Alkanes — Revision Notes

⚡ 30-Second Revision

General Formula: — $C_nH_{2n+2}$
Hybridization: — All carbons are $sp^3$ (tetrahedral, $109.5^\circ$)
Reactivity: — Low (paraffins), primarily undergo substitution.
Free Radical Halogenation: — $R-H + X_2 \xrightarrow{h\nu} R-X + HX$

- Reactivity of H: $3^\circ > 2^\circ > 1^\circ$ - Reactivity of X: $F_2 > Cl_2 > Br_2 > I_2$

Wurtz Reaction: — $2R-X + 2Na \xrightarrow{Dry,Ether} R-R + 2NaX$ (best for symmetrical alkanes)
Decarboxylation: — $R-COONa + NaOH \xrightarrow{CaO, Delta} R-H + Na_2CO_3$
Hydrogenation: — $R-CH=CH_2 + H_2 \xrightarrow{Ni/Pd/Pt} R-CH_2-CH_3$
Boiling Point Trend: — Increases with molecular mass, decreases with branching.

2-Minute Revision

Alkanes are saturated hydrocarbons with the general formula $C_nH_{2n+2}$, characterized by $sp^3$ hybridized carbon atoms forming only single bonds. Their tetrahedral geometry and free rotation around C-C bonds lead to conformational isomers.

Due to strong, nonpolar C-C and C-H bonds, they are relatively unreactive, hence called paraffins. Key preparation methods include Wurtz reaction (coupling alkyl halides, best for symmetrical alkanes), decarboxylation of carboxylic acid salts with soda lime, and hydrogenation of alkenes/alkynes using catalysts like Ni, Pd, or Pt.

Physically, alkanes are nonpolar, insoluble in water, and their boiling points increase with molecular mass but decrease with increased branching. The most important chemical reaction is free radical halogenation, where hydrogen atoms are substituted by halogens under UV light.

Remember the reactivity order of hydrogens ($3^\circ > 2^\circ > 1^\circ$) for predicting major products. Alkanes also undergo complete combustion to produce $CO_2$ and $H_2O$, and pyrolysis (cracking) at high temperatures to yield smaller hydrocarbons.

5-Minute Revision

Alkanes, the simplest class of hydrocarbons, are defined by their saturation – containing only C-C and C-H single bonds. Their general formula is $C_nH_{2n+2}$, and all carbons are $sp^3$ hybridized, resulting in a tetrahedral arrangement ($109.5^\circ$ bond angles). This allows for free rotation around C-C bonds, leading to conformations like staggered and eclipsed forms.

Nomenclature: Follow IUPAC rules: longest chain, lowest locants for substituents, alphabetical order. Example: $CH_3CH(CH_3)CH_2CH_3$ is 2-methylbutane.

Preparation Methods:

1
Hydrogenation: — Alkenes/alkynes $\xrightarrow{H_2, Ni/Pd/Pt}$ Alkanes. E.g., $CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3-CH_3$.
2
Wurtz Reaction: — $2R-X + 2Na \xrightarrow{Dry,Ether} R-R + 2NaX$. Best for symmetrical alkanes. E.g., $2CH_3CH_2Br + 2Na \rightarrow CH_3CH_2CH_2CH_3$.
3
Decarboxylation: — $R-COONa + NaOH \xrightarrow{CaO, Delta} R-H + Na_2CO_3$. E.g., $CH_3COONa + NaOH \rightarrow CH_4 + Na_2CO_3$.
4
Reduction of Alkyl Halides: — $R-X \xrightarrow{Zn/HCl} R-H$. E.g., $CH_3Cl \xrightarrow{Zn/HCl} CH_4$.

Physical Properties: Nonpolar, insoluble in water. Boiling points increase with molecular mass but decrease with branching (e.g., n-pentane > isopentane > neopentane).

Chemical Properties:

1
Free Radical Halogenation: — Substitution of H by X (Cl, Br) under UV light. Mechanism: Initiation ($X_2 \xrightarrow{h\nu} 2Xcdot$), Propagation ($R-H + Xcdot \rightarrow Rcdot + HX$; $Rcdot + X_2 \rightarrow R-X + Xcdot$), Termination (radicals combine). Reactivity of H: $3^\circ > 2^\circ > 1^\circ$. E.g., Monochlorination of propane yields 2-chloropropane as the major product.
2
Combustion: — $C_nH_{2n+2} + \left(\frac{3n+1}{2}\right)O_2 \rightarrow nCO_2 + (n+1)H_2O + Heat$.
3
Pyrolysis (Cracking): — High temp decomposition into smaller alkanes and alkenes. E.g., $C_6H_{14} \xrightarrow{Delta} C_4H_8 + C_2H_6$.

