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Isomers of Pentane

The term isomer is used for molecules having same molecular formula but different structural arrangement of the hydrocarbon main chain and the respective functional groups. Isomerism is visible from butane or the hydrocarbon with four carbon atoms in main chain. The physical state of straight chain alkanes at room temperature varies with their molecular size. Butane is considered as the simplest form of isomer but more specific and structurally vivid is pentane.

The C-1 to C-4 compounds are gases and the C-5 to C-16 compounds are liquids and anything beyond are all solid in nature. The criteria shifts to the branched or straight chain molecules and how difficult it is to escape into vapour state and thus the idea of isomerism. 

Till we have a proper straight chain hydrocarbon which can also exist as a branched molecule we do not come across an isomeric form. Isomers of pentane is first of the true isomeric forms which gives a very specific and different structure than the straight chain pentane molecule. 

 

Isomers of Pentane $C_{5}H_{12}$

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For any straight chain alkanes and for other families of straight chain compounds, the boiling points increase on a regular basis with increase in chain length and this is completely depending upon the molecular size as molecular surface area increase with increase in molecular size. 

Larger the surface are, greater are the attractive van der Waal forces between molecules and this also means that more energy is required to separate the individual molecules and allow to escape as vapours. 

Alkanes with increased branching have lower boiling points than the straight chain molecules or isomers to be more precise. The branched molecules are more compact with smaller surface areas. These branched molecules are nothing but the isomers of straight chain hydrocarbons.
The crude oil also shows significant share of alkanes with branched chain forms and each branched chain alkanes has the same formula as the normal corresponding straight chain alkane and hence each of the molecules can be considered as a structural isomer of the corresponding normal alkane. 

Due to the significant structural differences the molecules will also have different physical properties. These differences in properties depend completely upon the manner in which the participating atoms are arranged or spread across the molecule.

Example: the structural isomers of pentane like 2, 2, di-methyl propane and neo pentane have same molecular formula of C5H12 but the boiling points are completely different and hover around 36 C and 10 C respectively.

Based on these rules and physical properties of these straight chain and branched forms the isomers of pentane has been categorised into following forms:

$CH_{3} – CH_{2} – CH_{2} – CH_{2} – CH_{3}$ 

Normal pentane or n - pentane

$CH_{3} – C (CH_{3})_{2} – CH_{3}$ 

2, 2, di-methyl propane

$CH_{3} – CH (CH_{3}) – CH_{2} – CH_{3}$

2 methyl butane or iso-pentane 

As the number of carbon atoms increase the possible isomers increase rapidly and isomers of pentane being one of the more important volatile component of crude, the isomeric forms of the molecule also behave in different combustion rate.

Isomers of Pentane Structure

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In order to understand and draw the structures of pentane isomers, we need to get the carbon backbone first.
  • Once we draw the carbon backbone we also need to fill in the hydrogen atoms
  • One of these atoms of carbon is then separated from the chain and then attach it to the second carbon atom of the new chain.
  • Next, the separated carbon atom is again moved to the third carbon atom and so on.
  • A carbon atom attached to the second carbon of the main chain provides the same structure as a carbon attached to second to last carbon
  • Once all the possible methyl substituted isomers, we can shorten the chain by another carbon atom and begin with all possible isomers with two single carbon branches or one double carbon branch, which gives us only three possible isomeric forms for pentane.
In case we get to see a normal molecule of pentane, we get to see two methyl $(CH_{3})$ groups in the chain. There are three methylene $(CH_{2})$ as well but no methines (CH) in the chain. 

 
 
In case of a molecule of iso-pentane we get to see, 3 methyl functional groups, one methylene and one methine. 

 
 
In the last isomeric form of pentane neo pentane, we get to see four methyl groups, no methylene and no methine but a centrally placed carbon atom. 

 

Isomers of Pentane Boiling Points

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The isomeric forms of pentane are branched out and as discussed earlier the more the branches we get to see in a isomer, the lower the boiling point it possess.

Although same number of carbons and hydrogens are present in the molecule but the extended shape of an unbranched alkane provides more points of contact for any inter molecular associations. The shape of the isomer molecules indicates that the normal pentane with most extended structure and with largest surface area available for sticking to other molecules by induced dipole or dipole induced attractive force will have the highest boiling point.

Next to this normal form is the most compact of the isomers 2, 2, di methyl propane. This has a compact spherical form which engages in the fewest induced dipole or dipole induced attractions and possess the lowest boiling point.

These induced dipole or dipole induced attractions are individually very weak force but in an organic molecule the collective form contributes to inter molecular attraction and is the cause of the non-polar characteristics of alkanes.

The boiling point of normal pentane is 36 C, the iso pentane or 2 methyl butane has a boiling point of 28 C and finally the compact neo pentane or 2, 2, di methyl propane isomer has boiling point of 9 C.

Compared to this the cyclo pentane has a higher boiling point of 49.3 C than pentane which also indicates that there exist a greater force of attractions in the cyclic form than a straight chain alkane molecule.

How Many Isomers of Pentane?

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There are overall three isomeric forms of pentane and they are normal pentane with a straight 5 carbon atom chain with hydrogen filling in for tetra valency of carbon atoms. 

 
Next to this normal form is the four carbon atom chain where one methyl is attached to either second carbon of main chain or third of the carbon atoms but both of these will provide the same nomenclature of 2 methyl butane. 

 
The third and final isomeric structure is the 2, 2, di methyl propane which has three carbon main chain and two methyl groups attached to second carbon of the main chain. 

Pentane Isomers Condensed Formula

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The condensed formula for pentane and its isomers are as follows:

Normal pentane (n – pentane) $CH_{3} CH_{2} CH_{2} CH_{2} CH_{3}$

Iso pentane (2 methyl butane)

$CH_{3} CH (CH_{3}) CH_{2} CH_{3}$

The last of the isomer of pentane is neo pentane (2, 2, di methyl propane)

$CH_{3} C (CH_{3})_{2} CH_{3}$

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