Alkanes - Properties, Reactions, and Mechanisms

Properties of Alkanes

Physical Properties

Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. Their physical properties are largely determined by their molecular weight and the strength of London Dispersion Forces (LDFs).

• Boiling Point: Boiling points increase with increasing molecular weight due to stronger LDFs.
• Melting Point: Melting points also increase with increasing molecular weight.
• Solubility: Alkanes are non-polar and therefore insoluble in water but soluble in non-polar organic solvents.
• Density: Density generally increases with increasing molecular weight.

Chemical Properties

Alkanes are relatively unreactive due to the strength of the C-C and C-H bonds. They primarily undergo combustion and halogenation reactions.

Table of Physical Properties (Example - first 5 alkanes)

Alkane	Molecular Formula	Molecular Weight (g/mol)	Boiling Point (°C)	Melting Point (°C)
Methane	CH4	16	-161	-182
Ethane	C2H6	30	-88	-116
Propane	C3H8	44	-42	-96
Butane	C4H10	58	-0	-12
Pentane	C5H12	72	36	-110

Reactions of Alkanes

Combustion

Alkanes are excellent fuels and undergo complete combustion in the presence of excess oxygen, producing carbon dioxide and water.

The balanced chemical equation for the complete combustion of a generic alkane is:

$$C_n H_{2n+2} + \frac{n}{2} O_2 \rightarrow n CO_2 + n H_2O$$

Halogenation

Alkanes undergo substitution reactions with halogens (chlorine and bromine) in the presence of UV light (photohalogenation). This is a radical chain reaction.

Cracking (Dehydrogenation)

Large alkanes can be broken down into smaller, more useful alkanes and alkenes by heating them to high temperatures in the absence of oxygen. This process is called cracking.

For example, the cracking of propane yields propylene (an alkene) and methane.

$$C_3H_8 \rightarrow C_3H_6 + CH_4$$

Reaction Mechanisms

Photohalogenation

Photohalogenation of alkanes proceeds via a free radical chain mechanism, involving three main steps:

1. Initiation: UV light causes the homolytic cleavage of a halogen molecule (e.g., Cl₂) to form halogen radicals (e.g., Cl•).

$$Cl_2 \xrightarrow{UV} 2Cl \cdot$$

2. Propagation: A halogen radical abstracts a hydrogen atom from an alkane, forming a carbon radical and an alkane radical. The carbon radical then reacts with a halogen molecule to form a haloalkane and regenerate a halogen radical.
$$Cl \cdot + CH_4 \rightarrow HCl + CH_3 \cdot$$ $$CH_3 \cdot + Cl_2 \rightarrow CH_3Cl + Cl \cdot$$
3. Termination: Two radicals combine to form a stable molecule, stopping the chain reaction.
$$Cl \cdot + Cl \cdot \rightarrow Cl_2$$ $$CH_3 \cdot + CH_3 \cdot \rightarrow C_2H_6$$ $$Cl \cdot + CH_3 \cdot \rightarrow CH_3Cl$$

Combustion Mechanism (Simplified)

The combustion of alkanes is a complex process involving free radicals. A simplified mechanism involves the following steps:

1. Initiation: High temperature causes homolytic cleavage of bonds in methane (or other alkane) to form methyl radicals and hydrogen radicals.

$$CH_4 \xrightarrow{\Delta} CH_3 \cdot + H \cdot$$

2. Propagation: The methyl radical reacts with oxygen to form a carbon-centered radical and a peroxy radical. The peroxy radical then abstracts a hydrogen atom from another methane molecule, forming a hydroperoxy radical and regenerating a methyl radical.
$$CH_3 \cdot + O_2 \rightarrow CH_3OO \cdot$$ $$CH_3OO \cdot + CH_4 \rightarrow CH_3OOH + CH_3 \cdot$$
3. Termination: Combination of radicals to form stable products.