[mucker973]

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

This has the Br element hanging off the end of the chain
second isomer - 2-Bromo-3-methylpentane

This has the Br element hanging off the 2n or 4th carbon
third isomer - 3-Bromo-3-methylpentane

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?

[mucker973]

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)

[Borek]

You mean like that?

[mucker973]

You mean like that?

yes

[Borek]

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.

[mjc123]

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".

[mucker973]

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?

[mucker973]

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.

[mjc123]

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.)

[mucker973]

So... no one knows the answer to this?

[Borek]

You were already given the answer - according to IUPAC naming rules compound you have suggested is NOT bromo-3-methylpentane but something else.

[mucker973]

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?

[mjc123]

Yes you did. If you didn't understand it or otherwise found it unsatisfactory, say why.

[Borek]

Check the naming rules.

[mucker973]

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,