Proton NMR

Original post by otrivine

In the proton NMR spectrum I got CH3COOCH(CH3)2

and the mark scheme gets

(CH3)2CHCOOCH3

literally same but the other way round, why is that? in proton NMR spectrum do we start from the right going to left peaks

I may be wrong, but even if they're the other way around, are they still the same molecule? :smile:

What does the mark scheme say - does is allow other structures (such as yours), but with the same structural formula?

Reply 2

OP

Original post by zef1995

I may be wrong, but even if they're the other way around, are they still the same molecule? :smile:

What does the mark scheme say - does is allow other structures (such as yours), but with the same structural formula?

Not sure because I am not using the mark scheme past papers its the book question on page 102 , I presume you have the OCR A2 chemistry book? :smile:

Reply 3

Original post by otrivine

Not sure because I am not using the mark scheme past papers its the book question on page 102 , I presume you have the OCR A2 chemistry book? :smile:

Oh I see! Yes I do!
I personally think the molecule you've got it the same as the one in the mark scheme (even if they're written the opposite ways around), because when you draw each of them out, they have the same structure :smile:

Reply 4

OP

Original post by zef1995

Oh I see! Yes I do!
I personally think the molecule you've got it the same as the one in the mark scheme (even if they're written the opposite ways around), because when you draw each of them out, they have the same structure :smile:

Thanks

Yes because its same thing right

Reply 5

Original post by otrivine

Thanks

Yes because its same thing right

It appears so! I understand that the answers in the book can be confusing sometimes, but it looks as if you got it right anyway :smile:

Reply 6

username913907

It does matter! Bit of a technical thing, but which way round specifies which is which un the ester. You finish with the alcohol and start with the carbonyl segment. Can't for the life of me remember how to do the iupac names for these things.... But they are different molecules.

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Reply 7

OP

Original post by JMaydom

It does matter! Bit of a technical thing, but which way round specifies which is which un the ester. You finish with the alcohol and start with the carbonyl segment. Can't for the life of me remember how to do the iupac names for these things.... But they are different molecules.

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wait so in the proton NMR spectrum do I go from the right peaks to the left peaks

Reply 8

Original post by JMaydom

It does matter! Bit of a technical thing, but which way round specifies which is which un the ester. You finish with the alcohol and start with the carbonyl segment. Can't for the life of me remember how to do the iupac names for these things.... But they are different molecules.

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Original post by otrivine

wait so in the proton NMR spectrum do I go from the right peaks to the left peaks

Ah, my apologies :frown:

Reply 9

username913907

Original post by otrivine

wait so in the proton NMR spectrum do I go from the right peaks to the left peaks

What? You've clearly identified the molecule correctly (the hard bit) but you've written out the molecular formula of the molecule incorrectly. Have a little revision session on naming ester. It wouldn't normally matter which way round you wrote it but it does for esters.

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Reply 10

OP

Original post by JMaydom

What? You've clearly identified the molecule correctly (the hard bit) but you've written out the molecular formula of the molecule incorrectly. Have a little revision session on naming ester. It wouldn't normally matter which way round you wrote it but it does for esters.

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but for esters do you not start with CH3, I am confused now :frown:

Reply 11

Original post by JMaydom

It does matter! Bit of a technical thing, but which way round specifies which is which un the ester. You finish with the alcohol and start with the carbonyl segment. Can't for the life of me remember how to do the iupac names for these things.... But they are different molecules.

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Oh yeah, I see what you mean now! Thank you for clearing that up, as I told Otrivine the wrong thing! I'm sorry for any confusion I may have caused.

Thanks :smile:

Reply 12

OP

Original post by zef1995

Oh yeah, I see what you mean now! Thank you for clearing that up, as I told Otrivine the wrong thing! I'm sorry for any confusion I may have caused.

Thanks :smile:

Can you explain to me, totally lost now :redface:

Reply 13

Original post by otrivine

Can you explain to me, totally lost now :redface:

I will certainly try my best :smile:

I know I've said it already but I'm sorry for confusing you in the first place, really didn't mean to :/

Although I originally mentioned that when you draw out the molecules they have the same structure, I have tried again and it turns out that they don't. So, for the molecule you got, CH3COOCH(CH3)2, if you draw that out, the carbonyl group (C=O) is bonded to the methyl group, and the oxygen that is bonded to the carbonyl group (COO), is bonded to the carbon with two methyl groups attached to it.

However, with (CH3)2CHCOOCH3, the carbonyl group is bonded to the carbon with two methyl groups, instead of the oxygen, and the oxygen (not the one on C=O) is bonded to the methyl group.

It is a very small difference, but it is important! And as you have seen me demonstrate, a difference that can be difficult to spot :P

Anyway, I hope that helps :smile:

Original post by otrivine

Can you explain to me, totally lost now :redface:

In the first molecule your methyl carbon is directly attached to the ester group carbon atom but in the isomer the methyl carbon is attached to the oxygen of the ester group.

They are completely different molecules, with differeent names and different physical and chemical properties.

Your molecule is CH3COOCH(CH3)2 (2-propylethanoate), and the mark scheme requires (CH3)2CHCOOCH3, methyl (methyl)propanoate

Each isomer produces a slightly different NMR spectrum, mainly shown by the shifts of the singlet methyl group and the mulltiplet CH

EDIT: Looks like Zeff beat me to the draw!

(edited 10 years ago)

Reply 15

OP

Original post by charco

In the first molecule your methyl carbon is directly attached to the ester group carbon atom but in the isomer the methyl carbon is attached to the oxygen of the ester group.

They are completely different molecules, with differeent names and different physical and chemical properties.

Your molecule is CH3COOCH(CH3)2 (2-propylethanoate), and the mark scheme requires (CH3)2CHCOOCH3, methyl (methyl)propanoate

Each isomer produces a slightly different NMR spectrum, mainly shown by the shifts of the singlet methyl group and the mulltiplet CH

OK so using my NMR spectrum how would I make sure I am doing them in the right places

Original post by otrivine

OK so using my NMR spectrum how would I make sure I am doing them in the right places

Use the chemical shifts given by your spectrum and compare them to the data tables for NMR.

Reply 17

OP

Original post by charco

Use the chemical shifts given by your spectrum and compare them to the data tables for NMR.

so do you mean to start from the peak in the NMR spectrum that is to the right and work from right to left, so whatever chemical shift will the furtherest to the right that would be the starting material?

Original post by otrivine

so do you mean to start from the peak in the NMR spectrum that is to the right and work from right to left, so whatever chemical shift will the furtherest to the right that would be the starting material?

No, it doesn't matter which peak you start with. Your aim is to assign all of the peaks. What matters is getting the strructure correct.

CH3COOC2H5 is the same as C2H5OOCCH3

BUT is NOT the same as CH3OOCC2H5, which IS the same as C2H5COOCH3

Notice how the order of the -COO- in the ester group is important BUT that the molecule can be reversed without changing the structure...

Reply 19