I have realized that not all chains and double bonds are numbered. I have noticed that up until 3 carbons in a chain, the double bond is not numbered, but I am not sure about the chain.

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    $\begingroup$ Numbering is only necessary where the un-numbered name is ambiguous. I'm not really sure what your question is. $\endgroup$ – bon Jan 21 '16 at 11:21

When there are 2 carbons in a chain, the double bond will be at the number 'one' position. Therefore it's not always mentioned in the naming.

For example: Ethene It can be written as 1-ethene or eth-1-ene. Since the position of the double bond can be at no other places than the first, the position in the naming you saw, wasn't mentioned.

When 3 or more carbons are present, the double bond can be between any 2 of the carbons, ie. either the first position or the second. (but-1-ene/butene or but-2-ene)

Position of double bonds and that of branches, must always be mentioned while naming organic compounds.

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    $\begingroup$ Prop-1-ene and prop-2-ene are the same. $\endgroup$ – bon Jan 21 '16 at 13:32
  • $\begingroup$ Maybe you meant 4 Carbon -enes: (but-1-ene and but-2-ene) $\endgroup$ – Max Payne Jan 21 '16 at 13:48
  • $\begingroup$ Ahh yes sorry. I meant butene. $\endgroup$ – Shashank Agrawal Jan 21 '16 at 16:00

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