Organic nomenclature

[Deceitful Wings]

Hey everyone,
my teacher at school and chemistry tutor have conflicting views on how you should name organic molecules. 

My chem tutor says that for every molecule you must be specify the location of each functional group and double bond. That is, if you had a carboxyl group on the end of a pentane, the name of the compound would be 1-pentanoic acid.

However, my chem teacher at school says that its just pentanoic acid, and if you did say 1-pentanoic acid, you would have a mark taken off in the vcaa exam.

But my chem tutor says that the chem exams as of 2010 have been slightly altered in marking allocations so that you must specify the location of every functional group and double bond to obtain the mark, regardless of how obvious it is.

I AM SO CONFUSED! who would be right? and more importantly, what way does VCAA follow?

[charmanderp]

My advice to you would be to read the examiners reports;they specify there exactly what the expect, and I've changed my own personal style accordingly.

But in that case you don't need to put the number on, as you can't have a carboxylic acid group on any carbon other than the first one. You shouldn't lose a mark for it though.

If it's ambiguous (as in, the double bond/functional group could be on a carbon other than the first) you have to specify. For example it's 1-propanol; simply propanol is incorrect/

[Hellrocks]

I don't know about VCAA, but if you follow the IUPAC convention, the number is not needed if the functional group is obvious.
In any case, I remember I do tend to try and put the number whenever possible as it saves the hassle into thinking "is this one of those obvious cases?"
It is never wrong to put the number, if VCAA thinks so that is a bit ridiculous.

[jadams]

AFAIK, it is implied that the carboxylic acid group is on c1, making naming pentanoic acid as 1-pentanoic acid as redundant.

If you think about what is involved in the carboxylic acid group: a carbon atom doubled bonded to an oxygen atom, and also single bonded to a O-H group, it is impossible for this functional group to be situated in the middle of a standard chained alkane (due to a middle carbon atom being bonded to 2 other carbons, and therefore no room for the double bond and single bonded Oxygen atoms.)

Edit: Just realised after posting that it was mentioned above that the carboxylic acid group has to be on either end of the molecule, but I guess an explanation may be helpful.

[charmanderp]

As the two above have said, it's best to always be as specific as possible. You most likely will not lose marks for putting numbers in, even if they are redundant, so long as they're appropriate.

[jadams]

As the two above have said, it's best to always be as specific as possible. You most likely will not lose marks for putting numbers in, even if they are redundant, so long as they're appropriate.

I wonder what they prefer in regards to methyl propane. It is obviously implied the methyl side group must be bonded to the second carbon, so wouldn't 2-methyl propane be overly redundant?

[charmanderp]

Indeed it would, but you'd have to be dreadfully unlucky to have marks taken off for it.

[charmanderp]

TBH I think it poses more of a challenge in MC - you look through options A-D and are unable to find what you believe is the correct answer when it's really listed as a simplified version!

[jadams]

TBH I think it poses more of a challenge in MC - you look through options A-D and are unable to find what you believe is the correct answer when it's really listed as a simplified version!

Yup! I Like your sig btw

[mihir94]

You just have to be careful when naming carboxylic acids and alkanols.

This is what the 2004 VCAA report said: A common answer was 1-butanoic acid. While this was accepted, it should be emphasised to students that since 2-butanoic acid does not exist, the 1- is not necessary in the systematic name.

This is what the 2011 VCAA report said: Ethan-1-ol or 1-ethanol was a relatively common response for this question; however this is not the systematic name. There is no other option for the  –OH functional group but to be on C-1, hence the number is not part of the systematic name.

So i just put the number when it is necessary. As there is no such thing as 2-pentanoic acid, just put in pentananoic acid. As Hellrocks said 'follow the IUPAC convention'. You never know, if this year the exam is really easy and they are finding it difficult to differentiate people, they may come to this. Last year they didn't allow Ethan-1-ol or 1-ethanol.

[Tonychet2]

my advice is get a new tutor lol he is hell wrong

[Hutchoo]

Lmao, this thread made me realise how much of a gaybo my teacher is.

[Deceitful Wings]

LOOL
thanks for your help guys, it clarified a lot of stuff
do you reckon they will ask questions where there are two major functional groups on the same hydrocarbon(e.g. amine group and chloro group)? or something a little ambigous?
If so, which would be prioritised if they were on say, propane and they were on the second carbon?
i.e. What would the name of this molecule be?

    H      NH2  H
    |       |       |
H-C-----C-----C-H
    |       |       |
    H      Cl     H

[charmanderp]

Chloro, or any halogen, does not constitute a major functional group. If you had just no amine there and just a chlorine atom, would you call it propanchloro? Hopefully not!

This would be 2-chloropropan-2-amine.

[jadams]

i.e. What would the name of this molecule be?

    H      NH2  H
    |       |       |
H-C-----C-----C-H
    |       |       |
    H      Cl     H

Amino functional group has precedence over chloro...therefore it is 2-chloro propyl-2-amine.