Naming the pentane with phenyl group (or benzene with aryl group)

  • #1
Sunwoo Bae
59
4
TL;DR Summary
My textbook indicates that the following figure can be named in two ways, but I do not understand how the second one is a valid name.
1612370300975.png


I understand the first one, which indicates that there is a phenyl group in the second carbon of pentane.
But where did the 2 in 2-pentylbenzene come from?
Shouldn't if it be just pentylbenzene, since there is only one subtitute(pentane) on the benzene?
If not, would you explain why?

Thank you.
 

Answers and Replies

  • #2
TeethWhitener
Science Advisor
Gold Member
2,441
1,983
Neither is a great name for this compound. It's ambiguous (or rather would become ambiguous with more substituents) whether the 2 refers to the position relative to the pentane or relative to the benzene ring. IUPAC-wise, it should be pentan-2-yl benzene (cf. cumene, aka isopropylbenzene, as propan-2-yl benzene in the IUPAC scheme). The 2 is necessary because the benzene can attach to the pentane chain at the 1, 2, or 3 position (on the pentane).
 

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