Revision: Std. XI >> Hydrocarbons MAH-MHT CET (PCM/PCB) Hydrocarbons

Definitions [3]

Definition: Alkanes

Alkanes are hydrocarbons in which all the linkages between the carbon atoms are single covalent bonds.

Definition: Alkenes

Alkenes are unsaturated hydrocarbons containing at least one C=C double bond.

Definition: Alkynes

Alkynes are aliphatic unsaturated hydrocarbons containing at least one C≡C triple bond.

Key Points

Key Points: Alkanes

General formula: CₙH₂ₙ₊₂ (where n = number of carbon atoms)
Suffix used for IUPAC naming: –ane
e.g., Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈)
Alkanes exhibit chain isomerism due to absence of any functional group and the possibility of more than one chain type for the same molecular formula
e.g., C₅H₁₂ forms n-pentane, neo-pentane, and iso-pentane

Key Points: Preparation of Alkanes

Methods of Preparation:

Method	Reactants → Conditions → Product
Catalytic hydrogenation of alkene	\[\ce{\underset{Alkene}{C_{n}H_{2n}} + H2 ->[\underset{(room temp)}{Pt or Pd}][\underset{(high temp and high pressure)}{or Ni}] C_{n}H_{2n} + 2}\]
Catalytic hydrogenation of alkyne	\[\ce{\underset{Alkene}{C_{n}H_{2n - 2}} + 2H2 ->[\underset{(room temp)}{Pt or Pd}][\underset{(high temp and high pressure)}{or Ni}] C_{n}H_{2n} + 2}\]
Reduction of alkyl halide	\[\ce{CH3—I + \underset{Nascent}{2[H]} ->[Zn, HCI] CH4 + HI}\]
Wurtz Coupling reaction	\[\ce{CH3 — Br + 2Na + CH3 — Br ->[Dry ether] CH3 — CH3 + 2NaBr}\]
From Grignard reagent	\[\ce{CH3 — Mg — I + H2O ->[Dry ether] CH4 + MgI(OH)}\]
Soda lime decarboxylation	\[\ce{RCOONa + NaOH ->[CaO/Δ] RH + Na2CO3}\]
Kolbe's electrolysis	\[\ce{2RCOONa + H2O ->[Electrolysis] R — R + 2CO2 + 2NaOH + H2}\]

Key Points: Physical Properties of Alkanes

1. Polarity:

Alkanes are non-polar
Insoluble in polar solvents (water)
Soluble in non-polar solvents

2. Boiling point:

Increases with molecular mass

State at SATP:

C₁–C₄ → gases
C₅–C₁₇ → liquids
C₁₈+ → solids

Boiling point order (increasing):

Neo-pentane < iso-pentane < n-pentane

Trend:

Straight-chain > branched-chain
More branching → lower BP

3. Melting point:

Increases with molecular mass
Symmetrical molecules → higher MP
Even number of C atoms → higher MP than odd
Only intermolecular London forces are present

Key Points: Chemical Properties of Alkanes

Alkanes are saturated and relatively inert — they undergo only substitution reactions at C–H bonds.

Reaction	Conditions
Halogenation	\[\ce{CH4 + Cl2 ->[hv][-HCl] CH3Cl ->[hv][-HCl] CH2Cl2 ->[hv][-HCl] CHCl3 ->[hv][-HCl] CCl4}\]
Combustion	\[\ce{C_{n}H_{2n + 2} + \left(\frac{3n + 1}{2}\right)O_{2} ->[Complete][combustion] nCO2 + (n + 1)H2O}\] \[\ce{CH4(g) + 20_{2}(g) ->[Complete][combustion] CO2 + 2H2O}\]
Reforming / Aromatization	\[\ce{n-hexane ->[V2O5][\underset{12-20 atm}{773K}] C6H6 + 4H2}\]
Pyrolysis / Cracking

Key Points: Uses of Alkanes

The first four alkanes (CH₄, C₂H₆, C₃H₈, C₄H₁₀) are used as fuels (CNG, LPG).
Lower liquid alkanes used as solvents.
Used in making wax, ink, shoe polish, etc.
Alkanes with more than 35 carbon atoms are used for road surfacing (tar).

