I have three questions:

  1. To find the parent chain of a saturated hydrocarbon, we have to find the longest chain and then if we have a choice, we choose the one with the more substituents. But what if we still had a choice? What would we do next?

  2. Similarly, for numbering the chain, the last step is to number the chain such that the substituent which comes alphabetically first gets the lower number. But what if we again had a choice? I'm not sure here, but I think we can't possibly have another choice. Can anyone confirm this?

  3. If an alkyl substituent is attached to the parent chain by a double or triple bond, how would we name it?

Examples: Here are some examples which demonstrate the above 'tie-breakers'.

For question 1:

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The (green + blue) and (green + magenta) chains are the longest and both have two substituents.

enter image description here

The (green + blue) and (green + yellow) chains contain the maximum double bonds and are also the longest. They both have one substituent.

For question 2: As I said, I'm not sure if there can possibly be a 'tie-breaker' here.

For question 3:

enter image description here

The longest chain is the green one. I'm not sure how to name the substituent attached by the double bond.

  • $\begingroup$ could you atleast name any conflicting molecules for each question, it will also make it a lot easier for us to answer. $\endgroup$ Jan 16, 2013 at 3:12
  • $\begingroup$ In question 3 do you mean something like this $\endgroup$ Jan 16, 2013 at 3:14
  • $\begingroup$ @AdityaSriram Okay. And for your example: yes, I do mean something like that. $\endgroup$
    – Alraxite
    Jan 16, 2013 at 17:33

3 Answers 3


For Question 1

Example 1

Example 1
You should select the (green + blue) chain because, no matter which among the blue and yellow you pick, the numbering would not start from the green chain rather the yellow or blue.
Now if you pick the blue chain the substituent ($\ce{methyl}$) would get number 4, whereas if you picked the green chain the substituent($\ce{isopropyl\ or\ methylethyl}$) would get the number 5, so in accordance to the First Point of Difference Rule you should select the green + blue chain.
Also Worth noticing is the fact that no matter which chain you pick, the other chain which becomes the substituent would always get the number 7

So locants (4,7) are better than (5,7). $$\begin{matrix} \text{Better name:}& \text{7-(2-isopropylhexyl)-4-methylheptadecane}&\\ \text{Rather than:}& \text{5-isopropyl-7-(3-methylhexyl)heptadecane}& \end{matrix}$$

Example 2

Example 2

In this case according to the above mentioned rule, you should check the position of the double bond, now since there is a double bond already in the green chain at the first position it would get number 1 and the numbering would start from there.
If you choose the green + blue chain the other double bond would get number 5 and for the green + yellow chain the other double bond would get number 6. So you should go for the green + blue chain in this case.

So locants (1,5) are better than (1,6) with reference to double bonds. $$\begin{matrix} \text{Better name:}& \text{(5}\textit{Z}\text{)-5-ethenylhepta-1,5-diene}\\ \text{Rather than:}& \text{(3}\textit{Z}\text{)-3-ethylidenehepta-1,6-diene} \end{matrix}$$

For Question 2

If the chain has to be numbered and two substituents have the same locant the n you see the alphabetical order of the names of the groups/substituents and if they are also the same it only means that they are the same substituent and hence are written together with there locants separated by a (comma),.

For example in you would name the below compound as $\text{2,4-dichloropentane}$ Example 3

For Question 3

Example 1

Example 4 The substituent in this example would be named $\ce{ethylidene}$. $$ \begin{matrix} \text{Final name:}& \text{5-ethylidenenona-1,8-diene} \end{matrix} $$ Similar substituents can be named as

$$ \begin{array}{|c|c|c|c|} \hline \ce{Substituent& Saturated\ form& Name\ of& Name\ of \\ && saturated\ form& substituent \\ \hline R=CH2& R-CH3& Methyl& Methylidene\\ \hline R=CH-CH3& R-CH2-CH3& Ethyl& Ethylidene\\ \hline R-CH=CH2& R-CH2-CH3& Ethyl& Ethenyl\\ \hline R=CH-CH2-CH3& R-CH2-CH2-CH3& Propyl& Propylidene\\} \hline \begin{align} \ce{R=C&-CH3\\ |& \\ C&H3} \end{align}& \begin{array}{rl} \ce{R-C&-CH3\\ |\ &\\ C&H3} \end{array}& \ce{Isopropyl& Isopropylidene}\\ \hline \end{array}$$

  • 1
    $\begingroup$ Could you tell me your sources? $\endgroup$
    – Alraxite
    Jan 22, 2013 at 12:38
  • 1
    $\begingroup$ Question 1 , Example 1 and 2 only rely on the First point of difference rule, for question 2, it was trivial. $\endgroup$ Jan 22, 2013 at 16:03
  • 1
    $\begingroup$ Dont you think in example 2, numbering should start from the yellow double bond rather than the green one? This would give its substiuent(the blue chain) a lower number and still have the same number of carbon atoms in the parent chain when numbering srarts from green chain. $\endgroup$ May 3, 2015 at 17:27
  • $\begingroup$ Note that the preferred prefix is ‘propan-2-yl’ rather than ‘isopropyl’. $\endgroup$
    – user7951
    Apr 7, 2016 at 18:06
  • $\begingroup$ Also, magenta ≠ yellow. $\endgroup$ Apr 7, 2016 at 20:33

I think in case of #1, you should keep the chain which contains the overall less-complex substituents (which would be green and magenta). Stating it into a rule I guess it would mean comparing the longest substituent of both alternatives.

For question #2, you should chose the numbering that gives an overall lower numbering result, even if you had a choice based on the alphabetically first substituent.

I have no idea for question #3. My instinct tells me that naming that =C-CH3 as a substituent feels wrong and I should choose the shorter chain even if its against the rules. But there's nothing scientific about that.


Just to make things clearer, here are the IUPAC rules to be applied. Each rule must be evaluated in order until there is only one possibility for the main chain.

(For hydrocarbons, there is no principal group so we do not evaluate rule #1.)

  1. Maximum number of substituents corresponding to the principal group.
  2. Maximum length.
  3. Maximum number of double and triple bonds considered together.
  4. Maximum number of double bonds.
  5. Lowest locants for the principal groups (that is, for the suffix).
  6. Lowest locants for multiple bonds.
  7. Lowest locants for double bonds.
  8. Maximum number of substituents cited as prefixes.
  9. Lowest locants for all substituents in the principal chain cites as prefixes.
  10. The substituent first cited in alphabetical order.
  11. Lowest locants for the substituent cited first as prefix in alphabetical order

Edited to reflect changes in 2013 rules (thanks to Loong)

  • $\begingroup$ Could you cite a source? Also, what if there are still ties after evaluating all 11 rules? $\endgroup$
    – lirtosiast
    May 28, 2020 at 0:37

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