NEET Tip: Focus on reaction conditions, major products in halogenation, and the impact of branching on boiling points.

Prelims Revision Notes

Alkanes: NEET Revision Notes

1. Definition and Structure:

Saturated Hydrocarbons: — Contain only C-C and C-H single bonds.
General Formula: — $C_nH_{2n+2}$ (for acyclic alkanes).
Hybridization: — All carbon atoms are $sp^3$ hybridized.
Geometry: — Tetrahedral around each carbon, bond angles $\approx 109.5^\circ$.
Conformations: — Free rotation around C-C single bonds leads to different conformations (e.g., staggered and eclipsed for ethane), which are interconvertible.

2. Nomenclature (IUPAC):

Identify the longest continuous carbon chain (parent alkane).
Number the chain to give substituents the lowest possible locants.
Name substituents (alkyl groups) and list them alphabetically.
Use prefixes (di-, tri-, tetra-) for multiple identical substituents.

3. Isomerism:

Chain Isomerism: — Same molecular formula, different carbon skeleton (e.g., n-butane and isobutane).

4. Preparation Methods:

From Unsaturated Hydrocarbons (Hydrogenation): — Alkenes/Alkynes $\xrightarrow{H_2, Ni/Pd/Pt}$ Alkanes. (Sabatier-Senderens reaction).
Wurtz Reaction: — $2R-X + 2Na \xrightarrow{Dry,Ether} R-R + 2NaX$. Best for symmetrical alkanes. Not suitable for unsymmetrical alkanes (mixture of products).
Decarboxylation: — Sodium salt of carboxylic acid $\xrightarrow{NaOH/CaO, Delta}$ Alkane (one C less) + $Na_2CO_3$. (Soda lime is $NaOH + CaO$).
Kolbe's Electrolytic Method: — Electrolysis of aqueous sodium/potassium salt of carboxylic acid. Forms symmetrical alkanes at anode.
Reduction of Alkyl Halides: — $R-X \xrightarrow{Zn/HCl \text{ or } LiAlH_4} R-H$.

5. Physical Properties:

Nonpolar: — Due to small electronegativity difference between C and H.
Solubility: — Insoluble in water (polar), soluble in nonpolar organic solvents.
Density: — Less dense than water.
Boiling/Melting Points:

Increase with increasing molecular mass (stronger London dispersion forces). Decrease with increasing branching (reduced surface area, weaker London dispersion forces). E.g., n-pentane > isopentane > neopentane.

State: — $C_1-C_4$ gases, $C_5-C_{17}$ liquids, $C_{18+}$ solids.

6. Chemical Properties (Reactions):

Low Reactivity: — Due to strong, nonpolar C-C and C-H sigma bonds.
Free Radical Halogenation (Substitution): — $R-H + X_2 \xrightarrow{h\nu \text{ or } Delta} R-X + HX$.

* Mechanism: Initiation (homolytic cleavage), Propagation (chain reaction), Termination (radical combination). * Reactivity of H atoms: $3^\circ > 2^\circ > 1^\circ$ (due to radical stability). * Reactivity of Halogens: $F_2 > Cl_2 > Br_2 > I_2$ (Fluorination too violent, Iodination reversible).

Combustion: — $C_nH_{2n+2} + \left(\frac{3n+1}{2}\right)O_2 \rightarrow nCO_2 + (n+1)H_2O + Heat$. (Complete combustion).
Controlled Oxidation: — Under specific conditions (e.g., $CH_4 + O_2 \xrightarrow{Cu/523K/100atm} CH_3OH$).
Isomerization: — Straight-chain alkanes $\xrightarrow{AlCl_3/HCl, Delta}$ Branched-chain alkanes.
Pyrolysis (Cracking): — Alkanes $\xrightarrow{\text{High Temp, no air}}$ Smaller alkanes + alkenes + $H_2$. Important for petroleum refining.

7. Distinguishing Test: Alkanes do NOT decolorize bromine water or Baeyer's reagent (cold, dilute, alkaline $KMnO_4$), unlike alkenes/alkynes.

Vyyuha Quick Recall

For Alkane Reactions, remember 'CHIPS':

Combustion: Burns to $CO_2$ and $H_2O$.
Halogenation: Free radical substitution with $X_2$ and UV light ($3^\circ > 2^\circ > 1^\circ$ reactivity).
Isomerization: Straight to branched with $AlCl_3/HCl$.
Pyrolysis: Cracking into smaller hydrocarbons at high temps.
Synthesis (Wurtz, Decarboxylation, Hydrogenation): Key preparation methods.