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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Rutherford on June 30, 2012, 07:23:10 AM
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What is the name of the following alkane:
http://imageshack.us/photo/my-images/94/alk.png/
In the book it is 2,2,6,6,7-penthamethyloctane, but I would go with 3-ethyl-2,3,7,7-tetramethyloctane. I solved this so: when I find the longest row, I start from the end where the ethyl group is closest to, but how did they got 2,2,6,6,7-penthamethyloctane? What is correct?
{MOD Edit -- attach image}
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Yes, I think the book isn't right about this one. That one there on the 3rd carbon is clearly an ethyl and not a methyl
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What is the name of the following alkane:
http://imageshack.us/photo/my-images/94/alk.png/
In the book it is 2,2,6,6,7-penthamethyloctane, but I would go with 3-ethyl-2,3,7,7-tetramethyloctane. I solved this so: when I find the longest row, I start from the end where the ethyl group is closest to, but how did they got 2,2,6,6,7-penthamethyloctane? What is correct?
The name is 6-ethyl-2,2,6,7-tetramethyloctane according to ChemDraw.
find the longest chain then order the substituents alphabetically.
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That is similar as mine but I started from the other side to count. How to know the correct name, when the row starts to fork from 2nd carbon atom regardless from what side you start to count?
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That is similar as mine but I started from the other side to count. How to know the correct name, when the row starts to fork from 2nd carbon atom regardless from what side you start to count?
Then it does not really matter, important is the longest chain.
The name I posted is not similar to yours, it has an ethyl group in it!
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I didn't mean the book answer, mine answer was:
but I would go with 3-ethyl-2,3,7,7-tetramethyloctane
I don't think that it is the same compound. Do the 2 methyl group determine which side to start from?
While writting this reply I came to the next question: Does the quaternary carbon atom have advantage over the tertiary one in naming? I think that this would be a good explanation.
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I didn't mean the book answer, mine answer was: but I would go with 3-ethyl-2,3,7,7-tetramethyloctane
I don't think that it is the same compound. Do the 2 methyl group determine which side to start from?
While writting this reply I came to the next question: Does the quaternary carbon atom have advantage over the tertiary one in naming? I think that this would be a good explanation.
It is the same compound, according to ChemDraw just flipped 180°. I am not a nomenclature expert, but I believe ChemDraw is correct.
The quaternary carbon does not have any advantages.
As I said you determine the longest chain.
I think with the 6-ethyl…. you can derive the numbering sequence.
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But it is a general rule that 1 compound can have only 1 correct structure formula and name. For example, 2-methylbuthane is the same as 3-methylbuthane, but according to the IUPAC nomenclature, only the first name is correct. In this problem it should be the same (only 1 name is correct).
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But it is a general rule that 1 compound can have only 1 correct structure formula and name. For example, 2-methylbuthane is the same as 3-methylbuthane, but according to the IUPAC nomenclature, only the first name is correct. In this problem it should be the same (only 1 name is correct).
All you have done here is rotate the molecule by 180° and then by 90°. See the picture.
So nothing changes.
Actually there are at least two systems I know of for naming compounds, the IUPAC and the Chemical Abstracts system, both are different the last time I looked. Beilstein may be a third!
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The thing about quaternary atoms that I wrote is wrong, because I found that the compound that has octane number 100 is called 2,4,4-three(?)methylpenthane.
Still, I don't understand why the name I propose isn't correct (3-ethyl-2,3,7,7-tetramethyloctane), how to know in such cases where to start from?
In a more simple case, the counting of C atoms should always start from the C atom that is nearest to another C atom that makes the row forking (English isn't my mother language so I don't know if I used the best words), but in this case there are 2 possibilities.
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The thing about quaternary atoms that I wrote is wrong, because I found that the compound that has octane number 100 is called 2,4,4-three(?)methylpenthane.
Still, I don't understand why the name I propose isn't correct (3-ethyl-2,3,7,7-tetramethyloctane), how to know in such cases where to start from?
In a more simple case, the counting of C atoms should always start from the C atom that is nearest to another C atom that makes the row forking (English isn't my mother language so I don't know if I used the best words), but in this case there are 2 possibilities.
The two possibilities are the same molecule. See picture. The one on the right is your suggestion.
