Why are alkanes considered relatively unreactive?

Alkanes are considered relatively unreactive primarily because they consist only of strong, non-polar C-C and C-H single bonds. The small electronegativity difference between carbon and hydrogen means these bonds have very little polarity, making them resistant to attack by polar reagents like acids, bases, or oxidizing/reducing agents. Additionally, the absence of pi bonds or lone pairs of electrons means there are no readily available sites for electrophilic or nucleophilic attack, contributing to their overall stability under normal conditions.

How does branching affect the boiling point of alkanes?

Branching in alkanes generally decreases their boiling point. This is because branching makes the molecule more compact and spherical, reducing its overall surface area. A smaller surface area leads to fewer points of contact between adjacent molecules, thereby weakening the intermolecular van der Waals (London Dispersion) forces. Less energy is then required to overcome these weaker forces, resulting in a lower boiling point compared to a straight-chain isomer of the same molecular formula.

What is the role of UV light or heat in the halogenation of alkanes?

UV light or heat provides the necessary energy for the initiation step of the free radical halogenation reaction. Specifically, it causes the homolytic cleavage of the halogen molecule (e.g., $Cl_2$ or $Br_2$) into two highly reactive halogen free radicals. Without this initial energy input, the strong covalent bond in the halogen molecule would not break, and the chain reaction would not be able to start. It's a crucial condition for overcoming the activation energy barrier.

Why are alkanes insoluble in water but soluble in organic solvents?

Alkanes are non-polar molecules, while water is a highly polar solvent capable of extensive hydrogen bonding. According to the 'like dissolves like' principle, non-polar substances do not dissolve in polar solvents. For an alkane to dissolve in water, the strong hydrogen bonds between water molecules would need to be broken, and the weak van der Waals forces between alkane and water molecules would not provide enough energy to compensate for this disruption. Conversely, alkanes readily dissolve in non-polar organic solvents (like benzene or ether) because the intermolecular forces in both the alkane and the solvent are similar (van der Waals forces), making the mixing energetically favorable.

What is pyrolysis, and why is it important in the petroleum industry?

Pyrolysis, also known as cracking, is the process of heating larger alkane molecules to high temperatures (typically $400-700^circ C$) in the absence of air, causing them to break down into smaller alkanes and alkenes. This process is of immense industrial importance because it converts less valuable, long-chain hydrocarbons found in crude oil (like those in fuel oil or bitumen) into more valuable, shorter-chain hydrocarbons used as gasoline components (e.g., $C_5-C_{12}$ alkanes) and, crucially, into alkenes (e.g., ethene, propene) which are fundamental raw materials for the petrochemical industry, used in the production of plastics and other chemicals.

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Alkanes, being saturated hydrocarbons, are characterized by their relatively low reactivity due to the presence of strong, non-polar C-C and C-H sigma bonds. Their physical properties, such as boiling point, melting point, density, and solubility, are primarily governed by the strength of intermolecular van der Waals forces, which are influenced by molecular size and shape. Chemically, alkanes und…

Quick Summary

Alkanes are saturated hydrocarbons with the general formula $C_nH_{2n+2}$ , featuring only strong, non-polar C-C and C-H single bonds. Their physical properties are governed by weak van der Waals forces.

As chain length increases, boiling points, melting points, and density generally increase. Branching, however, decreases boiling points due to reduced surface area for intermolecular interactions. Alkanes are non-polar, making them insoluble in water but soluble in non-polar organic solvents.

Chemically, alkanes are relatively unreactive ('paraffins'). Their key reactions include complete combustion (producing $CO_2$ and $H_2O$ ) and incomplete combustion ( $CO$ or $C$ ), both highly exothermic.

Halogenation occurs via a free radical substitution mechanism under UV light or heat, showing selectivity for tertiary > secondary > primary hydrogen atoms. Pyrolysis (cracking) breaks larger alkanes into smaller alkanes and alkenes at high temperatures, vital for the petroleum industry.

Isomerisation converts straight-chain alkanes to branched ones using $AlCl_3/HCl$ catalyst, improving fuel quality. Aromatization converts higher alkanes to aromatic compounds. These properties define their utility as fuels, solvents, and chemical feedstocks.

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Key Concepts

Effect of Molecular Size and Branching on Boiling Point

The boiling point of alkanes is directly related to the strength of intermolecular van der Waals forces. As…

Free Radical Halogenation Mechanism

The halogenation of alkanes (e.g., with $Cl_2$ or $Br_2$ ) under UV light or heat proceeds via a free radical…

Selectivity in Halogenation and Alkyl Radical Stability

In the free radical halogenation of alkanes with more than one type of hydrogen atom (primary, secondary,…

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.

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

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