Hydrocarbon flash cards

1. 1.What are hydrocarbons? Give their broad classification.

   Compounds made only of carbon and hydrogen. Classified as saturated (alkanes), unsaturated (alkenes, alkynes), alicyclic (cyclic aliphatic) and aromatic hydrocarbons.

   Hint: Only C and H; based on bonding and rings.

2. 2.Give the general molecular formulae of alkanes, alkenes and alkynes.

   Alkanes $C_nH_{2n+2}$, alkenes $C_nH_{2n}$, alkynes $C_nH_{2n-2}$ (open-chain, one multiple bond for the latter two).

   Hint: Each degree of unsaturation removes 2 H.

3. 3.Why are alkanes called paraffins?

   From Latin parum affinis = little affinity. Alkanes are relatively unreactive toward common ionic reagents because of strong non-polar $C-C$ and $C-H$ sigma bonds.

   Hint: Latin meaning little affinity.

4. 4.Describe the hybridization and geometry of carbon in alkanes.

   Each carbon is $sp^3$ hybridized with tetrahedral geometry and bond angle $109.5^\circ$; all bonds are sigma bonds.

   Hint: Tetrahedral, $109.5^\circ$.

5. 5.What is the Wurtz reaction? Give its use and limitation.

   $2\,R\text{-}X + 2Na \xrightarrow{\text{dry ether}} R\text{-}R + 2NaX$. It gives symmetrical alkanes with even number of carbons. Limitation: mixed alkyl halides give a mixture, so it is unsuitable for unsymmetrical alkanes.

   Hint: Alkyl halide + Na in dry ether.

6. 6.How is an alkane prepared by hydrogenation of an alkene/alkyne?

   $C_nH_{2n} + H_2 \xrightarrow{Pt/Pd/Ni} C_nH_{2n+2}$ (Sabatier–Senderens reaction). Ni needs higher temperature; Pt and Pd act at room temperature.

   Hint: Add $H_2$ across the multiple bond, metal catalyst.

7. 7.What is decarboxylation of sodium salt of a carboxylic acid?

   $R\text{-}COONa + NaOH \xrightarrow{CaO,\ \Delta} R\text{-}H + Na_2CO_3$. The alkane formed has one carbon less than the acid.

   Hint: Soda-lime; loses $CO_2$ as carbonate.

8. 8.What is Kolbe's electrolytic method for alkanes?

   Electrolysis of aqueous sodium/potassium salt of a carboxylic acid gives an alkane at the anode: $2R\text{-}COO^- \rightarrow R\text{-}R + 2CO_2 + 2e^-$ (symmetrical alkane, even carbons).

   Hint: Electrolysis of carboxylate salt, alkane at anode.

9. 9.How does reduction of an alkyl halide give an alkane?

   $R\text{-}X + H_2 \xrightarrow{Zn/HCl \text{ or } Zn\text{-}Cu}$ or with $LiAlH_4 \rightarrow R\text{-}H$. Also reduction of $R\text{-}X$ by nascent hydrogen gives the alkane with the same number of carbons.

   Hint: Zn/acid or $LiAlH_4$ replaces X by H.

10. 10.Why is the boiling point of alkanes increasing with molecular mass but decreasing with branching?

    Larger molecules have greater surface area and stronger van der Waals forces (higher b.p.). Branching makes the molecule more spherical, reducing surface area and forces, so b.p. falls.

    Hint: Surface area controls dispersion forces.

11. 11.Which alkanes are gases, liquids and solids at room temperature?

    $C_1$–$C_4$ gases, $C_5$–$C_{17}$ liquids, $C_{18}$ onwards solids (approximate NCERT ranges).

    Hint: Physical state by chain length.

12. 12.What is meant by conformations of alkanes?

    The different spatial arrangements of atoms obtained by rotation about a $C-C$ single bond. They are non-separable, interconvertible arrangements (rotational isomers).

    Hint: Arise from rotation about $C-C$ sigma bond.

13. 13.Name the two extreme conformations of ethane and identify the more stable one.

    Staggered and eclipsed. Staggered is more stable (lower energy) because H atoms on the two carbons are farthest apart, minimizing torsional strain.

