Alkanes

What Are Alkanes? 

Alkanes are organic compounds composed of single-bonded carbon and hydrogen atoms. The formula for alkanes is CnH2n+2. They can be categorized into three groups which are:  chain alkanes, cycloalkanes, and branched alkanes.

The most basic family of compounds has been called alkanes. They comprise only hydrogen and carbon. Each carbon atom creates four bonds, and each hydrogen atom generates a solid bond. Chemists have been using the line-angle formulas since it is faster and easier to draw, unlike condensed structural formulas. The structural formulas for alkanes can indeed be written in a condensed form.


Hydrocarbons 

Simple alkane methane contains one carbon atom and four hydrogen atoms, and CH4 is its molecular formula. The hydrocarbon meaning is that these compounds exclusively consist of hydrogen and carbon atoms. Because this compound only has a single covalent bond, therefore, its chemical structure is

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Long Chain Alkane Molecules 

With the help of a single covalent bond, additional carbon atoms are connected to one another in a long chain alkane molecule. Each atom is bonded to adequate hydrogen atoms to form a total of 4 single covalent bonds. The whole long-chain structure is referred to as octane. The eight-carbon alkane has a molecular formula like C8H18 and a structural formula as shown in the image below

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List Of Different Alkanes 

Names of Alkanes 

Molecular Formula 

Structural formula 

Methane

(CH4)

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Methane

Ethane

(C2H6)

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Ethane

Propane

(C3H8)

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Propane

Butane

(C4H10)

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Butane

Pentane

(C5H12)

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Pentane

Hexane

(C6H14)

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Hexane

Heptane

(C7H16)

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Heptane

Octane

(C8H18)

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Octane

Nonane

(C9H20)

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Nonane

Decane

(C10H22)

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Decane


Physical Properties Of Alkanes 

  1. Solubility Properties 

  • Owing to a very small difference in electronegativity between carbon and hydrogen as well as the covalent property of the C-C bond or C-H bond, alkanes are commonly non-polar kinds of molecules.

  • As we usually witness, polar molecules seem to be soluble in polar solvents, whereas non-polar molecules are soluble in non-polar solvents. As a matter of fact, alkanes are hydrophobic in nature, which means that alkanes are insoluble in water.

  • Alkanes are soluble in organic solvents because the energy needed to overcome the established Van Der Waals forces and to create new Van Der Waals forces is quite comparable.

  1. Boiling Point Properties 

As we increase the molecular mass or the surface area of the molecule, there is a significant increase in the intermolecular Van Der Waals forces. There are two observations related to this process as follows: 

  • As the molecular weight of the alkanes increases, their boiling point also increases. 

  • Structural isomers have a lesser boiling point as compared to their equivalent straight-chain alkanes

  1. Melting Point Properties 

  • The melting point trends in alkanes, which are the hydrocarbon compounds, are also similar to that of the boiling point trend.

  • The melting point increases as the molecular weight of the alkane increases

  • It is due to the fact that the higher alkanes are in the solid-state, and hence it is more difficult to overcome the intermolecular forces of attraction, and as a result, the intermolecular forces are stronger and the melting point higher. 

  • It is commonly observed that even-numbered alkanes have a higher trend at the melting point as compared to odd-numbered alkanes. Also, the even-numbered alkanes are packed tightly in the solid phase, establishing a very well-organized structure that is difficult to break down.


Alkane Structural Formula 

The structural formulas for alkanes might well be written in condensed form. For example, the structural formula of pentane comprises three groups of CH2 methylene in the central portion of the chain. They can be grouped together, and the structural formula can be written down. The very first five alkane formulas with an unbranched chain are shown below: 

Name

Molecular Formula 

Condensed structural Formula 

Methane

CH4

CH4

Ethane

C2H4

CH3CH3

Propane

C3H8

CH3CH2CH3

Butane

C4H10

CH3(CH2)2CH3

Pentane

C5H12

CH3(CH2)3CH3



What Is The Formula Of Alkane?

Formulas of organic compounds provide the information of different properties and structures of those compounds. Molecular formulas, like that of octane, give the number and types of atoms in a molecule of a compound. The molecular formula of C8H18 may even be applied to many alkanes, each of which has unique physical, chemical , and toxicological properties. Compounds with the same molecular formula, and yet distinct structural formulas are known as structural isomers of that compound.

Now, many organic compounds may also be extracted from alkanes. In addition, numerous important elements of organic molecules encompass one or more alkane groups. Due to these factors, the names of so many organic compounds are based on the names of alkanes.


Alkane Formula For Branched Chains 

Just like other organic compounds, carbon atoms in alkanes will also form straight chains, branched chains, or rings. These three types of alkanes are termed as straight-chain alkanes, branched-chain alkanes, and cycloalkanes. 

The general molecular formula: 

  • For straight-chain alkanes and branched-chain alkanes: CnH2n+2 

  • For cyclic alkanes: CnH2n.


Example Questions

Question) Write down the structural formulas of one example alkane compound one of each type (Simple, branched and cyclic)

Answer) 

Straight chain alkanes 

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Branched-chain alkane: 2,3 Dimethyl Butane 

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Cyclic Alkane: Cyclo-Hexane 

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FAQ (Frequently Asked Questions)

1. What Are Alkyl Groups?

When an alkane with hydrogen is separated from a bond, it is called an alkyl group. This Alkyl group is very often referred to as the letter R, the same as the halogens, which are represented by the letter X. When a substituent group such as halogen attaches to an alkane molecule, a carbon-hydrogen bond of the molecule is transformed into a carbon-substituent bond. Here is a methane-chlorine reaction which can be written its general form as

R-H + X2 + (U.V Light) R-X + H-X

2. What Can Be The General Formula For The Structure Of Alkyne And What Can Be The Formula For The Simplest Alkyne?

Alkynes are compounds in a homologous series, with the general molecular formula which can be written as CnH2n‐2

Alkynes have always been triple bond carbon-carbon hydrocarbons. They do not exhibit any geometric or optical isomerism. Ethyne (HC-CH), often referred to as acetylene, is the easiest and simplest Alkyne.