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Core 6 > Applications of NMR Spectroscopy in Organic Chemistry > Flashcards

Applications of NMR Spectroscopy in Organic Chemistry Flashcards

1
Q

What is the equation to calculate coupling constants?

A

J (Hz) = Separation between two lines (ppm) x MHz of NMR spectrometer

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2
Q

What are the three main things to consider when looking at a H NMR spectrum?

A

1) Chemical Shift 2) Integration 3) Multiplicity

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3
Q

Is 10ppm downfield or upfield, is it shielded or deshielded and what functional group is usually present there?

A

Downfield, deshielded and aldehyde (Always start analysis as downfield as possible)

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4
Q

What causes coupling?

A

Orbital overlap

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5
Q

What type of protons do not couple to each other?

A

Equivalent protons

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6
Q

Hydrogen atoms also couple to which other atoms?

A

Fluorine and Phosphorus

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7
Q

What is the ppm of CDCl3 for a H NMR?

A

Trick question!!! It doesn’t have one. Residual CHCl3 appears as a singlet at ~7.27 ppm

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8
Q

What is the typical ppm range for aromatic protons?

A

8.5 - 6.5 ppm (large range due to many resonance forms)

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9
Q

What effect can make signals appear more downfield or upfield than expected?

A

Mesomeric effects can place positive charges on carbons resulting in a deshielding effect for the protons meaning their signals appear more downfield than expected. The same is observed with negative charges resulting in upfield signals

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10
Q

What are the two main things to consider when looking at a C NMR spectrum?

A

1) Chemical Shift 2) D.E.P.T (distortionless enhancement by polarisation transfer)

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11
Q

Rank these function groups in order of ppm on a C NMR and give a rough estimate to their range:
- Primary, Secondary and Tertiary Amines
- Ketones and Aldehydes
- C, CH ,CH2 and CH3
- Primary, Secondary and Tertiary Alcohols, Next to O
- Alkenes and Aromatics
- Esters and Amides

A

210 - 190: Ketones and Aldehydes
~170: Esters and Amides
150 - 100: Alkenes and Aromatics
100 - 50: Alcohols, Next to O
60 - 40: Amines
50 - 0: C, CH, CH2 and CH3

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12
Q

What is the ppm of CDCl3 for a C NMR?

A

77.0 - a 1:1:1 triplet. D has a spin of 1 therefore: 2(1) + 1 = 3 (Triplet)

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13
Q

What does the height of signals in C NMR roughly represent? How do quaternary carbons break this rule?

A

Proportional to how many carbons in a signal. A peak twice as high as another will have twice as many carbons. Quaternary carbons always appear smaller in height.

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14
Q

What happens to C, CH, CH2 and CH3 in DEPT-135?

A

C - absent
CH - point upwards
CH2 - point downwards
CH3 - point upwards

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15
Q

What happens to C, CH, CH2 and CH3 in DEPT-90?

A

C - absent
CH - point upwards
CH2 - absent
CH3 - absent

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16
Q

What happens to C, CH, CH2 and CH3 in DEPT-45?

A

C - absent
CH - point upwards
CH2 - point upwards
CH3 - point upwards

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17
Q

What is the typical ppm for CARBOXYLIC ACIDS?

A

12-10 ppm

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18
Q

What is the typical ppm for ALDEHYDES?

A

9.5-8.5 ppm

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19
Q

What is the typical ppm for AROMATICS?

A

8.5-6 ppm

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20
Q

What is the typical ppm for ALKENES?

A

6-4 ppm

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21
Q

What is the typical ppm for NEXT TO OXYGEN? (+ Cl and Br)

A

4-3 ppm

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22
Q

What is the typical ppm for NEXT TO NITROGEN?

A

3-2 ppm

23
Q

What is the typical ppm for NEXT TO C=C OR C=O? (and alkynes)

A

2.5-1.5 ppm

24
Q

What is the typical ppm for CH, CH2 AND CH3?

A

1.5-0.5 pp,

25
Q

What would a signal of 6.5 ppm for an othro-substituent suggest about the other substituents on the ring?

A

This value is lower than the value of benzene (7.3 ppm) therefore the hydrogen is more shielded than in benzene and so we would expect the substituents to be electron donating resulting in resonance and a negative charge on the carbon attached to the hydrogen (+M effect).

26
Q

Label these groups as either +M or -M:
- OMe
- Aldehyde
- NO2
- OH
- COOH
- NH2
- cyanide
- esters

A
  • OMe +M
  • Aldehyde -M
  • NO2 -M
  • OH +M
  • COOH -M
  • NH2 +M
  • cyanide -M
  • esters both +M and -M depending on which side is connected
27
Q

How are peaks different in pyrrole than in benzene?

A

The nitrogen lone pair is in the p-orbital and dissociated into the ring resulting in more shielding so the peaks are shifted upfield with the H next to N less upfield due to the -I effect

28
Q

How are peaks different in furan than in benzene?

A

The oxygen lone pair is in the p- orbital and dissociated into the ring resulting in more shielding so the peaks are shifted upfield, however the H next to O the same as benzene due to the -I effect

29
Q

How are peaks different in pyridine than in benzene?