Key Points: Alkenes

General formula: CₙH₂ₙ (where n = 2, 3, 4…)
Suffix for IUPAC naming: –ene
e.g., Ethene (CH₂=CH₂), Propene (CH₃–CH=CH₂)
The double bond consists of one σ bond and one π bond

Key Points: Preparation of Alkenes

Method	Example
Dehydration of alcohol	\[\ce{CH3CH2CH2OH ->[Conc H2SO4][160°C] CH3CH = CH2 + H2O}\]
Dehydrohalogenation of alkyl halide	\[\begin{array}{cc} \phantom{}\ce{CH3 - CH2 ->[Alc KOH][Δ] CH2 = CH2 + KBr + H2}\phantom{}\\ \|\phantom{.............................................}\\ \ce{Br}\phantom{...........................................} \end{array}\]
Dehalogenation of vicinal dihalide
From alkyne (partial reduction — gives cis)	\[\ce{CH ≡ CH + H2 ->[Pd/C] \underset{cis-alkene}{CH2 = CH2}}\]
From alkyne (gives trans)	\[\begin{array}{cc} \phantom{..........................................}\ce{R}\phantom{...........}\ce{H}\phantom{.}\\ \phantom{..........................................}\backslash\phantom{........}/\phantom{.}\\ \phantom{}\ce{R - C ≡ C - R ->[Na][Liquid NH3] C = C}\phantom{}\\ \phantom{..........................................}/\phantom{........}\backslash\phantom{.}\\ \phantom{..........................................}\underset{\text{trans-alkene}}{\ce{R}\phantom{...........}\ce{R}}\phantom{.} \end{array}\]

Key Points: Physical Properties of Alkenes

1. Solubility

Alkenes are non-polar
Insoluble in water
Soluble in non-polar organic solvents (e.g., hexane, benzene)

2. Boiling Point (BP)

Increases with molecular mass

More electrons → stronger London dispersion forces

Straight-chain > Branched-chain

Straight chains have larger surface area → stronger intermolecular forces

Cis-alkenes > Trans-alkenes (usually)

Cis is more polar → dipoles do not cancel → higher BP

Alkenes vs Alkanes (same number of carbons)

Alkenes have slightly lower BP

Reason:

π-bond leads to less effective electron distribution for dispersion forces
Slightly weaker intermolecular attractions

3. Melting Point (MP)

Trans-alkenes > Cis-alkenes

Trans is more symmetrical → packs better in crystal lattice → higher MP

Cis-alkenes

Less symmetrical → poorer packing → lower MP

In trans-alkenes:

Bond dipoles cancel → non-polar
Leads to tighter packing in solid state

Key Points: Chemical Properties of Alkenes

Alkenes undergo mainly electrophilic addition reactions due to the π bond (electron-rich site).

