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Topic: NMR: In (CH3)2CBrCH2Br how many signals are there?  (Read 11713 times)

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snoopy_tj

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NMR: In (CH3)2CBrCH2Br how many signals are there?
« on: April 02, 2006, 10:04:19 PM »
Hi all,

I did a search but couldn't find anything to answer my two questions. So hopefully someone can point me in the right direction :) Please if I am totally wrong please tell me and I will go back to the drawing board. I just really dont get NMR.

QUES 1) In (CH3)2CBrCH2Br how many signals are there? I thought the answer was 3 because of the electronegativity of the Br, but I am not 100% sure.

QUES 2) Please see attched. This is a differnt unrelated question to the above. I need to show the hydrogen that would appear in the lowest field in the 1Hn.m.r spectrum. I have circled the one I thought.
« Last Edit: April 03, 2006, 01:28:58 AM by snoopy_tj »
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Offline pantone159

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #1 on: April 03, 2006, 01:14:47 AM »
Quote from: snoopy_tj on April 02, 2006, 10:04:19 PM
1) In (CH3)2CBrCH2Br how many signals are there?

This doesn't match your picture.  If you drew this formula the right-hand two C atoms would each have a Br, so the middle one (the left of these two) would then have no H atoms attached.

This changes the answer.
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Offline mike

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #2 on: April 03, 2006, 01:27:24 AM »
I would have thought the Hydrogen that you circled would be the most HIGH field and that the hydrogen adjacent to the Br would be the most low field.

The moleculae you drew would have three signals but the one you wrote would have two.
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snoopy_tj

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #3 on: April 03, 2006, 01:29:44 AM »
Quote from: mike on April 03, 2006, 01:27:24 AM
I would have thought the Hydrogen that you circled would be the most HIGH field and that the hydrogen adjacent to the Br would be the most low field.

The moleculae you drew would have three signals but the one you wrote would have two.

Thanks for your reply :) I had a feeling I had it the wrong way around for question 2. I thought a low ppm was a high field...oops.

With Question 1 only having 2 signals, I thought that even though the H's on the two CH2 molecules were equivalent, that because one of the CH2 would be closer to the Br, that it would have a slightly differnt signal as the Br would have strong electron pulling properties and reduce the electron density causing it to resonance at a lower field.

If not how come this doesnt happen?
« Last Edit: April 03, 2006, 01:42:13 AM by snoopy_tj »
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Offline mike

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #4 on: April 03, 2006, 01:55:12 AM »
I don't know which one you mean would be closer :-[

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snoopy_tj

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #5 on: April 03, 2006, 02:12:11 AM »
Quote from: mike on April 03, 2006, 01:55:12 AM
I don't know which one you mean would be closer :-[



You picture explains it :) When it was in 2D It looked like one of them was closer, but your diagram has cleared it up for me :) Thanks!
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Offline Albert

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #6 on: April 03, 2006, 12:04:27 PM »
To sum up...
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Offline Borek

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #7 on: April 03, 2006, 12:14:33 PM »
Shouldn't 3 be H?
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Offline Albert

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #8 on: April 03, 2006, 12:15:52 PM »
Why?  ???
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Offline pantone159

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #9 on: April 03, 2006, 04:12:21 PM »
Quote from: Albert on April 03, 2006, 12:15:52 PM
Why?  ???

The OP's question had H where you have CH3 at the 3 position.

There are now three competing pictures for this molecule :)
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Offline pantone159

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #10 on: April 03, 2006, 04:17:12 PM »
Albert - Your graph has the two Me groups 1 and 2 with separate signals.
(I think so, it is a little hard to read.)
Why?  I would have figured they would be the same, and not split since there are no protons on the adjacent carbon.
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Offline Albert

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #11 on: April 03, 2006, 04:58:00 PM »
Quote from: Borek on April 03, 2006, 12:14:33 PM
Shouldn't 3 be H?

Oh, sorry Borek, now I've got it. Thank you Mark.

Here is the molecule Mike drew: just two signals (frequency 200 MHz).
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Offline Borek

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #12 on: April 03, 2006, 05:17:24 PM »
Even hi-res won't change it, at first I wasn't sure, but once I have drawn the structure it was obvious.
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Offline Albert

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #13 on: April 03, 2006, 05:49:21 PM »
Quote from: snoopy_tj on April 02, 2006, 10:04:19 PM
QUES 2) Please see attched. This is a differnt unrelated question to the above. I need to show the hydrogen that would appear in the lowest field in the 1Hn.m.r spectrum. I have circled the one I thought.

Yes, your answer is correct, being those hydrogens the most distant ones from the halogens.
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Offline Albert

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Re:NMR: In (CH3)2CBrCH2Br how many signals are there?
« Reply #14 on: April 03, 2006, 06:06:50 PM »
Quote from: Mark Kness on April 03, 2006, 04:17:12 PM
Albert - Your graph has the two Me groups 1 and 2 with separate signals.
(I think so, it is a little hard to read.)
Why?  I would have figured they would be the same, and not split since there are no protons on the adjacent carbon.


They aren't the same: they are diasterotopic methyl groups.
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