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Alkanes – Definition, Nomenclature, Preparation, Properties

  • Last Updated : 05 Oct, 2021

In natural science, a hydrocarbon is a natural atom comprising completely hydrogen and carbon. Hydrocarbons are an illustration of gathering 14 hydrides. Hydrocarbons are dreary and hydrophobic, with a slight scent. As a result of their diverse compound designs, it’s difficult, to sum up anymore. The greater part of anthropogenic hydrocarbon discharges come from the copying of petroleum derivatives, which incorporates both fuel creation and ignition. Ethylene, isoprene, and monoterpenes are largely normal hydrocarbons found in plant emanations.

What are Alkanes?

Alkanes are organic compounds made up of carbon and hydrogen atoms that are single-bonded. Alkanes have the formula CnH2n+2 and are classified into three groups: 

  • Chain alkanes, 
  • Cycloalkanes, and 
  • Branched alkanes.

Alkanes are a class of chemical compounds that have only one covalent link between carbon and hydrogen atoms. Single covalent bonds between carbon and hydrogen atoms make up this class of compounds. Coal gases, produced by destructive distillation of coal, LPG (Liquid Petroleum Gas), and CNG (Compressed Natural Gas) are examples of fuels. Additionally, kerosene oil is utilised as a household fuel. All of these fuels are made up of a combination of hydrocarbons, which are energy sources.

Nomenclature of Alkanes

CnH2n+2 is the general formula for alkanes, with n=1, 2, 3, 4. so on. Alkanes have a suffix- ane in the IUPAC system, and the prefix is determined by the number of carbon atoms. The following are some of the saturated hydrocarbons names:

Number of Carbon Atoms

Formula

IUPAC Name

1

CH4

Methane

2

C2H6

Ethane

3

C3H8

Propane

4

C4H10

Butane

5

C5H12

Pentane

6

C6H14

Hexane

7

C7H16

Heptane

8

C8H18

Octane

9

C9H20

Nonane

10

C10H22

Decane

Isomerism of Alkanes

Because there is only one method to link 1, 2, and 3 carbon atoms, the first three alkanes, methane, ethane, and propane, have just one structure. Butane, on the other hand, can have two isomers.

Isomerism of Alkanes: Butane

Pentane contains three isomers because five carbon atoms may be linked in three distinct ways.

Isomerism of Alkanes: Pentane

Hexane has five isomers, heptane has nine, octane has eighteen, nonane has thirty-five, and decane has seventy-five.

Preparation of Alkanes

Hydrogenation or reduction of unsaturated hydrocarbons is the process of adding hydrogen to an unsaturated hydrocarbon in the presence of a catalyst.

C2H4 + H2 → C2H6

C3H6 + H2 → C3H8

From alkyl halides 

Any of the following techniques can be used to convert alkyl halides into alkanes:

  • Grignard Reagent: Alkyl magnesium halides are formed when alkyl halides, particularly bromides and iodides, combine with magnesium metal in the presence of dry ethoxyethane (diethyl ether). Alkanes are formed when alkyl magnesium halides combine with water.

Preparation of Alkanes using Alkyl Halides: Grignard reagent

  • By Wurtz Reaction: The synthesis of alkanes is increased when an alkyl halide, particularly bromide and iodide, is treated with metallic sodium in the presence of dry ethyl ether.

Preparation of Alkanes using Alkyl Halides: Wurtz Reaction

From carboxylic acid

Any of the following procedures can be used to produce alkanes.

  • Decarboxylation: It’s an alkane preparation process done in the lab. A molecule of carbon dioxide is lost when a carboxylic acid is heated with soda lime (NaOH + CaO) at around 630 K, and an alkane with a carbon atom is fewer than carboxylic acid is produced.

Preparation of Alkanes from Carboxylic acid: Decarboxylation

  • Kolbe’s electrolytic method: When a concentrated aqueous potassium or sodium solution is used.

Preparation of Alkanes from Carboxylic acid: Kolbe’s electrolytic method

Physical Properties of Alkanes

The intermolecular force of attraction determines the physical characteristics of alkanes. Let’s have a look at some of the properties:

  • Boiling point- The straight-chain alkanes have higher boiling points as their molecular mass increases. This is due to a rise in the molecular size and surface area of the molecule, which raises the magnitude of van der Waals forces of attraction, and hence the boiling point. The first four members of the straight-chain alkanes are gases, the following thirteen are liquids, and the upper members are colourless waxy solids.

