Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

General Formula — $C_nH_{2n+2}$
Physical State — $C_1-C_4$ (gas), $C_5-C_{17}$ (liquid), $C_{18+}$ (solid)
Boiling Point (BP) — Increases with chain length. Decreases with branching.
Melting Point (MP) — Increases with chain length. Can be higher for symmetrical branched alkanes.
Density — Increases with chain length, less than water.
Solubility — Insoluble in water (non-polar), soluble in non-polar organic solvents.
Combustion — $C_nH_{2n+2} + (\frac{3n+1}{2})O_2 \longrightarrow nCO_2 + (n+1)H_2O$ (Complete)
Halogenation — Free radical substitution, $X_2$ (Cl, Br) + UV light/heat.

- Reactivity: $F_2 > Cl_2 > Br_2 > I_2$ - Selectivity: $3^circ H > 2^circ H > 1^circ H$ (due to radical stability)

Pyrolysis (Cracking) — High temp ($400-700^circ C$), no air. Larger alkanes $\longrightarrow$ smaller alkanes + alkenes.
Isomerisation — $AlCl_3/HCl$, $200^circ C$. n-alkane $\longrightarrow$ branched alkane.
Aromatization — $C_6+$ alkane, $Cr_2O_3/Al_2O_3$, $500-600^circ C$. Alkane $\longrightarrow$ Aromatic compound.

2-Minute Revision

Alkanes are saturated hydrocarbons ($C_nH_{2n+2}$) known for their low reactivity due to strong, non-polar C-C and C-H bonds. Their physical properties are governed by van der Waals forces. Boiling and melting points increase with chain length but decrease with branching (due to reduced surface area). They are less dense than water and are insoluble in water but soluble in non-polar organic solvents.

Chemically, alkanes undergo several key reactions:

1
Combustion — They burn to produce $CO_2$ and $H_2O$ (complete) or $CO/C$ and $H_2O$ (incomplete), releasing significant heat, making them excellent fuels.
2
Halogenation — This is a free radical substitution reaction with $Cl_2$ or $Br_2$ in the presence of UV light or heat. The reaction proceeds via initiation, propagation, and termination steps. Crucially, it exhibits selectivity: tertiary hydrogens are most reactive, followed by secondary, then primary, due to the stability of the intermediate alkyl radicals ($3^circ > 2^circ > 1^circ$).
3
Pyrolysis (Cracking) — Large alkanes break down into smaller alkanes and alkenes at high temperatures, a vital industrial process.
4
Isomerisation — Straight-chain alkanes convert to branched isomers using $AlCl_3/HCl$ catalyst, improving fuel quality.
5
Aromatization — Higher alkanes ($C_6+$) form aromatic compounds like benzene under specific catalytic conditions. Focus on these trends, conditions, and product predictions for NEET.

5-Minute Revision

Alkanes, with their general formula $C_nH_{2n+2}$, are saturated hydrocarbons characterized by strong, non-polar C-C and C-H single bonds, making them relatively unreactive. Their physical properties are primarily influenced by London Dispersion Forces (a type of van der Waals force), which increase with molecular size and surface area.

Physical Properties Summary:

State — $C_1-C_4$ are gases, $C_5-C_{17}$ are liquids, $C_{18+}$ are solids at room temperature.
Boiling Point (BP) — Increases with increasing carbon chain length. For isomers, branching decreases BP because it reduces the surface area for intermolecular contact, weakening van der Waals forces. E.g., n-pentane ($36^circ C$) > isopentane ($28^circ C$) > neopentane ($9.5^circ C$).
Melting Point (MP) — Generally increases with chain length. Highly symmetrical branched alkanes can have higher MPs than less symmetrical isomers due to efficient crystal packing.
Density — Increases with chain length but remains less than water ($<1,\text{g/mL}$). They float on water.
Solubility — Insoluble in polar solvents like water ('like dissolves like' principle) but soluble in non-polar organic solvents (e.g., benzene, ether).

Chemical Properties Summary:

1
Combustion — Alkanes are excellent fuels. Complete combustion with excess $O_2$ yields $CO_2$ and $H_2O$ (e.g., $CH_4 + 2O_2 \longrightarrow CO_2 + 2H_2O$). Incomplete combustion (limited $O_2$) produces $CO$ and/or $C$ (soot) along with $H_2O$.
2
Halogenation (Free Radical Substitution) — Reaction with $Cl_2$ or $Br_2$ in the presence of UV light ($h\nu$) or heat ($250-400^circ C$).

* Mechanism: Proceeds via a free radical chain mechanism (Initiation, Propagation, Termination). * Selectivity: The reactivity of hydrogen atoms towards abstraction by halogen radicals follows the order: $3^circ H > 2^circ H > 1^circ H$. This is due to the stability of the intermediate alkyl radicals ($3^circ > 2^circ > 1^circ$). For example, monochlorination of 2-methylpropane yields 2-chloro-2-methylpropane as the major product.

