Alkanes: Physical Properties
At roomtemperature, alkanes from C1 to C4 are gases, from C5 to C16 they are liquids and higher alkanes are solids and waxlike to some extent .
In saturated alkanes, only van der Waals forces are important as attractive forces. In general, alkanes show a relatively low reactivity.
However, protonation, oxygenation, pyrolysis, radiolysis, and photolysis are possible under extreme reaction conditions.
Oxidation of alkanes can also be carried out electrochemically. Lower alkanes in particular are highly flammable and form explosive mixtures (methane, benzene) with air (oxygen). Solubility of alkanes in water is very low.
The physical properties of alkanes follow a similar trend as seen in the regularity of alkane structures.
Thus, the larger the number of carbon atoms the higher the boiling points.
However, the melting point curve does not exhibit the same smooth gradation.
The melting point graph is not smooth, however.
Alkanes with even numbers of carbon atoms pack better into a solid structure, so that higher temperatures are needed to melt them.with odd numbers of carbon atoms do not pack as well, and they melt at lower temperatures.
Alkanes with even numbers of carbon atoms pack better into a solid structure, so that higher temperatures are needed to melt them.with odd numbers of carbon atoms do not pack as well, and they melt at lower temperatures.
A branched alkane generally melts at a higher temperature than the n-alkane with the same number of carbon atoms.
Branching of an alkane gives it a more compact three-dimensional
structure, which packs more easily into a solid structure and increases the melting point.
Boiling Points of Alkanes :
The boiling points increase smoothly with increasing numbers of carbon atoms and increasing molecular weights.
Larger molecules have larger surface areas, resulting in increased inter molecular van der Waals attractions.
As inter molecular interactions increases volatility decreases .Then vapour pressure decreases .With decrease in vapour pressure boiling point decreases.
These increased attractions must be overcome for vaporization and boiling to occur.
Thus, a larger molecule, with greater surface area and greater van der Waals attractions, boils at a higher temperature.
Branching of an alkane gives it a more compact three-dimensional
structure, which packs more easily into a solid structure and increases the melting point.
Boiling Points of Alkanes :
The boiling points increase smoothly with increasing numbers of carbon atoms and increasing molecular weights.
Larger molecules have larger surface areas, resulting in increased inter molecular van der Waals attractions.
As inter molecular interactions increases volatility decreases .Then vapour pressure decreases .With decrease in vapour pressure boiling point decreases.
These increased attractions must be overcome for vaporization and boiling to occur.
Thus, a larger molecule, with greater surface area and greater van der Waals attractions, boils at a higher temperature.
Boiling Points of A l kanes
Table 3-2 also gives the boiling points and melting points of the unbranched alkanes.
The boiling points increase smoothly with increasing numbers of carbon atoms and
increasing molecular weights. Larger molecules have larger surface areas, resulting in
increased intermolecular van der Waals attractions. These increased attractions must be
overcome for vaporization and boiling to occur. Thus, a larger molecule, with greater
surface area and greater van der Waals attractions, boils at a higher temperature.
Table 3-2 also gives the boiling points and melting points of the unbranched alkanes.
The boiling points increase smoothly with increasing numbers of carbon atoms and
increasing molecular weights. Larger molecules have larger surface areas, resulting in
increased intermolecular van der Waals attractions. These increased attractions must be
overcome for vaporization and boiling to occur. Thus, a larger molecule, with greater
surface area and greater van der Waals attractions, boils at a higher temperature.
| Name | Mp [°C] | Bp [°C] |
|---|---|---|
| Methane | -182.5 | -161.5 |
| Ethane | -183.3 | -88.6 |
| Propane | -187.7 | -42.1 |
| Butane | -138.4 | -0.5 |
| Pentane | -129.7 | 36.1 |
| Hexane | -95.3 | 68.7 |
| Heptane | -90.6 | 98.4 |
| Octane | -56.8 | 125.7 |
| Nonane | -53.5 | 150.8 |
| Decane | -29.7 | 174.1 |
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