Physical Properties of Alkanes and Their Variations

Hydrocarbons can be classified as alkanes, alkenes, alkynes and aromatic compounds. When asked “what are alkanes?”, we can say that alkanes are saturated acyclic/aliphatic hydrocarbons. This means these are open-chain compounds(branched or unbranched) and here each carbon atom is bonded to other atoms through a single covalent bond only. 

An alkane is also called paraffin. It consists of carbon and hydrogen atoms bonded by a single covalent bond in a tree-like structure. The general formula for alkanes is CH₂ₙ₊₂. Here we'll learn about the properties of alkanes and their variations. The structure of methane (CH₄),  is given below which is one of the most common alkanes.

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Properties of Alkanes

Let us now see what are the physical and chemical properties of alkanes.

Physical Properties of Alkanes

Some important physical properties of alkanes are:

1. Alkanes are colourless and odourless.

2. They possess weak Van Der Waals forces of attraction.

3. Alkanes having 1-4 carbon atoms are gases, then from 5-17 carbon atoms they are liquid and alkanes having 18 or more carbon atoms are solid at 298K.

4. Structure of alkanes:

In alkanes all the carbon atoms are sp³ hybridised which means that they form four sigma bonds with either carbon or hydrogen atoms. Their general formula Is CₙH2n+2. The bond angle between them is 109.5° and they exhibit tetrahedral geometry.

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5.  Melting and boiling point 

Shorter chain alkanes have low melting and boiling points but as the number of carbon atoms in the chain increases melting and boiling points rise.

• Boiling Point:

it increases with the increasing molecular weight as the Van Der Waals force increases with the increasing molecular weight. Straight chain alkanes have a higher boiling point than their structural isomers.

• Melting Point :

It also increases with increasing molecular weight because it is difficult to break the intermolecular forces of attraction between higher alkanes as they are generally solids. Even-numbered alkanes have a better packing in the solid phase than the odd ones as they form a well-organised structure which is difficult to break hence even-numbered alkanes have a higher melting point than odd-numbered ones.

6. Solubility

• Alkanes are generally non-polar molecules because of the covalent bonds between C-C and C-H and also because of the very small difference between electronegativities of carbon and hydrogen.

• We know that polar molecules are soluble in polar solvents and nonpolar molecules are soluble in non-polar solvents generally so this implies alkanes are insoluble in water or hydrophobic in nature.

• When a non-polar alkane is added to a polar solvent the water molecules are attracted to each other and alkane molecules are attracted to each other but water and alkane molecules do not attract each other.

• In organic solvents, they are soluble because the energy required to overcome existing Van Der Waals forces and to generate new Van Der Waals forces is quite comparable.

7. Alkanes have a lower density than water, they float on water. Density increases with an increase in molecular mass.

8. Apart from weak Van Der Waals forces, London forces, Dispersion forces, weak intermolecular forces act between the molecules of alkanes.

Chemical Properties of Alkanes 

Some important chemical properties of alkanes are given below.

1. Combustion of alkanes

In excess of oxygen O2, alkanes readily undergo combustion producing carbon dioxide gas, water and energy in the form of heat and light.

Alkane + Oxygen → Carbon Dioxide gas + Water +  Energy

C4H10(g) + 6½O2(g) → 4CO2(g)  + 5H2O(l) + 2874 KJ mol-1

The above reaction is the combustion reaction of butane. With the increasing molar mass of straight-chain alkane the energy released increases. Also with the increasing carbon chain length the combustion energy increases. In the absence of sufficient oxygen, alkanes try to undergo incomplete combustion. Incomplete combustion produces water and carbon monoxide or carbon.

2. Halogenation

Alkanes are less reactive. Without ultraviolet light, they do not react with halogens. With UV light halogenated alkane is produced. It is a substitution reaction in which one or more hydrogen atoms are substituted by halogen atoms.

CH3-CH2-CH3 + Br3 →  CH3-CH2-CHBr + HBr

Here propane is reacting with bromine. The general substitution reaction equation can be given as 

R-H +X2 → R-X + H-X

where R is a carbon chain and X is a halogen.

Uses of Alkanes

Because of the properties of alkanes, they are quite useful. Some of the uses are:

1. Propane and butane are used in propane gas burners, as propellants or aerosol sprays when liquified at low temperatures.

2. Pentane to octane fuels are good for an internal combustion engine.

3. Nonane to hexadecane have high viscosity and find use in diesel and aviation fuel.

4. Up to C-35 alkanes are used as paraffin wax candles, as anti-corrosive agents and in lubricating oil. Higher alkanes are cracked to smaller alkanes and then brought into use.

Alkanes are the basic hydrocarbons that are bonded through a single bond. They are saturated hydrocarbons. In this article, we have learned about the physical and chemical properties of alkanes. Also, we learned about the variation of physical properties. Therefore, this article develops the strong basic concept of organic chemistry for class 12 students.