1
Pyrolysis (Cracking) — Heating large alkanes ($C_6+$) to high temperatures ($400-700^circ C$) in the absence of air breaks them into smaller alkanes and alkenes. This is crucial for producing gasoline and petrochemical feedstocks.

* Example: $C_6H_{14} \xrightarrow{heat} C_4H_{10} + C_2H_4$.

1
Isomerisation — Straight-chain alkanes can be converted to branched-chain isomers using anhydrous $AlCl_3/HCl$ catalyst at $200^circ C$. This improves the octane rating of fuels.

* Example: n-Butane $\xrightarrow{AlCl_3/HCl}$ Isobutane.

1
Aromatization — Alkanes with six or more carbons can be converted to aromatic compounds (e.g., benzene from n-hexane) at high temperatures ($500-600^circ C$) over catalysts like $Cr_2O_3/Al_2O_3$.

Key takeaway for NEET: Understand the trends in physical properties, the specific conditions and reagents for each chemical reaction, and the selectivity rules for halogenation to predict major products.

Prelims Revision Notes

Alkanes: Physical and Chemical Properties (NEET Revision)

I. Physical Properties

General Formula — $C_nH_{2n+2}$ (Saturated hydrocarbons).
Intermolecular Forces — Primarily London Dispersion Forces (van der Waals forces).
Physical State (at $25^circ C$)

* $C_1-C_4$: Gases (e.g., Methane, Ethane, Propane, Butane) * $C_5-C_{17}$: Liquids (e.g., Pentane, Hexane, Octane) * $C_{18+}$: Solids (e.g., Paraffin wax)

Boiling Point (BP)

* Effect of Chain Length: Increases with increasing 'n' (molecular weight) due to stronger van der Waals forces (more electrons, larger surface area). * Effect of Branching: Decreases with increasing branching for isomers. Branching makes molecules more spherical, reducing surface area for intermolecular contact, thus weaker van der Waals forces. (e.g., n-pentane > isopentane > neopentane).

Melting Point (MP)

* Generally increases with chain length. * Symmetrical branched alkanes can have higher MPs due to efficient crystal packing, despite lower BPs.

Density — Increases with chain length, but always less than water ($<1,\text{g/mL}$). Alkanes float on water.
Solubility

* Insoluble in water: Alkanes are non-polar; water is polar. 'Like dissolves like' principle. * Soluble in non-polar organic solvents: (e.g., benzene, ether, $CCl_4$).

II. Chemical Properties (Reactivity: 'Paraffins' - low affinity)

Reason for low reactivity — Strong, non-polar C-C and C-H sigma bonds; absence of functional groups or pi bonds.

1
Combustion (Oxidation)

* Complete: Sufficient $O_2 \longrightarrow CO_2 + H_2O + \text{Heat}$. (Exothermic, fuels) * C_nH_{2n+2} + left(\frac{3n+1}{2}\right)O_2 \longrightarrow nCO_2 + (n+1)H_2O * Incomplete: Limited $O_2 \longrightarrow CO + H_2O$ and/or $C + H_2O$.

1
Halogenation (Free Radical Substitution)

* Reagents: $Cl_2$ or $Br_2$. * Conditions: UV light ($h\nu$) or high temperature ($250-400^circ C$). * Mechanism: Free Radical Chain Reaction (Initiation, Propagation, Termination). * Reactivity of Halogens: $F_2$ (too violent) > $Cl_2 > Br_2 > I_2$ (too slow/reversible).

* Selectivity: Reactivity of H-atoms for abstraction: $3^circ H > 2^circ H > 1^circ H$. * This is due to the stability of alkyl radicals: $3^circ > 2^circ > 1^circ$. * Product Prediction: Major product results from substitution of the most reactive H-atom.

1
Pyrolysis (Cracking)

* Conditions: High temperature ($400-700^circ C$), absence of air (or steam). * Reaction: Larger alkanes break into smaller alkanes and alkenes. * Importance: Industrial process for gasoline production and petrochemical feedstocks.

1
Isomerisation

* Conditions: Anhydrous $AlCl_3/HCl$ catalyst, $200^circ C$, $35,\text{atm}$. * Reaction: Straight-chain alkanes $\longrightarrow$ Branched-chain alkanes. * Importance: Improves octane number of gasoline.

1
Aromatization (Reforming)

* Conditions: $C_6+$ alkanes, $Cr_2O_3/Al_2O_3$ or $MoO_2/Al_2O_3$, $500-600^circ C$, high pressure. * Reaction: Dehydrogenation and cyclization to form aromatic compounds. * Example: n-Hexane $\longrightarrow$ Benzene + $4H_2$.

1
Reaction with Steam

* Example: $CH_4 + H_2O \xrightarrow{Ni, 800-900^circ C} CO + 3H_2$ (Syngas production).

Vyyuha Quick Recall

To remember the reactivity order of hydrogens in halogenation: Three Seconds Pass. (Tertiary > Secondary > Primary)