Exam Master Review Sheet - Alkanes

CHEMCA

EXAM MASTER FORMULA SHEET

Alkanes (Paraffins)

Strategic Summary for Hydrocarbons (JEE & NEET)

1. Preparation of Alkanes

Wurtz Reaction

Coupling of alkyl halides with Sodium. Best for symmetrical alkanes.

\[ \ce{2R-X + 2Na ->[\text{dry ether}] R-R + 2NaX} \]

*Methane cannot be prepared. $3^\circ$ halides undergo elimination instead.*

Corey-House Synthesis

Uses Lithium dialkyl cuprate (Gilman reagent). Best for unsymmetrical alkanes.

\[ \ce{R2CuLi + R'X -> R-R' + RCu + LiX} \]

Soda-Lime Decarboxylation

Loss of $\ce{CO2}$ from sodium salt of carboxylic acid. Step-down reaction (forms alkane with $n-1$ carbons).

\[ \ce{R-COONa + NaOH ->[CaO][\Delta] R-H + Na2CO3} \]

Soda-lime is $NaOH : CaO$ in $3:1$ ratio.

Kolbe's Electrolysis

Aqueous electrolysis of Na/K salts of fatty acids. Radical mechanism.

Anode: $\ce{R-R + CO2}$ Cathode: $\ce{H2 + NaOH}$

*pH of solution increases as reaction proceeds.*

Reduction Methods:

• Sabatier-Senderens: $\ce{R-CH=CH2 + H2 ->[Ni/Pt/Pd][\Delta] R-CH2-CH3}$
• Clemmensen Reduction: $\ce{>C=O ->[Zn-Hg][\text{conc. } HCl] >CH2}$
• Wolff-Kishner Reduction: $\ce{>C=O ->[NH2-NH2][KOH / Ethylene glycol] >CH2}$

The Universal Reducer: Red P / HI

Red P and highly concentrated HI at 423K reduces almost all functional groups (Alcohols, Aldehydes, Ketones, Carboxylic Acids, and Halides) directly into Alkanes.

2. Physical Property Trends

Boiling Point (B.P.)

• • $\text{B.P.} \propto \text{Molecular Weight}$
• • For isomers: $\text{B.P.} \propto \frac{1}{\text{Branching}}$ (Branching decreases surface area and van der Waals forces).
• n-pentane > isopentane > neopentane

Melting Point (M.P.)

• • $\text{M.P.} \propto \text{Packing Efficiency}$ in crystal lattice.
• • Alternation Effect: Alkanes with an Even number of carbon atoms have much better symmetry and pack closer in the crystal lattice than the next odd-numbered alkane, leading to abnormally higher M.P.

Physical State at RT: $C_1 - C_4$ are Gases | $C_5 - C_{17}$ are Liquids | $C_{18}+$ are Waxy Solids

3. Free Radical Halogenation

Halogen Reactivity Order:

$\ce{F2 > Cl2 > Br2 > I2}$

H-Atom Reactivity Order:

$\ce{3^\circ > 2^\circ > 1^\circ > CH4}$

Constraints:

• Fluorination: Too explosive. Done indirectly via Swarts reaction ($\ce{R-Cl + AgF -> R-F}$).
• Iodination: Very slow and reversible. $HI$ formed must be destroyed by strong oxidizing agents ($\ce{HNO3}$ or $\ce{HIO3}$).

Feature	Chlorination ($\ce{Cl2}$)	Bromination ($\ce{Br2}$)
Nature	Highly Reactive, Non-selective	Less Reactive, Highly Selective
Relative Reaction Rates	$3^\circ : 2^\circ : 1^\circ = 5 : 3.8 : 1$	$3^\circ : 2^\circ : 1^\circ = 1600 : 82 : 1$

Calculating Major Product % Yield:

\[ \text{Yield} \propto (\text{Relative Rate}) \times (\text{Number of equivalent H atoms}) \]

4. Oxidation & Rearrangement Reactions

Catalytic / Controlled Oxidation

To Alcohol:
$\ce{2CH4 + O2 ->[Cu / 523K / 100 atm] 2CH3OH}$

To Aldehyde:
$\ce{CH4 + O2 ->[Mo2O3 / \Delta] HCHO + H2O}$

To Carboxylic Acid:
$\ce{2R-CH3 + 3O2 ->[(CH3COO)2Mn / \Delta] 2R-COOH + 2H2O}$

KMnO$_4$ Exception: Alkanes normally resist $KMnO_4$. However, alkanes having a tertiary H-atom are oxidized to corresponding $3^\circ$ alcohols. (e.g., Isobutane $\xrightarrow{KMnO_4}$ tert-butyl alcohol).

Aromatization (Reforming)

Alkanes with $\ge 6$ carbons form benzene/homologues.

\[ \ce{n-Hexane ->[Cr2O3 / V2O5 / Mo2O3][773K, 10-20 atm] Benzene + 4H2} \]

Isomerization

Converts unbranched alkanes to branched isomers.

\[ \ce{n-Butane ->[anhyd. AlCl3 / HCl][\Delta] Isobutane} \]

5. Combustion, Nitration & Pyrolysis

General Combustion Formula

\[ \ce{C_{n}H_{2n+2} + \frac{3n+1}{2} O2 -> n CO2 + (n+1) H2O + Heat} \]

Nitration & Sulphonation

Requires extreme conditions (400-500°C) using conc. $HNO_3$ or fuming $H_2SO_4$. Occurs via Free Radical mechanism involving C-C bond cleavage. Forms a mixture of all possible nitroalkanes/sulphonic acids.

Pyrolysis (Cracking)

Decomposition of higher alkanes to a mixture of lower alkanes and alkenes upon strong heating (500-800°C) in absence of air. Free Radical mechanism.
$\ce{C6H14 ->[\Delta] C6H12 + H2}$ or $\ce{C4H8 + C2H6}$ etc.

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