Section B NMR and Structure Drawing Revision Quiz Questions and Answers PDF

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Download Section B NMR and Structure Drawing Revision Quiz Questions and Answers PDF

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Tuesday, 27 March 2018, 7:59 PM

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Tuesday, 27 March 2018, 10:45 PM 2 hours 46 mins

Question 1

A compound of molecular formula C H Cl Br gave the following NMR spectrum for which the integrations are from left to right 1 : 2.

Correct

Sketch the structure of the molecule and then press next to submit the SMILES string.

Marked out of 1.00

Difficulty: Easy

2 3

Answer:

Cl Cl Br

Your answer: ClC(Cl)CBr

2

Correct! Well done. It's 1-bromo-2,2-dichloroethane. These NMR problems are peculiarly addictive, a bit like Sudoku. The correct answer is: ClC(Cl)CBr.

Question 2

A compound of molecular formula C H O gave the following NMR spectrum for which the integrations are from left to right 2:3. Sketch

Correct

the structure of the molecule to submit the SMILES string as an answer.

Marked out of 1.00

Difficulty: Low

4 10

Answer:

O

Your answer: CCOCC

Correct! Well done. Of course it's diethyl ether. We'll have to try something harder soon. The correct answer is: CCOCC.

Question 3

A compound of molecular formula C H O gave the following NMR spectrum for which the integrations are from left to right 5:2:2:3.

Correct

Sketch the structure of the molecule and then press press submit.

9 12

Marked out of 1.00

Answer:

O

Your answer: CCOCc1ccccc1

Correct! Well done. It's benzyl ethyl ether. Hope you're getting the hang of this SMILES business.

These NMR problems are peculiarly addictive, a bit like Sudoku. The correct answer is: CCOCc1ccccc1.

Question 4

A compound of molecular formula C H O gave the following NMR spectrum for which the integrations are from left to right 2 : 1 : 6 : 3. 5 12

Incorrect Marked out of 1.00

Sketch the structure of the molecule to submit the SMILES string as an answer.

Answer:

Your answer: CCC(C)(C)O

Ouch no. That can't be right. First of all, you've suggested that this is an alcohol. Does that make sense? Alcohol protons very seldom show coupling and the one peak that integrates for 1H shows a complex coupling pattern - could the multiplets. Secondly, the two methyl groups must be equivalent, but they don't have a hydrogen on the next carbon. So they should not couple to anything else. So in your molecule there should be a singlet that integrates for six protons. Have a rethink and try again. The correct answer is: CCOC(C)C.

Question 5 Correct Marked out of 1.00

1

A compound of formula C H Cl gave a H NMR spectrum described as follows: 10 12

2

δ Multiplicity Integration 1.3 doublet

6H

5.6 quartet

2H

7.1 singlet

4H

Answer:

Cl Cl

Your answer: CC(Cl)c1ccc(C(C)Cl)cc1

Yes. Well done. The key is to spot the aromatic (there are so few hydrogens and the chemical shift around 7). And then it's pretty symmetrical so you can just split it in two and put the same thing on either end of the molecule. These problems take a bit of practice. If you're struggling your first starting point is to look carefully at the formula. Does it fit the general formula for an alkane? Are they hydrogens that seem to be missing? Does that imply a ring or a multiple bond... And then think about symmetry - if there aren't very many peaks that implies a very symmetrical structure. The correct answer is: CC(Cl)c1ccc(C(C)Cl)cc1.

Question 6

A compound of molecular formula C H O gave the following NMR spectrum for which the integrations are from left to right 2:2:3:3.

Correct

Sketch the structure of the molecule and then press submit.

Marked out of 1.00

If you cannot see the spectrum properly, click it to expand it into a new window.

8 10

Difficulty: Easy

Answer:

O

Your answer: COc1ccc(C)cc1

Correct! Well done. This is perhaps a little tricky because the protons on the aromatic ring are a bit more complicated than you might expect. That's because you do actually get coupling with protons around the ring and things get a little bit more complicated. These NMR problems are peculiarly addictive, a bit like Sudoku. A key strategy to use when trying these questions is to look closely at the molecular formula. How does the C : H ratio compare with the "rule" for alkanes, C nH 2n+2. Halogens will reduce the hydrogen count by one. But it should be easy to spot if there are a lot of hydrogens missing - this points towards a ring (two hydrogens down) or double bonds (two hydrogen down). Aromatic rings will be "wildly" low in hydrogen. The formula gives you a real head start before you actually look at the spectra themselves. The correct answer is: COc1ccc(C)cc1.

Question 7

A compound of molecular formula C H

Incorrect

Difficulty level: Hard

9 18

gave the following NMR spectrum for which the integrations are from left to right 4, 6, 6, 2.

Marked out of 1.00

Sketch the structure of the molecule and press submit.

Answer:

Your answer: CCC(C)=C=C(C)CC

This is a really tricky one. A key starting point is to look at the molecular formula. Notice how this does not fit with the alkane formula. That suggests that the molecule is a bit weirder than you might expect. Being an alkane, however, the chemical shifts don't provide much guidance so you need to work by a process of elimination and link things up, keeping in mind the symmetry which controls equivalence within the molecule.

