This article presents the reader a complete answer to the question what are the properties of Alkanes. It gives the description of chemical and physical properties of Alkanes. Alkanes vary with the molecular weight and the molecular structure. There are two main molecular arrangements in Alkanes; namely, acyclic alkanes (CnH2n+2) and cyclic alkanes (CnH2n). This article focuses mainly on the acyclic alkanes and their properties, and explains the differences in properties of branched and unbranched alkanes. Branched alkanes and un-branched alkanes possess different chemical and physical properties that describes their chemical reactivity, nature of bonding, density and solubility, reasons for the variations in boiling point and melting point. In general, this article answers, ‘how’ and ‘why’ the physical properties of alkanes change along the alkane series.
What are Alkanes
Alkanes contain only Carbon and hydrogen atoms. They have only single bonds between Carbon atoms (C-C bonds). They are called “saturated hydrocarbons.” Organic molecules that are formed with only Carbon and Hydrogen atoms are called “hydrocarbons.” According to the orbital hybridization model, all the carbon atoms in Alkanes have the SP3 hybridization. They form sigma bonds with Hydrogen atoms resulting the molecular geometry like a tetrahedron.
General molecular formula of Alkanes
Alkanes have the general molecular formula CnH2n+2. The smallest alkane is Methane (CH4).
Molecular structure of Alkanes
Acyclic alkanes: There is no ring formation in the structure. However, it can have branched or unbranched molecular arrangements. Unbranched Alkanes are sometimes called n-alkanes.
Cycloalkanes: There is a circular molecular arrangement in the structure. Cycloalkanes have the general formula CnH2n.
Chemical properties of Alkanes
Reactivity
Alkanes are inert to many chemical reagents. “Paraffin” is an old name for hydrocarbons. It is derived from the Latin word “parumaffinis,” which means “with little affinity”. The reason is Carbon–Carbon (C-C) and Carbon–Hydrogen (C-H) bonds are quite strong. It is very difficult to break their bonds unless alkanes are heated to fairly high temperatures. The C-H bonds are also strong, because Carbon and Hydrogen atoms have nearly the same electronegativity values.
Combustion
Alkanes can readily burn in the air. Reaction between Alkanes with excess Oxygen is called “combustion.” In this reaction, alkanes convert to Carbon dioxide (CO2) and water.
The combustion reactions are exothermic, which means they give off heat. Therefore, alkanes can be used as a source of energy.
Physical Properties of Alkanes
Alkanes exist in all three forms: as gases, liquids and solids. Methane, Ethane, propane, and butane are gases in room temperature. The unbranched structures of pentane, hexane, and heptane are liquids. Alkanes with higher molecular weight are solids.
CH4 C4H10 Gases
C5H12 C17H36 Liquids
Alkanes with higher molecular weight Soft solids
Solubility
Alkanes are a non-polar organic compound. Water is a polar solvent, so alkanes do not dissolve in water. They are said to be “hydrophobic” (means ‘water hating’) compounds. They are dissolved in non-polar or weakly polar organic solvents. Alkanes are used as good lubricants and preservatives for the metals because, they protect the metal surface from reaching water; it prevents corrosion.
Density
The densities of Alkanes are lower than the density of water. Their density value is nearly 0.7 g mL-1, considering the density of water as 1.0 g mL-1. For example, if we mix an Alkane with water, the Alkane layer separates on the top of the water, since Alkanes are less dense compared to water and they are insoluble in water.
Boiling points
For unbranched alkanes, the boiling point smoothly increases as the number of Carbon atoms and the molecular weight are increasing. Larger molecules have a larger surface area providing a greater ability to form van der waals interactions (London force interactions). Though these are weak intermolecular forces, they raise the boiling points and thus prevent vapourisation.
In general, branched alkanes have lower boiling points compared to the same unbranched alkanes, having the same number of Carbon atoms. The differences in boiling points arise since branched alkanes are more compact is a small surface area, and thus facilitating less surface area for London force interactions. This lowers the boiling points in branched alkanes.
Melting points
For n-alkanes, this follows the same variation as the melting points; melting point increases with the molecular weight. However, there is a slight difference in melting points between the alkanes with even number of Carbon atoms and odd number of Carbon atoms. Alkanes with even number of Carbon atoms have higher melting points, because they are packed well into a solid structure. Therefore, a higher temperature along the alkane series is required to melt them. Therefore, the variation of melting points does not show a smooth curve along the alkane series.
In general, branched alkanes have higher melting points than n-alkane with the same number of Carbon atoms. Branched structure gives a more compact 3D-structure. It easily packs into a solid structure with a high melting point.
Properties of Alkanes – Summary
Alkanes are hydrocarbons with the chemical formula CnH2n+2. All the Carbon atoms are SP3 hybridized, and form sigma bonds directing toward the corners of a tetrahedron. Both boiling point and melting point increase with the molecular weight. Branching of the chain has a great effect for both, melting point and boiling point, but in opposite ways. Branching of alkanes lowers the boiling point, conversely branching of alkanes raises the melting point. For an n-alkane series, the variation of boiling point and melting point shows an upward trending graph. Nevertheless, the graph for the melting points, does not have a smooth shape.
Alkanes are chemically stable and usually do not involve in chemical reactions. They are insoluble in polar solvents and soluble in non-polar or weakly polar organic solvents. Alkanes are less dense than water.
Alkanes show isomerism; there are several molecular structures for one molecular formula. Their physical and chemical properties change with the structure.
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