I’m a student taking organic chemistry now, and so far I’m surviving. The one concept I’ve struggled to wrap my brain around is exactly when one should use the n- prefix before a straight chain. I had thought it was used before chains of four or more carbons that do not require a number. Thus, it would be n-butane but 1-butanol or 1-butyne.
Now my teacher says that carboxylic acids do not require the n- even though they would not require a number! Can anybody help me here, when is the n- appropriate? Aldehydes? Carboxylic acids? I’m confused.
Any help is appreciated!
Rule 1 - never mix "n"s with numbers. If you are using the common system you can use “n” and prefixes such as iso, neo, tert, etc. or you can use the alpha, beta, gamma method of labeling carbons. But you never use numbers.
If you are using the IUPAC system then you use numbers and never use any of the above mentioned methods.
Rule 2 - when naming carboxylic acids you always begin with the longest straight chain which contains the carcoxy carbon. In other words…any butyric acid begins with a straight chain of 4 carbons, the “first” of which is the carboxy carbon. So it would be redundant to call it n-butyric acid since the “butyric” is aleady defined to be a straight chain.
I wish I could draw stuctures here but I can’t, so…
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draw a butyric acid (4 carbon, carboxylic group on first carbon)
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tack an ethyl group onto the alpha carbon (the carbon next to the carboxy carbon)
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the longest carbon chain is now 6 carbons, but we do not use this 6 carbon chain in naming the cmpd because the 6 carbon chain does not contain the carboxy carbon!!
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instead, the RULE is to look for the longest straight chain which does contain the carboxy carbon. We therefor call the above alpha-ethylbutyric acid. And since the “butyric” part is by definition a straight chain, we never have to worry about an “n” prefix.
Rule 3 - aldehydes get their names from the corresponding carboxylic acid. The above cmpd would give alpha-ethylbutyraldehyde.
Thats all I can think of right now. I hope it makes sense.
Thank you. I’m focusing on the IUPAC system, so if you can provide any help on that with regards to n- vs. numbers vs. nothing.
Thanks.
You NEVER NEVER NEVER use the “n” prefix in the IUPAC system.
Think about it.
In general cmpds are named based on the longest straight chain.
So if you just have butane, that means that you have a straight chain of 4 carbons. If you rearrange the cmpd such that it isn’t 4 straight carbons…then it is no longer butane. It’s now 2-methylpropane.
So n-butane and butane are exactly the same thing and you can leave off the “n”.
Same with pentane. If you call it pentane then it’s 5 straight carbons. If you rearrange it you can get 2-methylbutane or 2,2-methylpropane, but you can’t get any pentane except a straight chain pentane.
Treat ketones just like two regular cmpds tacked onto the same
The exceptions to the above are carboxylic acids and aldehydes. With carboxylic acids you name the cmpd based on the longest straight chain which also contains the carboxy carbon.
Everything which I said in my first post still holds. Now you just use the IUPAC nomenclature so butyric acid becomes butanoic acid and if you tack on the ethyl group it becomes 2-ethylbutanoic acid. The corresponding aldehyde is 2-ethylbutanal.
Throw all your "n"s out the window and don’t worry about them. Any time you think you need an “n” you will find that you can get along without it.
Treat ketones just like two regular cmpds tacked onto the same
functional group.