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In IUPAC naming double bonds take preference over an alkane chain of any length, right? So the above should be named as 2-ethylnon-1-ene.

However, when I verify with chemdraw it gives the answer as 3-methylenedecane. I cannot seem to find any mention of double bond side chains as alkylene anywhere on the internet... Is this common nomenclature, and if not why would chemdraw name it as this?

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  • $\begingroup$ Chemdraw is almost deliberately wrong for just this reason. "Cheating"...it is robust with trivial names and other unorthodox naming systems. They are technically correct however but you would be better off learning the rules you learned in class. $\endgroup$ – DrAzulene Jul 5 '16 at 20:27
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According to IUPAC rules first thing you need to see is longest carbon chain which is 10(deca) over here.

Then while numbering the Carbon double bond is given preference.

So above compound is 10 carbon chain in which 3rd carbon is double bonded.

Thus, 3-methylenedecane is correct name.

Few more such examples to make it more clear:

  1. 2,6,10-trimethyldodec-1-ene

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  1. 8-ethyldec-1-ene

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Example Source: ChemSink

Basic IUPAC rules:

  1. Find the longest chain in the molecule and name it.

  2. Name all groups attached to the chain as alkyl substituents.

  3. Number the carbons of the longest chain beginning with the end that is closest to a substituent.

  4. Write the name of the alkane by first arranging all of the substituents in numerical order (each preceded by the number of the carbon to which it is attached and a hyphen), and then add the name of the stem. Haloalkanes.

  5. The halogen is treated as an alkyl substituent, without any priority.

  6. Ethers are named as alkoxyalkanes. The first carbon of the alkoxy substituent is numbered as C-1 of that substituent.

Note that these rules are changed as of 2008. Older HW and exams have slightly different answers.

Rules Source: http://valhalla.chem.udel.edu/nomenclature.html

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It looks like you're correct.

This molecule has been given 2-ethyl-1-hexene or 2-ethylhex-1-ene enter image description here

Another name for your molecule would be 3-methylidenedecane.

Added from Wikipedia:

The trivial name carbene is the preferred IUPAC name.

The systematic names methylidene and dihydridocarbon are constructed according to the substitutive and additive nomenclatures, respectively.

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  • $\begingroup$ So do double bonds do not precedence over the longest chain length? $\endgroup$ – 1110101001 May 27 '15 at 4:14
  • $\begingroup$ I'm sure somebody will dig out the IUPAC guidelines to give you the definitive answer, but I just wanted to point out another possible name. $\endgroup$ – LDC3 May 27 '15 at 4:23
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    $\begingroup$ UC davis wiki still uses the old recommendations. The longest chain is now the first criterium. $\endgroup$ – Martin - マーチン♦ May 27 '15 at 7:40

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