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Proton NMR OCR- ester

http://www.ocr.org.uk/Images/131288-question-paper-unit-f324-rings-polymers-and-analysis.pdf

mark scheme http://www.ocr.org.uk/Images/135165-mark-scheme-unit-f324-rings-polymers-and-analysis-june.pdf

question 5)b) the last one on proton NMR, this was my structure its the same but mark scheme does not state.
(edited 11 years ago)
Scan0222.pdf320.4KB
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otrivine
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its the one I circled on bottom that is my structure
Original post by otrivine


Were you given a table of chemical shifts?

The only way to distinguish between the two structures is the chemical shift of the methyl and CH2 groups.

In your structure the methyl protons would be expected at over 3 and in the exam answer at about 2, but you could only be expected to know this with a table of shifts...
(edited 11 years ago)
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otrivine
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Original post by charco
Were you given a table of chemical shifts?

The only way to distinguish between the two structures is the chemical shift of the methyl and CH2 groups.

In your structure the methyl protons would be expected at over 3 and in the exam answer at about 2, but you could only be expected to know this with a table of shifts...



this is the data, we are provided http://www.ocr.org.uk/Images/74947-datasheet.pdf

will i get full marks for the structure, cause my knowledge of triplet all that was correct in order to get the correct groups attached to each other
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otrivine
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I am not sure, if my structure is correct?
Original post by otrivine
I am not sure, if my structure is correct?


No, you probably will not get full marks because you have the ester group the "wrong way round".

If you look at the shifts given in the data sheet you will see that the methyl group is attached to the carbonyl carbon, not to the ester oxygen ...
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Original post by charco
No, you probably will not get full marks because you have the ester group the "wrong way round".

If you look at the shifts given in the data sheet you will see that the methyl group is attached to the carbonyl carbon, not to the ester oxygen ...


do we use the chemical shifts to see what comes first? so does it go from small ppm to big ppm? thank you
Original post by otrivine
do we use the chemical shifts to see what comes first? so does it go from small ppm to big ppm? thank you


I don't understand your question.

You use the chemical shifts to differentiate between the groups you have identified from the integrals and splitting pattern, or to locate them in an environment.

You have a three proton singlet at about 2.2.

This corresponds to a CH3 group attached to an atom that does not contain a hydrogen (no splitting)

Your choices are CH3-CO-O

or

CH3-O-CO-

you use the data table to see which one fits ...
(edited 11 years ago)
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Original post by charco
I don't understand your question.

You use the chemical shifts to differentiate between the groups you have identified from the integrals and splitting pattern, or to locate them in an environment.


I understand, but in a proton NMR , how can you know what comes first!Like in my example above, I did everything correctly but just done the opposite putting ester at the end.

I think I get it know is it because from the chemical shift there is HC-C=O and there is no benzene ring ?
Original post by otrivine
I understand, but in a proton NMR , how can you know what comes first!Like in my example above, I did everything correctly but just done the opposite putting ester at the end.

I think I get it know is it because from the chemical shift there is HC-C=O and there is no benzene ring ?


Re-read my post above ...

I was adding to it while you were reading it!
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otrivine
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Original post by charco
Re-read my post above ...

I was adding to it while you were reading it!


oh so you work backwards cause in the NMR spectrum like you said 2.2ppm and do you go from right to left. Essentially?
Original post by otrivine
oh so you work backwards cause in the NMR spectrum like you said 2.2ppm and do you go from right to left. Essentially?


No, it doesn't matter which way you go ... there is no left and right!

It's a question of logic.

1. You know that there is a singlet with three hydrogen atoms = CH3

2. You know that this is attached to the ester group.

3. There are two possibilities, so you use the chemical shift to decide which it is.

Where does left and right come into it?
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Original post by charco
No, it doesn't matter which way you go ... there is no left and right!

It's a question of logic.

1. You know that there is a singlet with three hydrogen atoms = CH3

2. You know that this is attached to the ester group.

3. There are two possibilities, so you use the chemical shift to decide which it is.

Where does left and right come into it?



Ok, how do you know if the CH3 is attached to the C-O, is it by using the chemical shifts provided in the data
Original post by otrivine
Ok, how do you know if the CH3 is attached to the C-O, is it by using the chemical shifts provided in the data


yes
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otrivine
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Original post by charco
yes


Bingo! Do you consider that question to be the hardest for an A2 question?

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