Why 3-pentene incorrect?

Answer 1

You take the lowest number carbon from the outside of the molecule to determine the prefix, whether this be the left hand side or the right hand side. e.g.

C-C-C=C-C

is the same isomer as:

C-C=C-C-C

Both are 2-pentene.

The double bond on the third carbon will always be between a carbon that is closer to the end of the molecule (therefore lower in number), so it is this carbon that will dictate the prefix.

Therefore the only 2 isomers of Pentene are 1-pentene and 2-pentene.

Answer 2

3-pentene is incorrect because the parent chain for alkenes with five carbon atoms should be pentene, not 3-pentene. The numbering of the carbon atoms in the double bond starts from the end that gives the double bond the lowest number, so in this case, it would be pent-1-ene.