The correct answer is: CCC1(CC)[C@@H](C)[C@H]1C.

Question 8

A molecule of formula C H Cl gives the following spectrum. The peaks from left to right are in the ratio of 2: 1: 3. Sketch what you think

Correct

the molecule is.

10 12

Marked out of 1.00

Answer:

2

Your answer: CC(Cl)c1ccc(C(C)Cl)cc1

Yes. Well done. The simplicity of the spectrum tells you that it has to be pretty simple. Notice the strong shifting of the aromatic protons down to the region around 7 ppm. Notice also the substantial shift of the CH caused by the electronegative chlorine atome. These questions are a bit like Sudoku. There's a trick to them. If you've not got this one, then make sure you look closely at the molecular formula first and ask questions like "is it a substituted alkane?". If it's not then you need to think creatively about what kind of structure could give that sort of molecular formula and then see if there's anything in the NMR spectrum that helps you. Now you can start working on the connectivity and coupling. The correct answer is: CC(Cl)c1ccc(C(C)Cl)cc1.

Question 9

Here's the spectrum of a hydrocarbon C H . The peaks are in the ratio of 2 : 2 : 1 : 2 : 9 (from left to right). Can you work out what the

Not answered

molecule is?

Marked out of 1.00

Clicking on the image should open the spectrum into a new window for easier viewing.

12 16

Difficulty: Medium

Answer:

Your answer:

The combination of number of peaks with the integration, multiplicity and chemical shift can allow you to unravel even quite complex molecules like this one. The correct answer is: CC(C)(C)C=Cc1ccccc1.

Question 10

Look carefully at this spectrum. The molecular formula of the molecule is C H Cl . Can you identify it? The integration is shown in red at

Correct

the top of the spectrum.

Marked out of 1.00

Difficulty: Easy

4 7

Answer:

3

Cl

Cl Cl

Your answer: CC(Cl)C(C)(Cl)Cl

Good. Well done. The coupling pattern is pretty straightforward here and the peaks well spaced so it's not too hard this time. The correct answer is: CC(Cl)C(C)(Cl)Cl.

Question 11

Here's a spectrum. If the molecule has molecular formula C H ClO , can you work out what the molecule is? As usual, sketch the

Not answered

molecule using JMol – the SMILES string will be submitted automatically.

Marked out of 1.00

Be careful. This question has a twist to it.

4 7

Difficulty: Medium

Answer:

2

Your answer:

This is quite a tricky question. If you didn't get it because the peaks look weird, consider the possiblity of overlap… and then try again. The correct answer is: CCOC(=O)CCl.

Question 12

Can you identify the molecule based on the following NMR spectrum. The molecular formula is C H OS. Note that the spectrum shows

Incorrect

the expansion (i.e. blow-up") of the various multiplets.

Marked out of 1.00

Difficulty: Medium

4 8

Answer: Loading JME editor.... If this message does not get replaced by the JSME editor then you have not got javascript working in your browser. Your answer: CC(=O)C(C)=S

This is a pretty tricky question. The coupling is slightly more complicated that you might expect from some of the simple ideas presented in school and in lectures. The correct answer is: CSCCC=O.

Question 13 Correct Marked out of 1.00

A compound of formula C5H10 gave a 1H NMR spectrum described as follows: δ Multiplicity Integration 1.5 singlet

3H

1.6 singlet

3H

1.7 doublet

3H

5.6 quartet

1H

Answer: Loading JME editor.... If this message does not get replaced by the JSME editor then you have not got javascript working in your browser. Your answer: CC=C(C)C

Yes. Well done. Notice that there are no hydrogens that can couple so you only get singlets. These NMR problems are peculiarly addictive, a bit like Sudoku. These problems take a bit of practice. If you're struggling your first starting point is to look carefully at the formula. Does it fit the general formula for an alkane? Are they hydrogens that seem to be missing? Does that imply a ring or a multiple bond... The correct answer is: CC=C(C)C.

Question 14

Sketch the molecule of molecular formula C H

5 10

that would give this spectrum. Note that the four peaks have integrations of 1 : 3 : 3 : 3.

Correct Marked out of 1.00

Answer: Loading JME editor.... If this message does not get replaced by the JSME editor then you have not got javascript working in your browser. Your answer: CC=C(C)C

That is correct. Notice the severely shifted position of the alkene proton which is sitting right next to the pi-orbital. That causes some really weirdly unsymmetrical screening that shifts the proton strongly downfield. If you got it wrong, then here are some steps to help you think the problem. Consider the molecular formula carefully. Is it an alkane? If not, is there a functional group that might affect the chemical shift? Look at the integration ratio and see if you can group things. Look carefully at coupling and see how things should split. Now try again.

The correct answer is: C/C=C(C)\C.

Question 15 Incorrect

So here's a molecule. Its formula is C5H11NO. Look at the spectrum and then sketch what you think think that it fits. The integrations of the peaks are in the ratio of 4:4:3 (L -> R).

