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Organic Chemistry > H-NMR Spectroscopy > Flashcards

H-NMR Spectroscopy Flashcards

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1
Q

Standard chemical shift for CH3

A

0.9 ppm

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2
Q

Standard chemical shift for CH2

A

1.2 ppm

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3
Q

Standard chemical shift for CH

A

1.7 ppm

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4
Q

Chemical shift for an acid proton (COO-H)

A

about 12 ppm

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5
Q

Chemical shift for an aldehyde proton

A

about 10 ppm

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6
Q

Chemical shift for aromatic protons

A

6.5 - 8.5 ppm

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7
Q

Chemical shift for alkene protons

A

5.5 - 6.5 ppm

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8
Q

Chemical shift for saturated alkanes next to O (bonded to the O in esters, alcohols, or ethers).

A

3 - 4.5 ppm

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9
Q

Chemical shift for saturated alkanes not next to O (bonded to aryl, allyl, halogen, N, or carbonyl)

A

2 - 3 ppm

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10
Q

What is the effect of an alcohol or an ether O on the chemical shift of alpha protons?

A

+2 ppm

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11
Q

What is the effect on chemical shift of an alkene adjacent to the alkyl group containing the protons?

A

+1 ppm

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12
Q

What is the chemical shift range for protons on acids and aldehydes?

A

9 - 13 ppm

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13
Q

What does the location of a signal on a proton NMR spectrum tell us?

A

How shielded or deshielded a proton is.

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14
Q

What does splitting on a proton NMR signal tell us?

A

The number of protons on adjacent carbons.

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15
Q

When are two hydrogens on the same carbon not equivalent?

A

1) When they are in different electronic environments due to cis/trans positioning on an alkene.
2) When they are on in different electronic environments due to being axial/equatorial to a heteroatom or functional group on a substituted ring system.
3) When replacing one of the protons on an sp3 carbon with another molecule would make the molecule a diastereomer (i.e., when there is already another chiral center on the ring).

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16
Q

How does the electronegativity of an adjacent heteroatom affect the chemical shift of an alpha proton?

A

The more electronegative the heteroatom, the more deshielded the proton(s) and the greater the chemical shift.

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17
Q

What is the chemical shift for H3C–I?

A

2.1 ppm

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18
Q

What is the chemical shift for H3C–Br?

A

2.7 ppm

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19
Q

What is the chemical shift for H3C–Cl?

A

3.0 ppm

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20
Q

What is the chemical shift for H3C–F?

A

4.3 ppm

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21
Q

What is the chemical shift for H3C–NH2?

A

2.4 ppm

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22
Q

Which functional groups are worth about a 1 ppm downfield shift for adjacent carbon protons?

A

1) Alkenes
2) Alkynes
3) Carbonyls (acids, aldehydes, ketones, esters, and amides)
4) Nitriles (CN)
5) Sulfurs (SH and SR)

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23
Q

Which functional groups are worth about a 2 ppm downfield shift for adjacent carbon protons?

A

1) Oxygens (OH and OR)
2) Halogens
3) Amides
4) Sulfone

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24
Q

Which functional groups are worth about a 1.5 ppm downfield shift for adjacent carbon protons?

A

1) Aryl rings
2) Amines
3) Sulfoxide

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25
Q

Which functional groups are worth about a 2.5 ppm downfield shift for adjacent carbon protons?

A

1) Aryl ether (-OAr)

26
Q

Which functional groups are worth about a 3 ppm downfield shift for adjacent carbon protons?

A

1) Nitro groups (-NO2)
2) esters (-OCOR)
3) Fluoro (-F)

27
Q

What is it called when the electron density of an aromatic ring actually increases the magnetic field felt by protons on the ring?

A

Anisotropy

28
Q

What is the effect of anisotropy on the chemical shift of protons on an aromatic ring?

A

It increases chemical shift.

29
Q

What is the effect of EWG’s on the ring on the chemical shift of aromatic protons?

A

The chemical shift will be increased, especially in the ortho and para positions (observe a chemical shift greater than benzene).

30
Q

What is the effect of EDG’s on the ring on the chemical shift of aromatic protons?

A

The chemical shift will be decreased, especially in the ortho and para positions (observe a chemical shift less than benzene).

31
Q

What is the chemical shift of aromatic protons in benzene?

A

7.27 ppm

32
Q

What is the chemical shift range of alkene protons?

A

5.5 to 6.5 ppm

33
Q

What is the chemical shift range of alkyne protons?

