jane1234's chemistry questions!!

[jane1234]

Hey guys...

1. If you have pentan-2-one [CH3COCH2CH2CH3] and pentan-3-one [CH3CH2COCH2CH3] and analyse it with a mass spectrometer, the pentan-3-one will form a peak at 29 [CH3CH2] but pentan-2-one will not form a peak at 29... why is this??

2. (Relating to NMR) Predict the total number of peaks would would expect to see in the low resolution NMR spectrum of the following:
a) 1-chlorobutane [CH3CH2CH2CH2Cl]
b) 2-chlorobutane [CH3CH2CHClCH3]

I got 2 for the first and 3 for the second but the answers say 4 and 4. Why??


Thanks

[jane1234]

BUMP.

[luken93]

2) a)
       H    H   H   H
        |     |    |    |
 H - C - C - C - C - Cl
        |     |    |    | 
       H    H   H   H

Look at what they are next to. On the left hand side, the H on C4 are next to 2 Hydrogen atoms, so the CH3 group all share the same environment
On C3, They are next to a CH3 and a CH2, hence the C3 Hydrogens are another environment
On C2, They are next to a CH2 and a CH2Cl, hence the C2 Hydrogens are another environent.
On C1, they are next to a CH2 and a Cl atom on the same carbon, hence the C1 Hydrogens are another environment
All up, there are 4. It's similar for the other one.

[jane1234]

2) a)
       H    H   H   H
        |     |    |    |
 H - C - C - C - C - Cl
        |     |    |    | 
       H    H   H   H

Look at what they are next to. On the left hand side, the H on C4 are next to 2 Hydrogen atoms, so the CH3 group all share the same environment
On C3, They are next to a CH3 and a CH2, hence the C3 Hydrogens are another environment
On C2, They are next to a CH2 and a CH2Cl, hence the C2 Hydrogens are another environent.
On C1, they are next to a CH2 and a Cl atom on the same carbon, hence the C1 Hydrogens are another environment
All up, there are 4. It's similar for the other one.

Ah okay I think I get it now... so do you look at the whole group (ie the CH2 or the CH3) to decide which environments? For example, are all H's in a CH3 group going to be in the same environment??
Do you happen to know the answer to the first one as well??
Thanks

[luken93]

Yeah, usually all of the Hydrogens attached to the same Carbon will have the same environment, it's just a matter of looking what is next to them.

As for the 1st question, I would've thought it could produce one from the fragmentation of the ethyl ion, but obviously we are mistaken

[jane1234]

Still struggling with NMR...

On a 13C NMR spectrum, how many lines (presumably low res) will be shown for CH3CH2CH2CH2OH?

I said it was 3 but apparently not...

[luffy]

There are 5 hydrogen environments (from my knowledge), so there should be 5 peaks in low resolution proton NMR.

[jane1234]

There are 5 hydrogen environments (from my knowledge), so there should be 5 peaks in low resolution proton NMR.

Can you please explain why?

EDIT: It's a 13C NMR, not a hydrogen one so shouldn't you be looking at the carbon environments?

[luffy]

Oh woops - I read it wrong, I thought you were talking about proton NMR.

I haven't actually researched into carbon NMR as of yet, but I would assume the principles are the same.
Therefore, for carbon NMR, I would think there would be 4 carbon environments. The environments will be the carbon in CH3, the adjacent carbon of -CH2 -, the other carbon in CH2, which is in fact a different environment as it is bonded to carbons of different no.s of hydrogen bonds, and the carbon bonded to the -OH group.

If the theory behind Carbon NMR is the same, my answer should be right.

Hope I helped.

[luken93]

I'm kinda the same, but I would say 4. Although you're not looking at the Protons, in essence the Carbons are still affected by what is around them. Hence, since each environment has a different number of protons, I'd say 4?

But as you say, I don't think it's covered well enough in textbooks...

[Water]

For the 2nd one, 4 would be correct


CH3   CH2    CHCL     CH3



As you go across, the hydrogen environments + its neighbours when you go to each group from left to right is different to one another.

[jane1234]

Okay... so what's the difference between 1H and 13C NMR? As in, what should you look for in each? Because with the 13C I thought you just looked at the 4 bonds coming off each C... but do you have to take the other atoms into account and not just the immediate ones?

[Water]

I think, from the theory that I understand thus far,


C would also have four, because of the different shielding due to the atoms that surrounds it?

[jane1234]

So... for CH3CH2CH2CH2OH:

You could break it up like CH3 CH2 CH2 CH2OH

^^ But the CH2's are the same! Both are attached to 2 carbons and 2 hydrogens... Or do you always have to look at the next 'group' along (like the CH3 or the CH2OH)?

I just can't get my head around it :S This is why I didn't do physics

[luken93]

So... for CH3CH2CH2CH2OH:

You could break it up like CH3 CH2 CH2 CH2OH

^^ But the CH2's are the same! Both are attached to 2 carbons and 2 hydrogens... Or do you always have to look at the next 'group' along (like the CH3 or the CH2OH)?

I just can't get my head around it :S This is why I didn't do physics

No, the first CH2 is next to a CH3 and CH2, whereas the other one is next to a CH2 and CH2OH = Different.