Have a look at the following page!
http://en.wikipedia.org/wiki/IUPAC_nomenclature_of_organic_chemistry (http://en.wikipedia.org/wiki/IUPAC_nomenclature_of_organic_chemistry)
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It says there: ''If there is ambiguity in the position of the substituent, depending on which end of the alkane chain is counted as "1", then numbering is chosen so that the smallest number is used. For example, (CH3)2CHCH2CH3 (isopentane) is named 2-methylbutane, not 3-methylbutane.''
3-methylbutane is incorrect, 2-methylbutane is correct. This is ok, but in my problem it isn't so easy to chose where to start from.
Why is it named 6-ethyl-2,2,6,7-tetramethyloctane and not 3-ethyl-2,3,7,7-tetramethyloctane? Is my name correct, too?
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It says there: ''If there is ambiguity in the position of the substituent, depending on which end of the alkane chain is counted as "1", then numbering is chosen so that the smallest number is used. For example, (CH3)2CHCH2CH3 (isopentane) is named 2-methylbutane, not 3-methylbutane.''
3-methylbutane is incorrect, 2-methylbutane is correct. This is ok, but in my problem it isn't so easy to chose where to start from.
Why is it named 6-ethyl-2,2,6,7-tetramethyloctane and not 3-ethyl-2,3,7,7-tetramethyloctane? Is my name correct, too?
No you start from the right hand side, then all the methyl substituents have the low numbers.
6-ethyl is correct.
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Do you mean that methyl substituents should have the smallest possible numbers (2,2,6,7)? This doesn't agree with 2,4,4-threemethylpenthane (2,4,4 not 2,2,4).
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Do you mean that methyl substituents should have the smallest possible numbers (2,2,6,7)? This doesn't agree with 2,4,4-threemethylpenthane (2,4,4 not 2,2,4).
Quote from the link below.
The longest chain is numbered from one end to the other by Arabic numerals, the direction being so chosen as to give the lowest numbers possible to the side chains. 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.
Have a look at this page:
http://www.acdlabs.com/iupac/nomenclature/79/r79_36.htm (http://www.acdlabs.com/iupac/nomenclature/79/r79_36.htm)
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Could you explain it more clearly (the language barrier troubles me)?
-The longest chain is numbered from one end to the other by Arabic numerals, the direction being so chosen as to give the lowest numbers possible to the side chains.
Understood this.
-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.
Don' understand the meaning of locants, term, lowest number on the occasion, first difference - in this context.
-This principle is applied irrespective of the nature of the substituents.
"irrespective of the nature"-?
Thanks for the help till now, I saw that the 2nd example in rule A-2.2 is the thing I am looking for. The wording is difficult for me to understand.
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Could you explain it more clearly (the language barrier troubles me)?
-The longest chain is numbered from one end to the other by Arabic numerals, the direction being so chosen as to give the lowest numbers possible to the side chains.
Understood this.
-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.
Don' understand the meaning of locants, term, lowest number on the occasion, first difference - in this context.
-This principle is applied irrespective of the nature of the substituents.
"irrespective of the nature"-?
Thanks for the help till now, I saw that the 2nd example in rule A-2.2 is the thing I am looking for. The wording is difficult for me to understand.
This means that if the molecules you are comparing are the same you keep going until you find the first atom which is different, that molecule is then the lowest.
The last piece : This means it does not matter what the substituents are when doing this.
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The last part is ok. Now:
This means that if the molecules you are comparing are the same you keep going until you find the first atom which is different, that molecule is then the lowest.
What do you mean by different atom if all are carbons?
How the molecule to be lowest? Do you mean the shortest chain?
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The last part is ok. Now:
This means that if the molecules you are comparing are the same you keep going until you find the first atom which is different, that molecule is then the lowest.
What do you mean by different atom if all are carbons?
How the molecule to be lowest? Do you mean the shortest chain?
"the first atom which…." means the first atom that has a substituent on it, all else being equal. Imagine two molecules:
So go go along the longest chain, all the substituents appear on the same carbon atoms in the two molecules, suddenly somewhere along the chain you find a difference, that C atom suddenly has a substituent whereas in the other molecule it hasn't, further along the chain the substituents are all the same, the only difference is that one substituent. That is the numbering you then pick.