Reaction	Example
Addition of hydrogen (Hydrogenation)	\[\ce{H2C = CH2 ->[H2/Ni, Pt or Pd][523-573K] H3C - CH3}\]
Addition of halogen	\[\begin{array}{cc} \phantom{}\ce{H3CCH = CH2 + Cl — Cl ->[CCl4] CH3CH - CH2}\phantom{}\\ \phantom{....................................................}\|\phantom{.........}\|\phantom{}\\ \phantom{.....................................................}\ce{Cl}\phantom{.......}\ce{Cl}\phantom{} \end{array}\]
Addition of HX (Markovnikov's rule)	\[\begin{array}{cc} \phantom{..............................................................}\ce{Br}\phantom{}\\ \phantom{............................................................}\|\phantom{}\\ \phantom{}\ce{\underset{(For unsymmetrical allkene-Markownikoff’s rule)}{H3CCH = CH2 + HBr} -> H3C - CH - CH3}\phantom{} \end{array}\]
Addition of HBr (Anti-Markovnikov / Kharasch effect)	\[\begin{array}{cc} \phantom{}\ce{\underset{(In presence of peroxide-reverse of Markownikoff’s rule)}{H3CCH = CH2 + HBr} ->[Peroxide] CH3CH2CH2Br}\phantom{}\\ \end{array}\]
Hydration (addition of H₂SO₄/H₂O)	\[\begin{array}{cc} \ce{O}\phantom{..}\\ \|\|\phantom{..}\\ \phantom{}\ce{CH2 = CH2 + H - O - S - O - H -> C2H5HSO4}\phantom{}\\ \|\|\phantom{..}\\ \ce{O}\phantom{..} \end{array}\]
Oxidation (KMnO₄/H⁺)	\[\begin{array}{cc} \phantom{..........................}\ce{O}\phantom{}\\ \phantom{..........................}\|\|\\ \phantom{}\ce{H3C — CH = CH2 ->[{[O]}][KMnO4, {[H^{+}]}] H3C - C - OH + CO2 + H2O}\phantom{} \end{array}\]
Hydroxylation	\[\begin{array}{cc} \phantom{}\ce{H2C = CH2 + H2O + [O] ->[Dil.KMnO4][273K] CH2 - CH2}\\ \phantom{.....................................................}\|\phantom{..........}\|\phantom{}\\ \phantom{........................................................}\ce{OH}\phantom{.....}\ce{OH}\phantom{} \end{array}\]
Ozonolysis	\[\begin{array}{cc} \phantom{.....}\ce{H3C}\phantom{....................................}\ce{H3C}\phantom{............................}\\ \phantom{.....}\backslash\phantom{.........................................}\backslash\phantom{.....................}\\ \phantom{..........}\ce{C = CH2 + O3 ->[Zn/H2O] \phantom{.......}C = O + HCHO}\phantom{}\\ \phantom{......}/\phantom{..........................................}/\phantom{.....................}\\ \phantom{...............}\ce{H3C}\phantom{......................................}\ce{H3C}\phantom{......................................} \end{array}\]
Polymerisation
Hydroboration-oxidation	\[\begin{array}{cc} \phantom{...............}\ce{H}\phantom{....}\ce{H}\phantom{.............................................}\ce{H}\phantom{....}\ce{H}\phantom{..........................}\ce{H}\phantom{....}\ce{H}\phantom{............................}\\ \phantom{.............}\|\phantom{......}\|\phantom{..............................................}\|\phantom{......}\|\phantom{............................}\|\phantom{......}\|\phantom{..........................}\\ \phantom{}\ce{6(H - C = C - H) + (BH3)2 ->[THF] 2(H - C - C)3 - B ->[H2O2][OH^Θ] H - C - C - H + B(OH)3}\phantom{}\\ \phantom{..........................................}\|\phantom{......}\|\phantom{............................}\|\phantom{......}\|\\ \phantom{..............................................}\ce{H}\phantom{.....}\ce{H}\phantom{..........................}\ce{H}\phantom{.....}\ce{OH}\phantom{.} \end{array}\]

Key Points: Uses of Alkenes

Used in synthesis of alcohols, plastics, detergents, fuels
Used for artificial ripening of fruits (like mangoes) — ethylene gas

Key Points: Alkynes

General formula: CₙH₂ₙ₋₂
Suffix for IUPAC naming: –yne
e.g., Propyne (CH₃–C≡CH), Butyne (CH₃–CH₂–C≡CH)
C₂H₂ is acetylene (common name); IUPAC name is ethyne
The triple bond consists of one σ bond and two π bonds

Isomerism in Alkynes:

Alkynes show position isomerism (type of structural isomerism)
e.g., 1-Butyne and 2-Butyne

Key Points: Preparation of Alkynes

From calcium carbide:

By the action of water on calcium carbide, Cu₂C₂ or BaC_2, acetylene is formed.