    Hint: H atoms far apart vs directly behind.

14. 14.What is torsional strain in ethane conformations?

    The strain (repulsion) arising when bonding electron pairs on adjacent carbons come close, as in the eclipsed form. It is maximum in eclipsed and minimum in staggered ethane.

    Hint: Repulsion between eclipsing bond pairs.

15. 15.By how much (approximately) is staggered ethane more stable than eclipsed?

    About $12.5\ \text{kJ mol}^{-1}$. This small energy barrier allows nearly free rotation about the $C-C$ bond at room temperature.

    Hint: Roughly 12.5 kJ/mol energy barrier.

16. 16.Why can conformers of ethane not be isolated?

    The energy barrier to rotation ($\sim 12.5\ \text{kJ mol}^{-1}$) is too small; thermal energy at room temperature allows rapid interconversion, so individual conformers cannot be separated.

    Hint: Rotation barrier is very low.

17. 17.What type of reaction do alkanes typically undergo and why?

    Free-radical substitution reactions (e.g., halogenation). Being non-polar and saturated with strong sigma bonds, they resist ionic addition and prefer radical substitution.

    Hint: Substitution, via free radicals.

18. 18.Write the overall reaction for chlorination of methane.

    $CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$; further substitution gives $CH_2Cl_2$, $CHCl_3$ and $CCl_4$.

    Hint: Successive replacement of H by Cl in sunlight.

19. 19.Describe the mechanism steps of methane halogenation.

    Initiation: $Cl_2 \xrightarrow{h\nu} 2Cl^\bullet$. Propagation: $Cl^\bullet + CH_4 \rightarrow CH_3^\bullet + HCl$; $CH_3^\bullet + Cl_2 \rightarrow CH_3Cl + Cl^\bullet$. Termination: radicals combine, e.g. $Cl^\bullet + Cl^\bullet \rightarrow Cl_2$.

    Hint: Initiation, propagation, termination (free radical chain).

20. 20.Why is the halogenation of alkanes a chain reaction?

    In the propagation step each consumed radical regenerates another radical, so one initiation event triggers many product-forming cycles until termination.

    Hint: Radicals are regenerated in propagation.

21. 21.What is the order of reactivity of halogens in alkane halogenation?

    $F_2 > Cl_2 > Br_2 > I_2$. Fluorination is explosive/violent, iodination is very slow and reversible (needs oxidizing agent to proceed).

    Hint: Reactivity decreases down the group.

22. 22.What is the ease of substitution of different hydrogens during halogenation?

    Tertiary > secondary > primary $C-H$, because the stability of the intermediate free radical follows $3^\circ > 2^\circ > 1^\circ$.

    Hint: Follows free-radical stability order.

23. 23.What is controlled oxidation of methane products?

    $CH_4 + O_2 \xrightarrow{Cu,\ 523K,\ 100\ atm} CH_3OH$; $2CH_4 + O_2 \xrightarrow{Mo_2O_3} 2HCHO$. Partial oxidation gives alcohols/aldehydes.

    Hint: Catalytic partial oxidation of methane.

24. 24.What is the combustion reaction of alkanes?

    $C_nH_{2n+2} + \left(\tfrac{3n+1}{2}\right)O_2 \rightarrow n\,CO_2 + (n+1)H_2O + \text{heat}$. Complete combustion in excess air is highly exothermic.

    Hint: Burn in $O_2$ to $CO_2 + H_2O$.

25. 25.What is isomerisation of alkanes?

    $n$-alkanes rearrange to branched alkanes on heating with anhydrous $AlCl_3/HCl$, e.g. $n\text{-butane} \rightarrow$ isobutane. Used to raise octane number of petrol.

    Hint: $n$-alkane to branched with $AlCl_3$.

26. 26.What is aromatization (reforming) of alkanes?

    Straight-chain alkanes with 6+ carbons dehydrogenate and cyclize over $V_2O_5$ or $Pt$/$Mo$ oxide at high T/P to give aromatics, e.g. $n$-hexane $\rightarrow$ benzene.

    Hint: $n$-hexane to benzene; cyclize + dehydrogenate.

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