"That molecule is lowest", that was my fault I meant to say that is then the molecule which has the lowest numbering.
You always look for the longest chain.
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What is that difference you are talking about in mine problem?
When looking at the molecule now, I see 2 long chains, both with 8 C atoms. The first one would be 3-ethyl-2,3,7,7-tetramethyloctane or 6-ethyl-2,2,6,7-tetramethyloctane (still don't understand why is the 2nd correct), the second one would be 3-isopropyl-3,7,7-threemethyloctane, but because the nearest substituent is at C atom number 3 and in the previous it is at C atom number 2, the second one is surely not correct.
The dilema still remains at 6-ethyl-2,2,6,7-tetramethyloctane and 3-ethyl-2,3,7,7-tetramethyloctane that I can't understand. If the difference between the C atoms that have substituents should be the least possible, then mine name would be better. 3+2+3+7+7<6+2+2+6+7, but this is not correct for 2,4,4-threemethylpenthane.
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I thought, and maybe understood "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."
-The first difference between 2 C atoms that have an alkyl group in 6-ethyl-2,2,6,7-tetramethyloctane would be 0 (2-2), in mine case it would be 1 (3-2). The difference is lower in 6-ethyl-2,2,6,7-tetramethyloctane, so it would be the correct name. But again there is no logic when this is applied to 2,4,4-threemethylpenthane, so maybe it is wrong.
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I thought, and maybe understood "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."
-The first difference between 2 C atoms that have an alkyl group in 6-ethyl-2,2,6,7-tetramethyloctane would be 0 (2-2), in mine case it would be 1 (3-2). The difference is lower in 6-ethyl-2,2,6,7-tetramethyloctane, so it would be the correct name. But again there is no logic when this is applied to 2,4,4-threemethylpenthane, so maybe it is wrong.
Sorry I don't understand what is wrong about 2,4,4,-trimethylpentane.
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Posted by: discodermolide
Sorry I don't understand what is wrong about 2,4,4,-trimethylpentane.
IUPAC rule - the lowest locants
2,2,4 is lower then 2,4,4
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Then this is a mistake in the book, as 2,4,4-trimethylpentane is wrong. This example was confusing me to understand the other problem I posted and the statements in the links that discodermolide posted. Anyway, thanks very much for the help disco.
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Found from few other sources that the compound is 2,2,4-trimethylpentane, so I am 100% sure now, that there was a mistake in my book (a pretty misleading one). After all, I think that the logical conclusion would be:
Does the quaternary carbon atom have advantage over the tertiary one in naming? I think that this would be a good explanation.
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Could you explain it more clearly (the language barrier troubles me)?
-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.
Don' understand the meaning of locants, term, lowest number on the occasion, first difference - in this context.
-This principle is applied irrespective of the nature of the substituents.
"irrespective of the nature"-?
Let me try to walk you through these two questions. Starting with the second - "irrespective of the nature of the substituents" means that for the purposes of applying this rule, it doesn't matter whether the substituents you are trying to number are ethyl or methyl groups, all you care about is which carbon the substituent is attached to.
Now the first question:
"locants" - you are trying to describe the position of your substituents on a chain of 8 carbon. Each carbon atom on the chain is a possible locant for a substituent, and the locants are numbered 1 through 8.
"terms" - these are the descriptions of the substituents. For your example, numbered one way, the terms (in numerical order) are 2-methyl, 2-methyl, 6-methyl, 6-ethyl, and 7 methyl, while numbered the other way, the terms are 2-methyl, 3-methyl, 3-ethyl, 7-methyl, and 7-methyl.
With me so far?
Now, you want to compare your terms "term by term" - that means comparing the first term in one series, with the first term in the other series, and the second with the second, and so on. So, first term -
2-methyl compared with 2-methyl - there is no difference.
Second term -
2-methyl with 3-methyl - there is a difference!
Ding, ding, ding, we have a winner. It doesn't matter what the rest of the substituents look like, all you are concerned with is the "lowest number on the occasion of the first difference." The second term shows a difference, and the lowest number (2) gets the priority. So the name would be 6-ethyl-2,2,6,7-tetramethyl octane.
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Thanks for the detailed explanation.