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Preparation of Alkanes

Last updated date: 01st Mar 2024
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An Introduction to the Preparation of Alkanes

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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  • From Unsaturated Hydrocarbons (Sabatier and Senderens Method)

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℃.

\[CH_2= CH_2+H_2\xrightarrow{Ni235^{\circ}C}CH_3-CH_3\]

  • From Haloalkanes 

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

  • 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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  • By Reduction 

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

  • By Reduction of Aldehydes and Ketones 

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

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  • By Grignard’s Reagent 

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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  • By Salts of Carboxylic Acid or Decarboxylation 

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

  • Kolbe’s Reaction – On electrolysis, an aqueous concentrated solution of salts of carboxylic acid gives alkanes. 

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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  • By Metal Carbides 

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. 

FAQs on Preparation of Alkanes

1.What are some applications of alkanes?

Below are some applications of alkanes: 

  • Alkanes can be used in the production of shoes, glue, and many other leather products. 

  • Alkane is used by the police to identify hidden fingerprints. 

  • Hexane, a common type of alkane, is used to extract cooking oils. 

  • Alkanes are added to pesticides to make them more effective. 

  • The fuels in cigarette lighters also contain alkane. 

  • Butane is an alkane used in producing artificial rubber, which is used in making truck treads. 

2.How to prepare alkane from haloalkanes?

There are two methods you can use to prepare alkane from haloalkanes. These are as follows: 

  • Wurtz Reaction

In this method, alkyl halides are heated with sodium metal in the presence of dry ether. The alkanes produced in this method have double the number of C - atoms than in alkyl halides. 

  • Reduction of alkyl halides (RX)

In this method, alkanes are produced when haloalkanes bond, i.e. R-X, is heated to reduce substances like ether, Pd/H2, Pt/H2, Z. 

3.What can alkanes react with?

Alkanes cannot react with most reagents because of two reasons. First, the carbon–hydrogen bond and carbon–carbon bond are significantly strong due to their orbital overlap. Second, the carbon–hydrogen bond has similar electronegativity, which is why they cannot make alkane molecules acidic or basic. Due to this, the carbon–hydrogen bond has less polarity which means proton loss is quite difficult, making the alkane a poor acid. Moreover, due to the lack of nonbonded electron pairs on one of the atoms in the carbon–hydrogen bond, alkanes become poor bases. 

4.Explain the physical properties of alkanes.

Below are some of the physical properties of alkane: 

  • Alkane exists in the form of solid, liquid, and gases at room temperature. Methane, ethane, and propane are gaseous alkanes. From pentane to hexadecane, all the alkanes are liquids. Homologues that are larger than hexadecane are solid forms of alkane. 

  • Branched alkanes usually have a lower boiling point than unbranched ones. 

  • Solid alkanes are softer with low melting points because of the strong and repulsive forces created between electrons. 

  • Alkanes are insoluble in water as there is less attraction between water molecules and alkane molecules. 

5.Explain the uses of alkanes.

Alkanes are used in a number of areas and industries for different purposes. For starters, alkanes is used in refining crude oil to remove its dark colour, bad odour, and sulphur compounds. Even natural gases use light alkane, producing fewer pollutants as compared to petroleum fuels. Below are some other areas where alkanes are used: 

  • Ovens

  • Fireplaces

  • Fuel Cells

  • Outdoor lights

  • Heating/cooling house 

  • Plastic goods

  • Fertilisers 

  • Motor fuels (used as LPG or CNG that are cheaper and do not cause much pollution)

  • Fabric