IUPAC name of a branched chain hydrocarbon

[loom91]

Hi,

I'm just starting high-school organic chemistry and as is customary I've begun with nomenclature of organic compounds. Till now I've covered aliphatic hydrocarbons and will be starting on nomenclature of functional groups. The following problem from K.L. Chugh's Senior Chemistry Vol. II is creating a lot of trouble. So far neither other students nor a teacher have managed to confidently give an answer, though he is an inorganic rather than organic chemist.

The problem gives a drawing of the structure, which I've attached. The two possible solutions I arrived at:-

1)5-(1-ethylpropyl)-2,2,7-trimethyloctane
2)6-ehtyl-2,2-dimethyl-5-(2-methylpropyl)octane

As you can see, the two names differ in the selection of the parent chain. The rules for selecting parent chain in a hydrocarbon that we have been taught are that it must be the longest continuous chain, and a tie must be broken by selecting the one with more branches attached. But here both chains are of equal length and have the same number of branches, so which one should I select? The book gives (2) as the correct answer while I initially arrived at (1), but the answers in Chugh are notoriously unreliable.

Thanks for your help.

Molu

 

[GCT]

I see nine as the longest carbon chain in that drawing you've attached to your original post. However, in considering between the two choices which you've shown, number 2 is probably the correct one since it starts the naming off with a lower number designating a substituent; that is 2 instead of 5.

 

[loom91]

I see nine as the longest carbon chain in that drawing you've attached to your original post. However, in considering between the two choices which you've shown, number 2 is probably the correct one since it starts the naming off with a lower number designating a substituent; that is 2 instead of 5.

I'm sorry, the drawing was wrong. I've attached the correct drawing now. As for first point of difference, wouldn't the 5 in (1) be lower than 6 in (2)? We must start from the beginning of the name? Thanks.

Molu

 

[loom91]

Can someone please tell me?

 

[jzq]

Start numbering putting the most substituents near the beginning. Also the parent chain is of course one with the most carbons and most substituents. Remember to name substituents in ABC order. Just because there are two methyls which would come out to be dimethyl doesn't mean that you still put classify it under "M". You got the idea right with the first drawing. Like GCT said. Number 2 is close to being correct. You need to reorder the substituents. Now apply that to your other problems. Note: (2-methylpropyl) is the IUPAC name and isobutyl is the common name. I'm surprised that they offer organic chemistry courses in high school. What high school do you go to?

 

[loom91]

Start numbering putting the most substituents near the beginning. Also the parent chain is of course one with the most carbons and most substituents. Remember to name substituents in ABC order. Just because there are two methyls which would come out to be dimethyl doesn't mean that you still put classify it under "M". You got the idea right with the first drawing. Like GCT said. Number 2 is close to being correct. You need to reorder the substituents. Now apply that to your other problems. Note: (2-methylpropyl) is the IUPAC name and isobutyl is the common name. I'm surprised that they offer organic chemistry courses in high school. What high school do you go to?

But that is the problem! The different parent chains selected in (1) and (2) are of equal length and have equal number of substituents attached to them. What rule should I follow in this case to decide between the two? Note that the first drawing was wrong, it's the molecule in the second drawing that I'm talking about. Why do you consider (2) to be preferable? Should not (1) win according to first point of difference rule? As far as I can see, in (1) and (2) the substituents are already arranged in alphabetical order after ignoring the multiplying prefixes.

I'm surprised that some high schools apparently don't teach organic chemistry! All curriculum, state or national, followed in India teach Physical, Inorganic and Organic chemistry in 11th and 12th grade. I can't imagine a syllabus excluding such a huge part of chemistry! We start organic at 9th grade, though that's very basic. We start a systematic study in 11th grade.

I don't study in any special school. In fact the curriculum offered by our state educational board http://en.wikipedia.org/wiki/West_Bengal_Council_of_Higher_Secondary_Education" is considered by a lot of students like me to be designed for cognitively challenged students. It's too elementary and backward in outlook. IITJEE has a much better syllabus and that's the one me and my teachers mainly follow.

I study OChem from KL Chugh (for a HS syllabus oriented study) and Morrison & Boyd (for a more intellectually rewarding IITJEE oriented study). M&B is the standard book used for IITJEE. The problem is that my M&B is the old fifth edition (acquired used) and therefore not the best resource for nomenclature. Written in 1989, 4 years before the 1993 recommendations, it mostly uses trivial names such as methyl chloride and ethyl alcohol. The Chugh, while much newer, is unfortunately rather unreliable, unclear and contradictory. I attempted reading the Blue Book online, but since it was not written to be a textbook I soon lost myself in the unhelpfully structured labyrinth.

