Introduction to Alkanes and their general properties

Hydrocarbons

The branch of chemistry which deals with the carbon and hydrogen derivatives is called hydrocarbons.

They can be classified into two broad categories:

Aliphatic: These are open-chain molecules, meaning the carbon atoms are arranged in straight or branched chains. They do not contain aromatic rings.

Aromatic: These hydrocarbons have at least one benzene ring (a ring of six carbon atoms with alternating double and single bonds). Benzene rings provide special stability due to the delocalization of $-$electrons.

Subcategories of Aliphatic Hydrocarbons:

Saturated: These hydrocarbons contain only single bonds (sigma bonds) between carbon atoms. Examples include alkanes (straight or branched-chain hydrocarbons) and cycloalkanes (closed ring structures, e.g., cyclopropane, cyclobutane, and cyclohexane).

Unsaturated: These contain one or more double or triple bonds ($\pi$π-bonds) between carbon atoms. Examples include alkenes (with double bonds) and alkynes (with triple bonds). The presence of these bonds makes them more reactive than saturated hydrocarbons.

 

Alkanes (Paraffins):

Simplest form: Methane (CH₄) is the simplest alkane. It is known as a marsh gas because it is found in marshy or muddy environments.

Geometry: Methane and all alkanes exhibit a tetrahedral geometry due to sp³ hybridization of carbon atoms, where the bond angles are approximately 109.5°.

General formula: Alkanes follow the general formula

Homologous series: A series of compounds represented by the general formula with the addition of CH₂ group is called Homologous series.

Alkyl groups: An alkyl group is formed when one hydrogen atom is removed from an alkane. For example, if we remove one hydrogen from methane (CH₄), we get a methyl group (CH₃). Similarly, removing one hydrogen from ethane (C₂H₆) gives an ethyl group (C₂H₅).

General Properties of Alkanes:

Colour and State:

Small alkanes (C₁ to C₄) are gases at room temperature.

Medium-sized alkanes (C₅ to C₁₇) are liquids.

Larger alkanes (C₁₈ and above) are solids.

 

Boiling Point:

Straight chains: As the number of carbon atoms in the straight-chain alkanes increases, their boiling points increase in a fairly regular manner. This is because larger molecules have greater surface area and stronger van der Waals forces (intermolecular forces).

Branched chains: Branched-chain alkanes have lower boiling points compared to their straight-chain counterparts. This is because branched molecules are more compact and have less surface area, reducing van der Waals forces.

 

Melting Point (M.P.):

In alkanes, the melting point increases as the length of the carbon chain increases, similar to the trend in boiling points.

Branched alkanes tend to have higher melting points compared to straight-chain alkanes of the same molecular weight. This is due to the fact that branched molecules can pack more tightly into a crystal lattice, leading to stronger intermolecular forces.

 

Solubility:

Alkanes are non-polar molecules. As a result, they dissolve well in non-polar solvents like hexane, benzene, and ether.

They do not dissolve in polar solvents like water because there is no attraction between the polar water molecules and non-polar alkane molecules. In chemistry, this is summarized by the phrase "like dissolves like," meaning non-polar solutes dissolve in non-polar solvents, and polar solutes dissolve in polar solvents.

Example: Water (H₂O) is a polar solvent, while alkanes like hexane are non-polar, so they do not mix.

 

Density:

The density of alkanes depends on their molecular weight. As the molecular weight of an alkane increases, its density also increases, but alkanes have low densities compared to other organic compounds.

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Alkanes generally have densities in the range of 0.6-0.8 g/ml less than water, so they float on water. For example, oil (a mixture of hydrocarbons) floats on water because its density is lower.