[ritwik08]

NC-CH₂-CH₂-COOH
Will the C of CN be counted in main chain?

[macman104]

No.  The CN group is a cyano group, and it is not counted as part of the parent chain.

[ritwik08]

No.  The CN group is a cyano group, and it is not counted as part of the parent chain.

Thats exactly what I thought. But my teacher has marked me wrong for not counting the carbon in CN. Can you supplement this argument with a reliable source please?

[macman104]

What did you put as your answer that was marked wrong?

[azmanam]

I think it depends if the nitrile is the highest priority substituent or not.  If it is, the carbon is included in the backbone and it is named as a 1-nitrile.  If there is a higher priority substituent, the carbon is not included in the chain and the C-N triple bond is named as a cyano group.

In this example, I would say the acid is highest priority and the compound should be named 3-cyano-propanoic acid (ChemDraw agrees).

http://www.chem.ucalgary.ca/courses/351/orgnom/nitriles/nitriles-01.html

[ritwik08]

What did you put as your answer that was marked wrong?

I put:
3-cyano-propan-1-oic acid
but he says it should be
4-cyano-butan-1-oic acid

[macman104]

I disagree with your professor.

Go to this site:

http://www.lookchem.com/cas-160/16051-87-9.html

Give him the CAS number and tell him to double-check if he doesn't believe you.

[azmanam]

fyi - While not technically incorrect, the -1- before the -oic acid is redundant.  Carboxylic acids, by definition, must be at the 1 position of the longest linear chain.

Even ChemSpider -- notorious for having every possible name of a compound (and some impossible names) -- says that compound is 3-cyanopropanoic acid only.

http://www.chemspider.com/Chemical-Structure.231083.html

[Borek]

Carboxylic acids, by definition, must be at the 1 position of the longest linear chain.

What about...

[azmanam]

touche.  Gotta pull out the exceptions, don't you? 

So the acid defines the parent chain, and is by definition at position 1.  That might be a more precise way to say it.  Your example would be 2-ethylbutanoic acid.

[Borek]

touche.  Gotta pull out the exceptions, don't you? 

That's the way my brain is wired. I don't even have to think about counterexamples, they just lie around

[ritwik08]

Thanks guys for those proofs. Thanks once again. I will give him the CAS number, but there is little chance that he would pay heed to my argument.

[DrCMS]

Your teacher is confused and wrong.

Heres why:

With only the CN group there the carbon of the CN is included in the longest chain name with the CN denoted as a nitrile.

NC-CH₂-CH₂-CH₃  is butanenitrile

With the acid group as well the longest chain is named for the acid with the CN as a cyano group

NC-CH₂-CH₂-COOH  is 3-cyanopropanoic acid

See this for the IUPAC proof of this:

http://www.acdlabs.com/iupac/nomenclature/93/r93_557.htm