Why are alkanes called paraffins?

Alkanes are often referred to as paraffins, a term derived from the Latin words 'parum' (meaning little) and 'affinis' (meaning affinity). This nomenclature accurately reflects their chemical inertness and low reactivity under normal conditions. The reason for this low reactivity lies in their molecular structure: alkanes consist solely of strong carbon-carbon and carbon-hydrogen single bonds, which are nonpolar and difficult to break. Furthermore, they lack any functional groups, such as double bonds, triple bonds, or heteroatoms with lone pairs, that would typically provide sites for chemical attack by common reagents like acids, bases, or oxidizing agents.

How does branching affect the boiling point of alkanes?

Branching in alkanes significantly affects their boiling points. As the degree of branching increases for a given molecular formula, the boiling point decreases. This is because branching leads to a more compact, spherical shape for the molecule, which reduces the surface area available for intermolecular contact. Consequently, the strength of the London dispersion forces (van der Waals forces) between molecules decreases. Weaker intermolecular forces require less energy to overcome, resulting in lower boiling points. For example, n-pentane has a higher boiling point than isopentane (2-methylbutane), which in turn has a higher boiling point than neopentane (2,2-dimethylpropane).

What is the significance of the Wurtz reaction in alkane synthesis, and what are its limitations?

The Wurtz reaction is a classic method for synthesizing alkanes by coupling two alkyl halide molecules using sodium metal in dry ether. Its significance lies in its ability to form carbon-carbon bonds, effectively increasing the chain length. It is particularly useful for preparing symmetrical alkanes with an even number of carbon atoms. However, it has a major limitation: if two different alkyl halides are used to prepare an unsymmetrical alkane, a mixture of three different alkanes will be produced (R-R, R'-R', and R-R'). Separating these products, which often have similar boiling points, is challenging and makes the reaction impractical for synthesizing unsymmetrical alkanes efficiently.

Explain the role of UV light in the halogenation of alkanes.

UV light plays a crucial role in initiating the free radical halogenation of alkanes. This reaction proceeds via a free radical mechanism, which requires the formation of highly reactive free radicals to start the chain reaction. UV light provides the necessary energy (photons) to homolytically cleave the halogen molecule (e.g., $Cl_2$ or $Br_2$) into two halogen free radicals. Homolytic cleavage means each atom involved in the bond breaking retains one electron from the shared pair. Without UV light, the halogen molecule is stable, and the reaction with alkanes, which are themselves quite stable, would not proceed under normal conditions due to a high activation energy barrier.

Why is methane not prepared by the Wurtz reaction?

Methane ($CH_4$) cannot be prepared by the Wurtz reaction because the Wurtz reaction involves the coupling of two alkyl halide molecules to form a new carbon-carbon bond, effectively doubling the carbon chain or combining two different chains. The smallest alkane that can be formed by the Wurtz reaction is ethane ($CH_3-CH_3$), which results from the coupling of two methyl halide molecules ($CH_3X$). Methane has only one carbon atom, and its formation does not involve the creation of a C-C bond through coupling. Therefore, the fundamental mechanism of the Wurtz reaction is incompatible with the synthesis of methane.

What is pyrolysis or cracking of alkanes, and why is it important?

Pyrolysis, also known as cracking, is the process of heating higher alkanes to very high temperatures (typically $700-800, ext{K}$) in the absence of air, causing them to break down into smaller alkanes, alkenes, and hydrogen. This is a free radical process. Its importance is immense in the petroleum industry. Crude oil contains a large proportion of heavy, long-chain alkanes, which are less valuable as fuels. Cracking converts these less useful heavy fractions into more valuable, shorter-chain hydrocarbons like gasoline (petrol) components and alkenes. Alkenes, in turn, are crucial raw materials for the synthesis of polymers (plastics) and various other organic chemicals, making cracking a cornerstone of petrochemical production.

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Alkanes

Chemistry

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Version 1Updated 22 Mar 2026

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Alkanes are saturated acyclic hydrocarbons consisting only of single carbon-carbon and carbon-hydrogen bonds. They belong to the homologous series with the general formula $C_nH_{2n+2}$ , where 'n' represents the number of carbon atoms. Due to the presence of only sigma bonds and the absence of any functional groups, alkanes are relatively unreactive and are often referred to as paraffins (from Lat…

Quick Summary

Alkanes are saturated hydrocarbons, meaning they contain only carbon-carbon single bonds and carbon-hydrogen bonds. Their general formula is $C_nH_{2n+2}$ . Each carbon atom is $sp^3$ hybridized, resulting in a tetrahedral geometry with $109.

5^circ$ bond angles. They are relatively unreactive, hence called paraffins. Alkanes exhibit structural isomerism (chain isomerism) and conformational isomerism due to free rotation around C-C single bonds.

Key preparation methods include hydrogenation of unsaturated hydrocarbons (Sabatier-Senderens), Wurtz reaction (for symmetrical alkanes), decarboxylation of carboxylic acids (using soda lime), Kolbe's electrolytic method, and reduction of alkyl halides.

Physically, they are nonpolar, insoluble in water, and their boiling points increase with molecular mass but decrease with branching. Chemically, their most important reactions are free radical halogenation (requiring UV light, $3^circ > 2^circ > 1^circ$ reactivity for H), complete combustion (producing $CO_2$ and $H_2O$ ), and pyrolysis (cracking) to yield smaller hydrocarbons.

They are widely used as fuels, solvents, and lubricants.

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Key Concepts

Free Radical Halogenation Mechanism

This reaction is a cornerstone for understanding alkane reactivity. It involves three distinct stages: 1.…

Wurtz Reaction and its Limitations

The Wurtz reaction is a powerful synthetic tool for forming new carbon-carbon bonds. It involves treating an…

Effect of Branching on Physical Properties

The physical properties of alkanes, particularly boiling point and melting point, are significantly…

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

u} 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.

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.

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