July 01, 2022, 06:11:53 PM
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Topic: Mysterious Isomer of Bromo-3-methylpentane  (Read 2315 times)

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Offline mucker973

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Re: Mysterious Isomer of Bromo-3-methylpentane
« Reply #15 on: May 04, 2022, 11:48:02 PM »
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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

Offline mucker973

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Re: Mysterious Isomer of Bromo-3-methylpentane
« Reply #16 on: May 05, 2022, 12:02:55 AM »
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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?

Offline mjc123

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Re: Mysterious Isomer of Bromo-3-methylpentane
« Reply #17 on: May 05, 2022, 05:31:05 PM »
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".

Offline mucker973

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Re: Mysterious Isomer of Bromo-3-methylpentane
« Reply #18 on: May 05, 2022, 07:42:13 PM »
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,

Offline mjc123

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Re: Mysterious Isomer of Bromo-3-methylpentane
« Reply #19 on: May 06, 2022, 04:54:37 PM »
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.

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