Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: mucker973 on May 01, 2022, 07:23:33 AM
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I am studying chemistry at uni and I had a question which asked me write the skeletal formula of all 3 isomers of the above compound. I worked them out myself, then I went and researched them on the net to confirm I was correct. But I came up with a 4th isomer myself and can't find the flaw in my logic. I cannot find this 4th isomer anywhere on the Internet so I am confident I am wrong in this, but I just want someone to explain to me where my logic is flawed. Here are the facts what I was presented with:
chemical formula: C6H13Br
first isomer - 1-Bromo-3-methylpentane
(https://pubchem.ncbi.nlm.nih.gov/image/imgsrv.fcgi?cid=39899&t=l)
This has the Br element hanging off the end of the chain
second isomer - 2-Bromo-3-methylpentane
(https://pubchem.ncbi.nlm.nih.gov/image/imgsrv.fcgi?cid=19612097&t=l)
This has the Br element hanging off the 2n or 4th carbon
third isomer - 3-Bromo-3-methylpentane
(https://pubchem.ncbi.nlm.nih.gov/image/imgsrv.fcgi?cid=91414&t=l)
This has the Br element coming off the 3rd carbon in the opposite direction of double carbon branched chain.
I came up with a 4th, it is the same structure as the 3rd isomer but instead of the Br being where it is there, I had it on the opposite side - at the very end of the C-C branch.
Why is this not a valid isomer?
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I bit more context to anticipate some answers. I know the opposite has a CH3 there. My understanding is that you can just replace one H with a Br though (which is what is done for all other isomers)
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You mean like that?
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You mean like that?
yes
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Can you list the question exactly as it was asked? C6H13Br has many more isomers, for some reason all you listed have one tertiary carbon, which makes me think we are not told everything.
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If you were asked for the structural isomers of "bromo-3-methylpentane", those are the three. The fourth you identified would not be called "bromo-3-methylpentane" but "3-bromomethylpentane".
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If you were asked for the structural isomers of "bromo-3-methylpentane", those are the three. The fourth you identified would not be called "bromo-3-methylpentane" but "3-bromomethylpentane".
ok, but why?
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If you were asked for the structural isomers of "bromo-3-methylpentane", those are the three. The fourth you identified would not be called "bromo-3-methylpentane" but "3-bromomethylpentane".
There are 2 parts to the question. The first one, i answered:
3-Methylpentane is a colourless liquid with a characteristic smell that reacts with bromine liquid (a diatomic molecule) in the presence of a catalyst. A bromo-3-methylpentane liquid compound is formed through the substitution of one hydrogen with bromine.
Write a balanced chemical equation for this reaction and explain if it requires balancing. Hydrogen bromide gas is also a product of this chemical reaction. Note: ignore the presence of the catalyst.
Then the second part:
Bromo-3-methylpentane has three isomers: 1-Bromo-3-methylpentane, 2-Bromo-3-methylpentane, 3-Bromo-3-methylpentane. Draw the skeletal formula of the isomers and write the chemical formula for each isomer.
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ok, but why?
(1-, 2- or 3-)bromo-3-methyl pentane is a pentane with two substituents - a bromine atom and a methyl group. 3-bromomethylpentane has one substituent, a bromomethyl (CH2Br) group. (Incidentally, I don't see why it shouldn't be one of the products of the bromination reaction.)
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So... no one knows the answer to this?
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You were already given the answer - according to IUPAC naming rules compound you have suggested is NOT bromo-3-methylpentane but something else.
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You were already given the answer - according to IUPAC naming rules compound you have suggested is NOT bromo-3-methylpentane but something else.
yep but then I was asking why it becomes a different compound just because the Br atom is on the position it is. Didn't get an answer on that one?
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Yes you did. If you didn't understand it or otherwise found it unsatisfactory, say why.
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Check the naming rules.
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Ok, so I think it is because I don't understand the naming rules. I haven't covered that topic yet in my studies, can you direct to a link which explains that?
thanks,
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ok, but why?
(1-, 2- or 3-)bromo-3-methyl pentane is a pentane with two substituents - a bromine atom and a methyl group. 3-bromomethylpentane has one substituent, a bromomethyl (CH2Br) group. (Incidentally, I don't see why it shouldn't be one of the products of the bromination reaction.)
Ok i Can see you did indeed answer it here. Sorry, for some reason when I did the refresh of the page this answer did not appear or I somehow missed it. Let me have google on this and I'll post back if I still don't understand it. thanks
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Incidentally, I don't see why it shouldn't be one of the products of the bromination reaction.
Also, what do you mean by this?
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In principle, all four isomers are similar; they all result from replacing one of the hydrogens of 3-methylpentane with a bromine atom, and as far as I can see there is no reason why one of them should not be formed while the other three are. However, the naming rules lead to three of them being named as bromo-3-methylpentane and one as 3-bromomethylpentane. The point is that the longest carbon chain is taken as the base name (pentane). The first 3 isomers have two separate substituents (Br and CH3) directly attached to the pentane chain, and are named as 1-, 2- or 3-bromo-3-methylpentane. However, in the fourth isomer, the bromine isn't attached to the pentane chain, but to the methyl substituent, so it is a pentane with one substituent, bromomethyl (CH2Br), so is called 3-(bromomethyl)pentane - perhaps it helps to include those brackets to emphasise that it is one substituent (bromomethyl), not two (bromo and methyl).
But Nature doesn't know the IUPAC naming rules, and 3-(bromomethyl)pentane is not a fundamentally different kind of compound from the other three. You were right to think of it, and I think it should be included; it just isn't named as a "bromo-3-methylpentane".
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Thanks for clearing that up. I suppose "technically" I was wong because if you look at the original question (pasted about two posts in) it does only ask for the 3 isomers. But here is my issue, to answer it correctly it requires me to have an understanding of the naming rules etc that you just explained. Now, it makes sense. But, I've gone back over my course material and no point does it cover how the naming works. So based on the knowledge I've been given alone through my course, there is no way I could have known the 4th isomer I came up with was wrong. I am very annoyed with my uni course and I think it is very subpar. I have already found mistakes all over it and this is another example of poor information. What makes this all the more worse - it is a questio on the final exam paper for this module, which carries more weight in my grading than all other exams I have done since... I am going to putting in a complaint.
thanks again,
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The rules for naming organic compounds are a fundamental topic that any course worth its salt must cover. It is perfectly reasonable for a university final exam to assume you know the rules; it is completely unreasonable for the course not to teach them.