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I have to draw the structure for trans-3,4-dichlorocyclopentene.

What position should I draw the double bond at? Should I draw it as molecule 2? Because there's a trans.

Or should I draw it as shown in molecule 1 because it didn't state?

enter image description here

I feel that molecule 2 in the diagram is correct. What do you think? Where are my mistakes?

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    $\begingroup$ Use logic. Why the first atom is the first, how do you think? Why we didn't start numbering from someplace else? $\endgroup$ – Ivan Neretin May 24 '17 at 5:52
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The issue here really is that the name 'trans-3,4-dichlorocyclopentene' is not really correct, as it is unspecific. It lacks stereo-descriptors, and the locant for the double bond.

Assuming that the double bond starts at the first carbon, which is usually the case if it is omitted, there are two enatiomers still possible:

  1. (3R,4R)-3,4-Dichlorocyclopent-1-ene

    structural formula of (3R,4R)-3,4-Dichlorocyclopent-1-ene

    See for details: National Center for Biotechnology Information. PubChem Compound Database; CID=12430197.

  2. (3S,4S)-3,4-Dichlorocyclopent-1-ene

    structural formula of (3S,4S)-3,4-Dichlorocyclopent-1-ene

    See for details: National Center for Biotechnology Information. PubChem Compound Database; CID=131015828.

So the structure you have drawn as 1 is (3R,4R)-3,4-dichlorocyclopent-1-ene.

The structure you have drawn as 2 doesn't really exist, as the double bond is planar, but ignoring that it would be 1,2-dichlorocyclopent-1-ene.

1,2-dichlorocyclopent-1-ene

See for details: National Center for Biotechnology Information. PubChem Compound Database; CID=10606793.

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