[sd79812]

Two pentene I assume the third carbon is the middle one. You remove one hydrogen from that third carbon and you form a double bond from the 2nd to third carbon. I'm following the rule for iso, why or why not is two-penetene isopentene?

[mjc123]

There is no "rule" about a "middle carbon". It just happens in your example that there is a distinction between the terminal carbons (removing H from which gives n-propyl) and the middle one - the sole non-terminal carbon. For molecules with more than one non-terminal C, there is no rule about the middle one giving iso. In fact, apart from isopropyl, iso- names are derived from isoalkanes, which are branched. Thus isobutene is 2-methylpropene (not 2-butene). Isopentene is 3-methyl-1-pentene (why that rather than any other derivative of isopentane, I'm not sure; perhaps the honour goes to the least substituted alkene?). 2-pentene is not isopentene.

[Babcock_Hall]

The amino acids leucine and isoleucine both have branched side chains.