Another problem is that my teacher is an inorganic chemist, researching on kinetics and reaction mechanisms. The last time he studied organic was during post graduation (his PhD was on inorganic reaction mechanism), and in University he teaches only inorganic and physical. He expectably does not feel very motivated to study organic anew only to teach high school students. He has forgotten the details of organic nomenclature.

Other parts of nomenclature which are not entirely clear to me are compounds containing multiple functional groups and compounds containing rings (aromatic or alicyclic). Can you give me some pointers? Thanks for the help.

Molu

 

[loom91]

Please, someone help me! I'm completely confused!

 

[cristo]

But that is the problem! The different parent chains selected in (1) and (2) are of equal length and have equal number of substituents attached to them. What rule should I follow in this case to decide between the two? Note that the first drawing was wrong, it's the molecule in the second drawing that I'm talking about. Why do you consider (2) to be preferable? Should not (1) win according to first point of difference rule?

Chain number (1) has the substituents branching off on C atoms number 2,2,5,7. Chain (2) has branches on C atoms number 2,2,5,6. You pick chain number (2) since it has the lowest numbering system

 

[loom91]

But when applying first point of difference rule, do we mean first when counted along the parent chain (in 2) or first as reported in the name, that is in alphabetical order (in 1)?

 

[cristo]

But when applying first point of difference rule, do we mean first when counted along the parent chain (in 2) or first as reported in the name, that is in alphabetical order (in 1)?

Here you have two different names for the same diagram, used by selecting two different parent chains. The first point of difference rule only comes into play when naming the structure using the same parent chain, and says that the first time a difference in numbering occurs, then the method that gives the lowest numbering is used.

If this case occurs again, you should pick the two parent chains, as you have done above. Then name them, using the appropriate rules. Then compare the two (like we do here) and choose the name with the lowest overall numbering system

 

[loom91]

By lowest overall, do you mean the sum of locants? How would I compare two diffrent strings of numbers to see which is lowest overall?

 

[loom91]

What is the criteria for finding lowest overall?

 

[cristo]

I guess in this instant, when you are comparing two names with different parent chains, that you use the one with the lowest sum. However, this rule does not hold when deciding a name for a molecule where you have a clear choice for parent chain. In this case, you would use the first point of difference rule.

 

[loom91]

Thanks. Can you give me a reference/link to the part of IUPAC recommendations where this is discussed?

 

[cristo]

http://www.acdlabs.com/iupac/nomenclature/79/r79_36.htm

See rule 2.6: If chains of equal length are competing for selection as main chain...

 

[loom91]

Thanks for the link. It uses the ambiguous wording "(b) The chain whose side chains have the lowest-numbered locants." You interpret lowest-numbered as least sum. But previously it is said: "When series of locants containing the same number of terms are compared term by term, that series is "lowest" which contains the lowest number on the occasion of the first difference. This principle is applied irrespective of the nature of the substituents."

It appears to me that the use of lowest-sum rule is not recommended in any situation. In that case (1) seems to win over (2) because 5 is lower than 6. What do you think?

 

[cristo]

On most occasions it turns out that the "lowest sum" rule is the same as the first difference rule. On this occasion, they both seem the same to me! I don't really get what you're trying to say.

Let's talk through the numbering the two different ways. For chain (1), we number from the right, taking the horizontal(ish) chain as the parent chain. Numbering this way gives us a system 2,2,5,7.

Now for chain (2), we again number from the right, but this time go up on the 5th C atom. So, we obtain a system 2,2,5,6. The first point of difference is in the last number, and since 6<7 we adopt the name (2).

 

[loom91]

On most occasions it turns out that the "lowest sum" rule is the same as the first difference rule. On this occasion, they both seem the same to me! I don't really get what you're trying to say.

Let's talk through the numbering the two different ways. For chain (1), we number from the right, taking the horizontal(ish) chain as the parent chain. Numbering this way gives us a system 2,2,5,7.

Now for chain (2), we again number from the right, but this time go up on the 5th C atom. So, we obtain a system 2,2,5,6. The first point of difference is in the last number, and since 6<7 we adopt the name (2).

But often they are not the same (for moderately to highly branched molecules). You are counting the chains along the chains. Is this the right way to count? Or should we count in the order the locants appear in the name? Thanks.

 

[cristo]

For the majority of high school problems, however, the two methods will results in the same outcome. And yes, you count along the chain, not along the name.