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How many structural isomers can you draw for pentane?

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Last updated date: 28th Mar 2024
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MVSAT 2024
Answer
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Hint: Structural isomers are those which have the same molecular formula and different bonding patterns and atomic organization. Pentane is an organic compound with 5 carbon atoms attached with single bonds. So, just see the number of arrangements formed by pentane.

Complete step by step answer:
-Pentane is a 5 carbon organic compound and since the suffix is –ane we can say that there is only single bonds (it is a saturated molecule). There are no double or triple bonds (it is not unsaturated).
-The molecular formula of Pentane is: \[{C_5}{H_{12}}\]
-The 5 carbon atoms of pentane can be arranged in following distinct forms:
n-pentane or pentane: in this first arrangement all the 5 carbon atoms are arranged one after the other to form a single chain with no branching. It is known as n-pentane and its molecular formula is \[{C_5}{H_{12}}\]. Its structural formula is:
                                  
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Isopentane or 2-methylbutane: in this type of arrangement 4 carbon atoms are arranged in a straight chain and the 5th carbon atom is attached to the 2nd carbon atom of the straight chain to form a branch. This is known as isopentane and the IUPAC name is 2-methylbutane. Its molecular formula is \[{C_5}{H_{12}}\].
Its structural formula is:
                                          
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Neopentane or dimethylpropane: in this type of arrangement 3 carbon atoms are arranged one after the other to form a straight chain, while the other 2 carbon atoms are attached to the middle carbon atom of the straight chain to form 2 branches. It is known as neopentane and the IUPAC name is dimethylpropane. Its molecular formula is \[{C_5}{H_{12}}\] .
Its structural formula is:
                                    
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No other patterns or types of structures are possible for pentane. So, we can say that pentane has 3 structural isomers: 1) n-pentane
2) isopentane (2-methylbutane)
3) neopentane (dimethylpropane)

Note: While naming any molecule the carbon chain should be numbered from the side where the branching is closer and not the other way round. Basically , we need to give the branch or functional group the lowest number possible. This helps us ensure that all of our isomers are distinct and not similar.
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 (A) Numbering done here is wrong.
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(B) Numbering done here is correct.
So, don’t confuse the above 2 isomers as different counting them as 2 isomers, they both are the same , the only difference is the numbering of the carbon atoms which is wrong in (A) and correct in (B).
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