  • Melting Point- Alkanes’ melting points follow the same pattern as their boiling points. With increasing molecular mass, the melting point of alkanes rises. The melting point of alkanes is determined by their form and size, as well as the density of their molecules. As a result, it’s been discovered that even-numbered alkanes have a greater melting point trend than odd-numbered alkanes.

  • Solubility- Due to the tiny electronegativity among carbon and hydrogen and the covalent property of the C–C bond and C–H bond, alkanes are non-polar sort of particles. Alkanes are insoluble in polar solvents like water, liquor, and so forth, however, are profoundly dissolvable in non-polar solvents like petrol, ether, benzene, carbon tetrachloride, and so on.
  • Density- The densities of alkanes increment with an increment in the sub-atomic masses as far as the possible worth of 0.8 g cm–3 is reached, which implies that all alkanes are lighter than water.

Reactions of Alkanes

  • Substitution reactions- A substitution reaction occurs when a hydrogen atom in a hydrocarbon is replaced by an atom or a group of atoms. Because C–C and C–H have a sigma bond, alkanes only undergo substitution reactions. The following are some examples of substitution reactions:
    • Halogenation- Halogenation is the process of exchanging a hydrogen atom with a halogen such as F, Cl, Br, l. It can be done by heating the mixture between 520 and 670 degrees Celsius, or by using UV radiation.
    • Nitration- Nitration is the process of replacing a hydrogen atom with a nitro group. Alkanes do not react with nitric acid at usual temperatures, but when a combination of an alkane and fuming HNO3 vapours is heated at 423 – 673 K under pressure, alkanes are nitrated.

CH4HNO3 → CH3–NO2 + H2O

  • Oxidation- Alkanes burn in the air when heated, generating carbon dioxide and water. This is the combustion process, which results in the release of a huge quantity of heat energy.

CH4+2O2→2CO2+H2O ; ΔcH∘ = –80KJmol–1

When alkanes are heated with inadequate oxygen, incomplete combustion occurs, resulting in carbon monoxide and unburned carbon.

2CH4 + 3O2 → 2CO + 4H2O ; 

CH4+O2→C+2H2O

Uses of Alkanes

  1. Natural gas is made up of methane. In both households and enterprises, LPG (a combination of butane and isobutane) is utilised as a fuel.
  2. Carbon black, which is used in printing inks, painting, and automotive tyres, is made from alkanes.
  3. Alcohols, aldehydes, and carboxylic acids are produced via catalytic oxidation of alkanes.
  4. Gasoline, kerosene oil, diesel, lubricating oils, and paraffin wax are all examples of higher alkanes.
  5. Certain halogen derivatives, such as chloroform and carbon tetrachloride, are made from alkanes and are utilised as solvents in industry and laboratories.

Sample Questions

Question 1:  What are the properties of alkanes?

Answer:

Alkanes have three physical properties: they have a high melting point, a high boiling temperature, and they are soluble in non-polar solvents but insoluble in polar solvents.

Question 2: What are the first four alkanes?

Answer:

The first four alkanes are methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10). The simplest alkane is methane gas, which has the chemical formula CH4.

Question 3: What is Halogenation?

Answer:

 Halogenation is the process of exchanging a hydrogen atom with a halogen such as F, Cl, Br, l. It can be done by heating the mixture between 520 and 670 degrees Celsius, or by using UV radiation.

Question 4: What is Nitration?

Answer:

Nitration is the process of replacing a hydrogen atom with a nitro group. Alkanes do not react with nitric acid at usual temperatures, but when a combination of an alkane and fuming HNO3 vapours is heated at 423–673k under pressure, alkanes are nitrated.

 CH4HNO3 → CH3–NO2 + H2O

Question 5: Why alkanes are insoluble in water?

Answer:

Due to the tiny electronegativity among carbon and hydrogen and the covalent property of the C–C bond and C–H bond, alkanes are non-polar sort of particles. Alkanes are insoluble in polar solvents like water, liquor, and so forth, however are profoundly dissolvable in non-polar solvents like petrol, ether, benzene, carbon tetrachloride, and so on .


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