CaC₂ + 2H₂O → CH ≡ CH + Ca(OH)₂

\[\ce{CaCO3 ->[][\underset{}{-CO2}] CaO->[2C + Heat][\underset{}{}] CaC2->[H2O][\underset{}{}] CH ≡ CH}\]

Dehalogenation of 1,1,2,2-tetrahaloalkanes:

On treatment with Zn, tetrahalides get dehalogenated to give alkynes.

\[\ce{R - CBr2 - CHBr2 + 2Zn ->[Δ] R - \underset{Alkyne}{C ≡ CH} + 2ZnBr2}\]

\[\ce{R - CBr2 - CBr2 - R + 2Zn ->[Δ] R - \underset{Alkyne}{C ≡ C} - R + 2ZnBr2}\]

By dehydrohalogenation of vic and gem dihalides:

Alkynes are prepared by dehydrohalogenation of vic. and gem. dihalides with alc. KOH + NaNH₂ or KNH₂.

\[\ce{R - \underset{β}{C}H2 - CHX2 ->[(i) NaNH2/Δ][(ii) H^{+}] R - C ≡ CH}\]

\[\begin{array}{cc}
\ce{X}\phantom{.......................}\\
\phantom{}|\phantom{.......................}\\
\phantom{}\ce{R - \underset{β}{C}H2 - C - R ->[NaNH2/Δ] R - C ≡ CH}\phantom{}\\
\phantom{}|\phantom{.......................}\\
\ce{X}\phantom{.......................}
\end{array}\]

Key Points: Physical Properties of Alkynes

Odour: Alkynes are generally odourless, but acetylene smells of garlic due to phosphine impurity

Boiling and melting points: Slightly higher than those of corresponding alkenes and alkanes with similar carbon atoms

Order	BP/MP Comparison
BP of alkynes	Hex-1-yne > Pent-1-yne > But-1-yne > Propyne > Ethyne
MP of alkynes	Ethyne > Pent-1-yne > Propyne > But-1-yne > Hex-1-yne

Reason for higher BP: Linear structure around the triple bond allows electrons to come closer together, resulting in greater London forces

Solubility: Alkynes are soluble in organic solvents like benzene, CCl₄, and ether

Density: Increases with increase in molecular size

Non-polar molecules → insoluble in water

Key Points: Chemical Properties of Alkynes

Reaction	Examples
Acidity of terminal alkynes	\[\ce{HC ≡ CH + Na -> HC ≡ C^⊖Na^{+} + 2H2}\]
Addition of hydrogen (Hydrogenation)	\[\ce{CH3C ≡ CH + 2H2 ->[Ni] CH3CH2CH3}\]
Addition of halogen	\[\begin{array}{cc} \phantom{..........................}\ce{Br}\phantom{...}\ce{Br}\phantom{}\\ \phantom{.........................}\|\phantom{......}\|\phantom{}\\ \phantom{}\ce{HC ≡ CH ->[2Br2][CCl4] H - C - C - H}\phantom{}\\ \phantom{.........................}\|\phantom{......}\|\phantom{}\\ \phantom{..........................}\ce{Br}\phantom{...}\ce{Br}\phantom{} \end{array}\]
Addition of HX	\[\ce{HC ≡ CH + 2HBr -> CH3CHBr2}\]
Hydration (Hydration of alkynes)	\[\ce{HC ≡ CH + H2O ->[1\% HgSO4][40\% H2SO4] [\underset{Unstable}{CH2 = CHOH}] ⇌ \underset{\underset{compound}{Carbonyl}}{CH3CHO}}\]

Key Points: Uses of Alkynes

Used in the manufacture of polymers, synthetic rubber, and plastics
Used in the artificial ripening of fruits
Used in welding and cutting of metals (oxy-acetylene flame, temperature ~3500°C)

Key Points: Aromatic Hydrocarbons

Cyclic, planar hydrocarbons with delocalised π electrons.

Benzenoids contain benzene ring; Non-benzenoids are aromatic without benzene ring.

Structure of Benzene: