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Question
Which is not considered an isomer for pentane?

A) $\ce{CH3CH(CH3)CH2CH3}$

B) $\ce{CH3CH2CH(CH3)CH3}$

C) $\ce{CH3CH2CH2CH2CH3}$

D) $\ce{CH3CH=CHCH2CH3}$

I knew A and B were obviously isomers. I knew D was not an isomer, because the definition of isomers in my book states that isomers "are two DIFFERENT compounds with the SAME molecular formula" and that structural isomers are compounds with SAME molecular formula but DIFFERENT structure.

I was very confused with the choice C since it was IDENTICAL and did not match the definition. What confused me more is that there were practice questions in the book that gave like 4 examples of compounds and asked if they are isomers or identical compounds.

So, are identical compounds considered isomers or not?

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  • $\begingroup$ Related: Isomer Identification Using Condensed Structural Formulae $\endgroup$ – Loong Mar 11 '16 at 16:34
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    $\begingroup$ I'll point out that (A) and (B) are the same isomer - isopentane. (c) is not just pentane, but n-pentane. $\endgroup$ – MaxW Mar 11 '16 at 16:46
  • $\begingroup$ can you please clarify the confusion, the clear option for the above given example seems to be D, as C as stated by @MaxW is n-pentane, or what is generally meant when pentane is mentioned. $\endgroup$ – yawar Mar 11 '16 at 16:57
  • $\begingroup$ The question could have been worded more precisely. I think if it said "Which is not a pentane isomer?", then it would more readily fall into @Kraw's interpretation, which I think is what the question meant to be asking. $\endgroup$ – jerepierre Mar 11 '16 at 17:51
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Identical compounds are not considered to be isomers. However, the following explanation should clear up any confusion from your question.

A and B are both isopentane and C is n-pentane (normal pentane). These make up two of the three isomers of pentane, the third being neopentane ($\ce{CH3CH(CH3)2CH3}$). The question is referring to pentane as $\ce{C5H12}$, a group of isomers in which n-pentane ($\ce{CH3CH2CH2CH2CH3}$) is included and not identical to.

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  • $\begingroup$ The last sentence is wrong. A and B are identical as noted. $\endgroup$ – MaxW Mar 12 '16 at 22:18
  • $\begingroup$ Ah, I see how that was poorly worded. Thanks. $\endgroup$ – Kraw Mar 12 '16 at 23:05
  • $\begingroup$ Very concise - great answer. $\endgroup$ – orthocresol Mar 12 '16 at 23:36
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Updated 12-03-2016

Isomer(s) of a molecule are defined as molecules that have the same molecular formula, but have a different arrangement of the atoms in space

Here we see ...the same molecular formula... but different arrangement. Now lets take your example; pentane has the chemical formula $\ce{C5H12}$ but it has a few isomers, but the molecule 2,2 dimethylpropane has the same chemical formula as pentane ($\ce{C5H12}$), the difference being in their structure. So is the formula for 2 methylbutane. These molecules having the same formula but different structure are said to be isomers of one another, As a group of isomers of ($\ce{C5H12}$) they are reffered to as the isomers of pentane which includes n pentane So When the word isomer of something is mentioned, it will include the something and it's isomers. It's like an album with the thumbnail from the same album.

If you say isomers of 2,2 dimethylpropane it will include n pentane as well as neo pentane and iso pentane and 2,2 dimethylpropane.

. In the above case 'C' the pentane is n-pentane or normal pentane with no branches[ an isomer of pentane (or what you may refer to as the pentane itself, as per simple IUPAC nomenclature of alkanes) ].iso pentane also called as 2 methylbutane is an isomer of pentane as well. neo pentane or 2,2 dimethyl propane is also an isomer of pentane.

D in above case is identical to pentane in terms of carbon number, even in terms of general structure but has one less hydrogen, so the chemical formula is not same as it belongs to a different organic class of compounds (alkenes) and thus is not an isomer of pentane.

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