrier is very small, so rotation is fast at room temperature.

    Hint: Rapid interconversion, low barrier.

13. 13.By what mechanism does chlorination of methane proceed?

    Free-radical substitution (chain mechanism) via three steps: initiation, propagation, termination. Needs UV light or heat.

    Hint: Free radicals, needs $h\nu$.

14. 14.Write the initiation step of methane chlorination.

    $Cl_2 \xrightarrow{h\nu} 2\,Cl^\bullet$ — homolytic cleavage of the Cl–Cl bond by UV light generates chlorine radicals.

    Hint: Homolysis of the halogen.

15. 15.Write the two propagation steps of methane chlorination.

    $Cl^\bullet + CH_4 \rightarrow CH_3^\bullet + HCl$ then $CH_3^\bullet + Cl_2 \rightarrow CH_3Cl + Cl^\bullet$. The regenerated $Cl^\bullet$ continues the chain.

    Hint: Radical consumed and regenerated.

16. 16.Give two possible termination steps in methane halogenation.

    Any radical–radical combination, e.g. $Cl^\bullet + Cl^\bullet \rightarrow Cl_2$, $CH_3^\bullet + CH_3^\bullet \rightarrow C_2H_6$, or $CH_3^\bullet + Cl^\bullet \rightarrow CH_3Cl$.

    Hint: Two radicals combine, chain ends.

17. 17.Why does chlorination of methane give a mixture of products?

    Once $CH_3Cl$ forms it still has C–H bonds that undergo further substitution, giving $CH_2Cl_2$, $CHCl_3$ and $CCl_4$. Poly-substitution cannot be avoided.

    Hint: Product still has abstractable H.

18. 18.What is the order of ease of free-radical halogen substitution of C–H bonds?

    $3^\circ > 2^\circ > 1^\circ$ (tertiary hydrogen abstracted most easily) because a more substituted radical is more stable.

    Hint: More stable radical forms fastest.

19. 19.Compare reactivity and selectivity of chlorine vs bromine in radical halogenation.

    $Cl_2$ is more reactive but less selective; $Br_2$ is less reactive but highly selective (strongly prefers $3^\circ$ H). Fluorination is explosive, iodination is reversible/does not occur.

    Hint: Reactivity vs selectivity trade-off.

20. 20.What is the combustion reaction of an alkane and its general balanced form?

    $C_nH_{2n+2} + \left(\dfrac{3n+1}{2}\right)O_2 \rightarrow n\,CO_2 + (n+1)H_2O$. Complete combustion is highly exothermic.

    Hint: Complete oxidation to CO2 + H2O.

21. 21.What happens on incomplete combustion of alkanes (limited $O_2$)?

    Formation of carbon monoxide ($CO$) and/or carbon (soot) plus water, e.g. $2CH_4 + 3O_2 \rightarrow 2CO + 4H_2O$. Less energy released.

    Hint: Insufficient oxygen → CO/C.

22. 22.What is controlled oxidation of methane with a catalyst?

    $CH_4 + \tfrac{1}{2}O_2 \xrightarrow{Cu,523K,100atm} CH_3OH$ (methanol); with $Mo_2O_3$ it gives $HCHO$ (methanal). Partial oxidation gives specific oxygenated products.

    Hint: Cu → methanol; Mo2O3 → methanal.

23. 23.What is isomerisation of alkanes and its catalyst?

    Straight-chain alkane converts to branched-chain isomer on heating with anhydrous $AlCl_3/HCl$, e.g. n-hexane $\rightarrow$ 2-methylpentane. Raises octane number.

    Hint: n-alkane → branched, AlCl3.

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