Marked out of 1.00

Answer: Loading JME editor.... If this message does not get replaced by the JSME editor then you have not got javascript working in your browser. Your answer: CCC

The correct answer is: CN1CCOCC1.

Question 16

A molecule of formula C5H6O gives the following NMR spectrum. What could it be?

Incorrect Marked out of 1.00

Answer: Loading JME editor.... If this message does not get replaced by the JSME editor then you have not got javascript working in your browser. Your answer: C=CC=CC

The range of chemical shifts is something to ponder as it helps to give clues to the nature of the molecule. The correct answer is: CCC#CC([H])=O.

Question 17

Take a look at this spectrum. The molecule has the formula C6H16OSi. Sketch the molecule.

Incorrect

The peaks are in the ratio of 1: 6 : 9.

Marked out of 1.00

Answer: Loading JME editor.... If this message does not get replaced by the JSME editor then you have not got javascript working in your browser. Your answer: CC(C)S(C)(C)C

The correct answer is: C[Si](C)(OC(C)C)C.

Question 18

Can you identify the molecule that gives rise to the following 1H NMR spectrum? The molecular formula is C H O . The integrated area

Incorrect

is given in red above each peak.

Marked out of 1.00

Difficulty level: Easy.

8 10

2

Answer: Loading JME editor.... If this message does not get replaced by the JSME editor then you have not got javascript working in your browser. Your answer: COCc1ccc(COC)cc1

The molecular formula, combined with the vastly differing chemical shifts here really offer some serious clues… The correct answer is: COc1ccc(OC)cc1.

Question 19

A compound of molecular formula C H O gave the following NMR spectrum for which the integrations are from left to right 1, 2, 3, 6.

Incorrect

Sketch the structure of the molecule to submit the SMILES string as an answer.

5 12

Marked out of 1.00

Answer: Loading JME editor.... If this message does not get replaced by the JSME editor then you have not got javascript working in your browser. Your answer: CCC(C)C

Many people did not submit a response to this question. This suggests that you need more practice and I'd have a careful think about the problem now. Quite a few people found this question hard and we got a bewildering range of answers so we won't comment on individually. However there were many submissions of C 5 molecules with the wrong number of hydrogens - ketones, aldehydes and so on. That can't be right. Look at the molecular formula and compare the C : H ratio with the formula for an alkane C H . This is incredibly helpful. Notice that n 2n+2

the formula has an additional oxygen - it has to be like an alkane with an extra oxygen thrown in. That narrows things down to either an alcohol or an ether. Now you can narrow things down. Could this be an alcohol? If not, then it has to be an ether. Essentially you need to work by a process of elimination of clues to arrive at the answer. The correct answer is: CCOC(C)C.

Question 20

A compound of molecular formula C H

Correct

3. You can expand the spectrum by clicking on it. Sketch the structure of the molecule and then press submit.

Marked out of 1.00

Difficulty level: Medium

10 12

gave the following NMR spectrum for which the integrations are from left to right 2 : 2 : 1 : 1: 3 :

Answer: Loading JME editor.... If this message does not get replaced by the JSME editor then you have not got javascript working in your browser. Your answer: CC=Cc1ccc(C)cc1

Almost correct! You've spotted the aromatics and the double bond and connected them up correctly. But in fact that it's cis isomer, rather than the trans. But notice how the coupling is quite straightforward -you get a nice doublet for one proton. The other couples to both the olefinic CH and the methyl. Good stuff. These NMR problems are peculiarly addictive, a bit like Sudoku.

The correct answer is: C/C=C\c1ccc(C)cc1.

Question 21

Which proton environment (indicated in italics) would be most strongly deshielded (i.e. shifted to the highest ppm/chemical shift value)?

Correct Marked out of 1.00

Select one: a. CH3CH=CH 2 b. O(CH2 CH3) c. C H -H 6 5

That is correct. The protons attached to an aromatic ring will be strongly affected by the ring current associated with

the delocalised electrons of the aromatic ring. These typically come around a chemical shift of 7 - 7.5 ppm. d. CH3CH 2CH 3

Your answer is correct. This proved to be quite a tricky question that separated those who've really got a bit hooked on NMR from the rest. Chemical shift is subtle and you need to keep several factors in mind, from the weird impact of pi electrons on very close hydrogens, though to electronegative substituents. The correct answer is: C6 H5-H

Question 22

A compound of formula C5H10 gave a 1H NMR spectrum described as follows:

Correct Marked out of 1.00

δ Multiplicity Integration 1.5

singlet

3H

1.6

singlet

3H

1.7

doublet

3H

5.6

quartet

1H

Identify the molecule and write the SMILES string for the structure in the box below. You can use MARVIN sketch or the NIH Online structure editor to get the SMILES string (Click on "start structure editor").

Answer: CC=C(C)C

Yes. Well done. Notice that there are no hydrogens that can couple so you only get singlets. These problems take a bit of practice. If you're struggling your first starting point is to look carefully at the formula. Does it fit the general formula for an alkane? Are they hydrogens that seem to be missing? Does that imply a ring or a multiple bond... The correct answer is: CC=C(C)C

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