[Chemistry Class Notes] on Preparation of Alkanes Pdf for Exam

Alkanes are saturated hydrocarbons. These organic compounds are made up of carbon and hydrogen atoms. They possess a single bond between carbon–carbon and carbon–hydrogen atoms. Their general formula is CnH2n+2. Methane, ethane, propane, butane, pentane, etc. are examples of alkanes. Suffix ‘ane’ is added at the end of the name of each alkane. Alkanes can be prepared by various methods.

Methods of Preparation of Alkanes 

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Unsaturated hydrocarbons such as alkenes and alkynes on catalytic hydrogenation give alkanes. Generally, in these reactions, nickel and platinum are used as catalysts. General equation is given below –

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For example, ethene can be converted into ethane by hydrogenation in the presence of nickel (catalyst) at 200–300℃.

[CH_2= CH_2+H_2xrightarrow{Ni235^{circ}C}CH_3-CH_3]

Alkanes can be prepared by reduction of haloalkanes. One of these preparation methods of alkanes was given by Wurtz which is called Wurtz Reaction. 

When alkyl halide reacts with metallic sodium in presence of dry ether, it forms alkane with double the number of carbon atoms present in the alkyl halide of reactants. This reaction is called the Wurtz Reaction. 

Wurtz Reaction can be written as follows – 

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Alkanes can be prepared by reduction of alkyl halide. In these reactions, LiAlH4, H2/Pd or nascent hydrogen etc. are used as reducing agents. The general equation is given below taking nascent hydrogen as reducing agent –

R–X + 2H → R–H + HX

Alkanes can be prepared by reducing aldehydes and ketones by amalgamated zinc and concentrated hydrochloric acid. Reactions are given below –

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Alkanes can be prepared by hydrolysis of Grignard’s reagent in presence of dry ether. The general equation is given below –

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Alkanes can be prepared from salts of carboxylic acids. One of these methods includes Kolbe’s electrolytic method. 

2RCOONa → 2RCOO− + 2Na+

RCOO− gains electrons and forms unstable RCOO which gives R–R (alkane) and carbon dioxide gas. 

• By Heating Sodium Salts of Carboxylic Acids – Alkanes can be prepared by heating sodium salts of carboxylic acids with soda lime (mixture of NaOH and CaO). In this reaction, carbon dioxide is removed, thus the alkane obtained by this reaction has one carbon less than reactant salt. This reaction is called decarboxylation. Methane gas is prepared by this method in laboratories.  Reaction is given below –

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Alkanes can be prepared by reaction of metal carbide with pure water. In the reaction, metal carbides such as aluminium carbide (Al4C3), beryllium carbide (Be2C), etc. can be used. Reactions are given below –

Be2C + 4H2O → CH4 + 2Be(OH)2

Al4C3 + 12H2O → 4Al(OH)3 + 3CH4

So, this is all about the preparation of Alkanes. We hope you enjoyed learning about it. 

Properties of Alkane 

Below are the various properties of alkane: 

1. Solubility of Alkane: As there is only a little difference in electronegativity between hydrogen and carbon, the nature of alkane molecules is polar. Polar molecules are soluble in polar solutions only, which is why alkane is insoluble in water. 

2. Boiling Point: If there is an increase in the molecular weight, there will be an increase in the boiling point of alkane too. Moreover, straight-chain alkanes have a much higher boiling point than structural isomers. 

3. Melting Point: The change in molecular weight has the same effect on the melting point as it does on the boiling point of alkanes. Increased molecular weight will lead to an increase in the melting point of alkane.  

Uses of Alkane 

Alkane is used in various areas for different purposes. Below are some of the areas and industries where alkanes are widely used. 

• Alkanes are used in different types of fertilisers to substitute the chemical compounds taken from the soil by plants. 

• Ethane, which is a type of alkane, is used as a refrigerant in refrigeration systems as it is an odourless compound. Moreover, ethane can also be used for heating and cooking food. 

• Propane, another type of alkane, is a common ingredient in air fresheners by releasing a scented smell. It is also used in households to generate heat for cooking due to the lack of electricity or gas. 

• Since heptane has low reactivity with chemicals, it is widely used in school laboratories. It is also used in the pharmaceutical industry to manufacture a number of drugs and medicines. Moreover, law enforcement agencies can use heptane to process hidden fingerprints.

• Some alkanes are used in engines to prevent them from getting damaged. 

• Higher alkanes have high density, which means that they are more suitable to use as aviation fuels. 

• Alkanes act as a hydrogen solvent to help in the manufacturing of paints.