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

[Sunwoo Bae]

TL;DR

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.

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.

 

[TeethWhitener]

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).