A

About 2 ppm

34
Q

Why is the chemical shift of alkynes so much lower than for alkenes?

A

The pi system of alkenes enhances the magnetic field felt by the protons; the pi system of alkynes reduces the magnetic field felt by the protons (due to pi orbitals lining up perpendicular to the field).

35
Q

What is the chemical shift range for aldehyde protons?

A

9.0 - 10.0 ppm

36
Q

What is the chemical shift range for acid protons?

A

about 12 ppm

37
Q

Which signal is often very broad due to proton exchange between molecules?

A

Acid signals

38
Q

Why can protons attached to O and N be in a variety of chemical environments (and, therefore, their chemical shifts vary broadly)?

A

Because of proton exchange due to hydrogen bonding. Can broaden peaks, making them singlets.

39
Q

What is the chemical shift range for protons attached to oxygen?

A

2 - 5 ppm

40
Q

What is the chemical shift range for protons attached to nitrogen?

A

1.2 - 4 ppm

41
Q

Why is the signal for a proton with one neighboring hydrogen split into two peaks?

A

If the spin of the neighboring proton is aligned with the magnetic field, the chemical shift is increased; if it is aligned against the magnetic field, the proton’s chemical shit is decreased. Because their is a 50/50 chance that the spin of the neighboring proton is with (or against) the magnetic field, the proton’s signal is split into two peaks, one just above its chemical shift and one just below.

42
Q

If a proton’s peak is split by N equivalent protons, how many peaks is its signal split into?

A

N+1 peaks

43
Q

What should you suspect if you see a tertiary C-H peak with seven peaks?

A

An isopropyl group

44
Q

Which type of protons bonded to the same carbon will split each other?

A

Diastereotopic protons

45
Q

What will be true of the J value (or coupling constant) of protons that are coupled to each other?

A

The J value will be the same for both protons; their peaks will be split by the same distance.

46
Q

What part of an H-NMR spectrum provides a good method for determining cis and trans protons?

A

Coupling constants

47
Q

Why do trans hydrogens cause a greater coupling constant in each other than cis hydrogens?

A

Coupling is a through-bond effect, and trans hydrogens have good overlap between the C-H sigma bonding orbital of one hydrogen and the C-H sigma anti-bonding orbital of the other hydrogen. Cis hydrogens have poor overlap.

48
Q

What is the typical coupling constant range for trans hydrogens?

A

15-18 Hz

49
Q

What is the typical coupling constant range for cis hydrogens?

A

8-12 Hz

50
Q

What happens if a proton’s signal is split by more than one set of hydrogens?

A

You’ll get complex coupling. The largest coupling constant will split the signal into N+1 peaks. The next larges will split each peak from the first split into another set of N+1 peaks.

51
Q

What sort of coupling will a 1,4 di-substituted aromatic ring have?

A

A pair of doublets that lean (not perfectly 1:1 in height).

52
Q

At what chemical shift are benzene protons observed?

A

7.2 ppm

53
Q

If you have a pair of leaning doublets, both just below 7.2 ppm?

A

An aromatic ring with two EDG’s.

54
Q

What sort of signal will you have for an aromatic ring with the same group 1,4-disubstituted on the ring?

A

A singlet.

55
Q

If a proton is split by 1 other proton, how many peaks will its signal have? What will the ratio of their heights be?

A

2 peaks in a 1:1 ratio

56
Q

If a proton is split by 2 equivalent protons, how many peaks will its signal have? What will the ratio of their heights be?

A

3 peaks in a 1:2:1 ratio

57
Q

If a proton is split by 3 equivalent protons, how many peaks will its signal have? What will the ratio of their heights be?

A

4 peaks in a 1:3:3:1 ratio

58
Q

If a proton is split by 4 equivalent protons, how many peaks will its signal have? What will the ratio of their heights be?

A

5 peaks in a 1:4:6:4:1 ratio

59
Q

If a proton is split by 5 equivalent protons, how many peaks will its signal have? What will the ratio of their heights be?

A

6 peaks in a 1:5:10:10:5:1 ratio

60
Q

If a proton is split by 6 equivalent protons, how many peaks will its signal have? What will the ratio of their heights be?

A

7 peaks in a 1:6:15:20:15:6:1 ratio

61
Q

What functional group should you suspect if you see a septet slightly higher than a doublet in the alkyl hydrogen range?

A

an isopropyl group

62
Q

What functional group should you suspect if you see a quartet slightly higher than a triplet in the alkyl hydrogen range?

A

an ethyl group

Organic Chemistry (7 decks)

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