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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.

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could you atleast name any conflicting molecules for each question, it will also make it a lot easier for us to answer. –  Aditya Sriram Jan 16 '13 at 3:12
    
In question 3 do you mean something like this –  Aditya Sriram Jan 16 '13 at 3:14
    
@AdityaSriram Okay. And for your example: yes, I do mean something like that. –  Alraxite Jan 16 '13 at 17:33

2 Answers 2


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). $$\begin{matrix} \ce{Better\ name:& 7(2-Isopropylhexyl)-4-methyl\ heptadecane&\\ Rather\ than:& 5-isopropyl-7(3-Methylhexyl)\ heptadecane&} \end{matrix}$$


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.
Similar to the previous example no matter which chain you choose the other one always gets the same number that is 5.

So locants (1,5) are better than (1,6) with reference to double bonds. $$\begin{matrix} \ce{Better\ Name:& 5-Ethenyl\ hepta-1,5-diene \\ Rather\ than:& 5-Ethylidene\ hepta-1,6-diene} \end{matrix}$$


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 $\ce{2,4-dichloropentane}$ Example 3


For Question 3

Example 1

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

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

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Could you tell me your sources? –  Alraxite Jan 22 '13 at 12:38
    
Question 1 , Example 1 and 2 only rely on the First point of difference rule, for question 2, it was trivial. –  Aditya Sriram Jan 22 '13 at 16:03

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.

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