A

The nitrogen lone pair is sp2 hybridised and doesnt dissociate into the ring resulting in no change in the peaks. The H next to N undergoes both the -M and -I effect resulting in a very deshielded and downfield peak

30
Q

What is the difference between a triplet and an apparent triplet and what is the actual splitting pattern of an apparent triplet?

A

Triplet - coupling to two equivalent protons, 1:2:1 intensity ratio
Apparent triplet - coupling to two different protons with identical J values by chance, 1:2:1 intensity ratio
Apparent triplets are actually double doublets (dd) and must be labelled as such in literature

31
Q

For double double doublets (ddd), what are the 8 different possibilities of splitting pattern and how many lines do they each have?

A

1) L, M, S - 8 lines
2) L, M, S - 8 lines
3) L, M, S - 7 lines
4) L, S, S - 6 lines
5) L, S, S - 6 lines
6) L, L, S - 6 lines
7) L, S, S - 5 lines
8) L, L, L - 4 lines

32
Q

For 8 line double double doublets (ddd), how is each J value measured?

A

1 to 2 = S, 1 to 3 = M, if 1 to 2 = 3 to 4 then:
1 to 5 = L
If not then:
1 to 4 = L

33
Q

For 7 line double double doublets (ddd), how is each J value measured?

A

1 to 2 = S, 1 to 3 = M, 1 to 4/5 = L

34
Q

For 6 line double double doublets (ddd), how is each J value measured?

A

1:2:1:1:2:1 - If 1 to 2/3 = 2/3 to 4 then: 1 to 5 = L
1:2:1:1:2:1 - If 1 to 2/3 is not = 2/3 to 4 then: 1 to 4 = L
1:1:2:2:1:1 - 1 to 2 = S, 1 to 3/4 = L

35
Q

For 5 line double double doublets (ddd), how is each J value measured?

A

1 to 2/3 AND 1 2/3 to 4/5 = S
1 to 4/5 = L

36
Q

For 4 line double double doublets (ddd), how is each J value measured?

A

1 to 2/3/4 = L

37
Q

What are the 2 factors that change 2 bond coupling constants?

A

1) adjacent electronegative atoms reduce 2J value
2) adjacent pi-systems (C=C, Ar, C=O) increase 2J value

38
Q

Why do terminal alkenes have very small 2J values?

A

The pi-system interrupts the ability for orbitals to overlap and so reduced coupling is observed (often no coupling at all 0Hz)

39
Q

Where is geminal coupling observed? (2J coupling)

A

In compounds with 1 chiral centre and in terminal alkenes

40
Q

What are the typical 2J coupling values for these compounds:
- Standard coupling
- adjacent to an electronegative atom
- adjacent to a pi-system
- terminal alkenes

A

12Hz
10.5Hz
16Hz
0-3Hz

41
Q

What do 3J coupling values depend on?

A

The dihedral angle of the hydrogens

42
Q

What is the average value of 3J coupling constant and, considering the structure of free-rotating molecules, why is this the value?

A

The value of the 3J coupling constant is dependent on the dihedral angle and for a free rotating system, that typically adopts staggered confirmations at 60, 120 and 180 degrees, the average of the 3J value is 7 Hz

43
Q

From highest to lowest 3J value, rank these different types of coupling and give an approximate value:
- equatorial-equatorial
- axial-axial
- axial equatorial
- cis alkenes
- trans alkenes

A
  • trans alkenes (14-16Hz)
  • axial-axial (9-12Hz)
  • cis alkenes (8-10Hz)
  • axial-equatorial AND equatorial-equatorial (2-6Hz)
44
Q

What 2 things can give rise to 4J coupling?

A

1) A pi-system (aromatic or double bond)
2) a “W” shape

45
Q

What is a diastereoisomer and what does it require a molecule to have?

A

Two molecules with the same chemical formula but differ in space and ARE super-imposable mirror images (not enantiomers). Requires at least 1 chiral centre

46
Q

How are diastereotopic protons distinguished between in a H NMR?

A

It isn’t possible to distinguish a pair of diastereotopic protons

47
Q

Why are OH signals (and to a lesser extent NH signals) unpredictable and how can they identified?

A
  • Varying chemical shifts
  • Sometimes broad, sometimes sharp
  • Sometimes show coupling
    A D2O shake
48
Q

How and why can OH (NH) coupling sometimes be observed?

A

A proton from the solvent can exchange with OH proton - slowing this exchange makes coupling observable
- Rigorously drying NMR solvent (CDCl3)
- Low concentration of compound in NMR sample
- Use a hydrogen bonding solvent (d6-DMSO)

49
Q

What is the Nuclear Overhauser Effect? (NOE)

A

Dipole-dipole interactions between saturated and observed nuclei. Only occurs for H close in space

50
Q

When can NOEs be used?

A

Useful in structural determination
- Alkenes
- Cyclic compounds where conformation is fixed
- Complex biomolecules (protiens)

51
Q

What does a 2D H-H COSY NMR show?

A

Protons that are coupled to each other

52
Q

What does a 2D H-C COSY NMR show?

A

Protons that are bonded to specific carbons

53
Q

What does a 2D H-H NOESY NMR show?

A

Protons that are close in space to each other

Core 6 (4 decks)

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