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Is there a difference between common naming of alkenes vs alkynes? Why is Ethylene the common name for an alkene and acetylene for an alkyne? Isn’t acet- the common form of naming ethyl groups anyway so why is ethyl used in a common name?

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The official IUPAC name for ethyl with a double bond is ethene, but this is a relatively recent change. Before the official name (as an exception) was ethylene. I’m not sure about propene/propylene but am fairly confident the official IUPAC name for propyl double bond is propene and not propylene. However, these compounds are often referred to with the unofficial name as per tradition.

For alkynes, the IUPAC name is, for ethyl, the name is ethyne.

Don’t remember to specify the location of the double/triple bond (ex: 2 - ethene)

For alkenes, the formula is $\ce{C_nH2_{n+2}}$ and for alkynes is $\ce{C_nH_{2n}}$.

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    $\begingroup$ To be precise, the non-systematic name “ethylene” was still retained in the 1979 IUPAC recommendations but was no longer recommended in the 1993 recommendations. $\endgroup$ – Loong Jun 19 '18 at 19:29
  • $\begingroup$ Is there a 2-ethene? For acyclic cmpds: alkanes, CnH2n+2; alkenes, CnH2n; alkynes, CnH2n-2. $\endgroup$ – user55119 Jul 20 '18 at 12:31

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