Hydrocarbons can be classified as alkanes, alkenes, alkynes and aromatic compounds. When asked “what are alkanes?”, we can say that alkanes are the 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 CₙH₂ₙ+2. Here we'll learn about the properties of alkanes and their variations.
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Above structure is of ethane(C2H6), which is one of the most common alkanes.
Let us now see what are the physical and chemical properties of alkanes.
Some important physical properties of alkanes are:
Alkanes are colourless and odourless.
They possess weak Van Der Waals forces of attraction.
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.
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.
Bond angle between them is 109.5° and they exhibit tetrahedral geometry.
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The figure above is the 3-D representation of methane.
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 higher melting point than odd-numbered.
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 increase in molecular mass.
8) Apart from weak Van Der Walls forces, London forces, Dispersion forces, weak intermolecular forces act between the molecules of alkanes.
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
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.
Alkanes are less reactive. Without ultraviolet light, they do not react with the 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 + Br2 → 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 halogen.
Uses of Alkanes
Because of the properties of alkanes, they are quite useful. Some of the uses are:
Propane and butane are used in propane gas burners, as propellants or aerosol sprays when liquified at low temperature.
Pentane to octane fuels is good fuel for an internal combustion engine.
Nonane to hexadecane have high viscosity and find use in diesel and aviation fuel.
Up to C35 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.
1. Why are alkanes called paraffin?
Paraffins are taken from the Latin language. It is formed by two words parum meaning little and affinis meaning reactivity because of their little affinity towards a general reagent. We can say that alkanes are inert and undergo reactions under drastic conditions. This happens because alkanes form only single bonds between carbon and hydrogen atoms which are relatively strong and difficult to break and also carbon and hydrogen have similar electronegativities which give molecules that are nonpolar. Therefore, they undergo limited reactions, undergo specific conditions only and are called paraffin.
2. How are alkanes named?
Alkanes are named by IUPAC norms. All alkanes have a common suffix 'ane' and the number of carbon atoms determine the name. First 10 alkanes are given below.
C1 - methane
C2 - ethane
C3 - propane
C4 - butane
C5 - pendant
C6 - hexane
C7 - heptane
C8 - octane
C9 - nonane
C10 - decane
They are generally written as n-hexane, n-heptane etc. if they normal chain alkanes but when the isomers of alkanes are written we use the prefixes iso and neo. When all carbons except one form a continuous chain we use iso and when all but two carbons which are a part of terminal tert- butyl group form a continuous chain we use the prefix neo.