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Pentane Isomers

A molecule can represent with the help of molecular and structural formula. Molecular formula represents the number of constituent atoms in the molecule. It does not provide any information about structure and bonding of atoms in the given molecule. On the other hand a structural formula gives the information about bonding of atoms in the molecule and also arrangement of constituent atoms. The structural formula can be written in the form of condensed structural formula, line-angle formula etc.

The condensed structural formula indicates the number of atoms involve in molecule. Like condense formula of ethane is C2H6. It indicates that it has 2 carbon atoms and 6 H atoms.

The line angle structural formula indicates carbon atoms by dots which are attached together with line. There is no H atom indicates in molecule. The molecules which have same molecular formula but different structural formula are called as isomerism and this phenomenon is called as isomerism. Isomers can be classified as constitutional and stereoisomers. Constitutional isomers are also called as structural isomers. They have different molecular formula but same structural formula. Constitutional isomers can be chain isomers, functional isomers, metamers, tautomers and positional isomers. Stereoisomers are also called as spatial isomers as they have different arrangement of bonded atoms in three dimensional spaces. They can be further classified as geometrical and optical isomers. Geometrical isomers are formed due to restricted rotation of C=C bond and can be two types; cis and trans-isomers. Optical isomers have different optical properties and can be dextro- and levorotatory isomers. 



Pentane Isomers Structural Formulas

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Constitutional isomers have same molecular formula but different structural formula. They can be further classified as chain, position, functional, metamers and tautomers. Chain isomers have same molecular formulae and have different number of carbon atoms in the parent chain. 

Chain isomerism is very common isomerism with alkanes as they have long carbon chain. Position isomerism is characteristic of those organic compounds which have same functional group at two different positions. The position of double or triple bond can also determine the positional isomerism.

Functional isomers have different functional groups but same molecular formulae. For example; alcohol and ether compounds are functional isomers of each other. Similarly carboxylic acid and ester with same molecular formulae are functional isomers of each other. Metamerism is mainly found with ether and amines. The alkyl group on both sides of given functional groups are different in these isomers. Lets discuss isomers of pentane. Since pentane is an alkane with 5 carbon atoms. The molecular formula of alkane with 5 carbon atoms will be C5H12. The structural formula for pentane is given below.


Alkanes do not have any other functional group so they can best show the chain isomers. With C5H12 molecular formula, most possible chain isomers can be 4 or 3 carbon atoms. Remaining carbon atoms will arrange as side chain bonded at parent carbon chain. 

Pentane Isomers Structural Formulas

How Many Structural Isomers of Pentane?

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The molecular formula for pentane is $C_{5}H_{12}$ in which 5 carbon atoms are arranged in straight chain with all single covalent bonds between carbon atoms. With same molecular formula, another possible structure is isopentane or 2 - methylbutane. Isobutane contains a parent chain with 4 carbon atoms and one methyl group as side chain 2 - position. So we can say that it is one of the chain isomer of pentane. Another possible isomer is neopentane with same molecular formula. The IUPAC name of neopentane is 2, 2 - dimethylpropane. It contains a parent chain of 3 carbon atoms and two methyl groups are bonded at 2-position of the molecule. So we say that pentane can show 3 structural isomers. 

Pentane, Isopentane & Neopentane

3 Isomers of Pentane

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Pentane can show chain isomerism with molecular formula C5H12. Three isomers with this molecular formula are pentane, 2 - methylbutane and 2, 2-

Pentane, 2 - methylbutane & 2, 2 - dimethylpropane

These three isomers have same molecular formula with different number of carbon atoms in the parent chain.

Isomers of Pentane Boiling Points

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We know that isomers of pentane have same molecular formula so they must have same molecular mass. But they show different chemical and physical properties. This is because of different structural formulae of them. They have different arrangement of atoms in the molecule and also different number of carbon atoms in the parent chain. The bonding and arrangement of atoms in the molecule affect the chemical and physical properties of molecules. For example, the boiling point and melting point of 3 isomers of pentane are different. 

Boiling point and melting points of molecules mainly depends on the branching and bonding of atoms in the molecule. A straight chain compound has strong London dispersion forces between molecules so their boiling and melting points are higher compare to branch compounds with same molecular formula. For example the boiling point of pentane is 36°C which is higher compare to isobutane (27.9°) and neopentane (9.5°C).  

Boiling Points of Pentane, 2 - methylbutane & 2, 2 - dimethylpropane

Isomers of Pentane Structure

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So we can say that chain isomerism is a common phenomenon for alkanes. Chain isomers have different number of carbon atoms in the parent chain of the molecule. Overall chain isomers have different branching of carbon atoms. 

For example; the molecular formula for pentane is $C_{5}H_{12}$. With this molecular formula, three chain isomers are possible. Pentane, isobutane and neopentane are three isomers with given molecular formula. Out of these, pentane is a straight chain compound whereas isobutane and neopentane are branched alkanes. 

Due to different branching in these isomers, they exhibit different chemical and physical properties. Physical properties like boiling point, melting point depend on structural formula of molecule. As branching increases in molecule, the melting point and boiling point of molecule decreases due to weak molecular interactions. So straight and branched chain isomers can easily identified with the help of their physical properties. Three isomers with molecular formula $C_{5}H_{12}$ also exhibit different physical properties due to same reason. 
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