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Orgo. I > Spectroscopy > Flashcards

Spectroscopy Flashcards

1
Q

Visual difference between a quartet and a doublet of doublets

A

Doublet of doublets are all the same height

Quartets have split heights

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2
Q

Splitting effect of ethene’s trans H’s

A

Unusually large splitting patterns

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3
Q

Splitting effect of ethene’s cis H’s

A

“Normal” splitting patterns

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4
Q

Splitting effect of ethene’s geminal H’s

A

Unusually small splitting patterns

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5
Q

Why might a pair of H’s attached to the same C not be equivalent?

A

Two possible reasons:

1) Restricted rotation from the C being part of a pi bond or ring. This causes the H’s attached to that C to split each other

2) The C the two H’s are on is a stereocenter. Again this causes the H’s to split each other

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6
Q

A doublet of doublets’ 4 peaks are evenly spaced. What H’s are causing this splitting pattern?

A

Cis + trans

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7
Q

A doublet of doublets’ has 2 pairs of peaks spaced relatively far apart. What H’s are causing this splitting pattern?

A

Trans + gem

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8
Q

A doublet of doublets’ has 2 pairs of peaks spaced relatively close together. What H’s are causing this splitting pattern?

A

Cis + gem

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9
Q

Why is CDCl3 used in 13C and 1H NMR?

A

CDCl3 is a solvent that dissolves the reaction so it can be fed into the NMR machine

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10
Q

Does CDCl3 show up in 13C-NMR?

A

A bit, yest

It usually appears as a thicker, short triplet line at 77 ppm.

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11
Q

Does CDCl3 show up in 1H-NMR?

A

No.

CDCl3 is deuterated, hence the D instead of H. 1H-NMR cannot detect deuterated H’s, so it doesn’t show up.

However, in rare cases when it does, it presents as a peak at 7.2 ppm.

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12
Q

What does the D stand for in CDCl3?

A

Deuterium

CDCl3 is just deuterated CHCl3. It’s used b/c it can dissolve the reaction (necessary to run the NMR) but usually doesn’t show up on the spectrum (minimal effects on the data)

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13
Q

IHD (aka degrees of unsaturation) formula

A 
         2
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14
Q

What does IHD (aka degrees of unsaturation) tell us?

A

The sum of all pi bonds and/or rings are present in a molecule

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15
Q

Your molecule has an IHD of 4. What should you immediately think of?

A

Benzene.

Benzene’s ring counts as 1, and it’s 3 pi bonds count as 3, totalling it to 4

Also, it’s more common to see benzenes than alkenes. Welcome to organic chemistry.

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16
Q

What does a broad singlet indicate?

A

An acidic proton

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17
Q

Why do acidic protons appear as broad singlets on 1H-NMR spectrums?

A

The acidic proton isn’t attached to the molecule at all times, it’s busy jumping on and off in the molecule’s/rxn’s equilibrium. Because it’s only present part of the time it’s peak is short and squat. Also, it’s not present enough to either be split or to split anything else, so it always appears a singlet regardless of whether it has neighbors or not

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18
Q

Your molecule has an IDH (aka degrees of unsaturation) of 4 or more. Which is more likely, benzene+friends or only alkene (pi bonds)?

A

Benzene+friends

Benzene’s are more common than alkenes in orgo. It’s highly unlikely that an IHD of > 4 will be due to only alkenes and not a benzene + additional pi bonds

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19
Q

What are the 3 benzene substituent patterns? (for 2 substituents only…)

A

Ortho
Meta
Para

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20
Q

Benzene substituent (2) pattern:
Ortho

A

1’ X
2’ Y

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21
Q

Benzene substituent (2) pattern:
Meta

A

1’ X
3’ Y

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22
Q

Benzene substituent (2) pattern:
Para

A

1’ X
4’ Y

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23
Q

Do the 4 H’s on ortho, meta, and para benzenes have the same splitting patterns?

A

No

The different benzene substituent placements will differentially affect H splitting patterns

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24
Q

Ortho H splitting patterns

A

1’ X
2’ Y
3’ doublet
4’ triplet
5’ triplet
6’ doublet

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25
Meta H splitting patterns
1' X 2' singlet 3' Y 4' doublet 5' triplet 6' doublet
26
Para H splitting patterns
1' X 2' doublet 3' doublet 4' Y 5' doublet 6' doublet
27
Your 1H-NMR has 3 peaks with an integration of 1, 1, and 3. How many H's are present in your molecule?
Can't be determined from integration. Integration is a ratio, not an exact count. The molecule could have 5 H's, 10, 15, etc. Look for the starting material (formula, initial structure) to accurately determine how many H's are present.
28
Where (in a molecule) do doublets of doublets occur?
In rings, alkenes, or in/near stereocenters. Places with no free rotation.
29
You know from your given molecular formula that your molecule has 4 carbons. Your 13C-NMR has 4 distinct peaks (5 including CDCl3's triplet). What does this tell you about the molecule's structure?
Each carbon is unique, hence the 4 peaks. There is no symmetry in the molecule
30
Why do we use 13C for NMRs and not 12C?
Because the atom for NMR must have a non-zero spin number, meaning that the isotope must have unequal numbers of protons and neutrons 12C has 6 protons and 6 neutrons, so its spin number is 0. NMR just doesn't work for isotopes with spin numbers of 0
31
What is the most common type of coupling observed in 1H-NMR?
Vicinal coupling (J3), aka 3-bond coupling
32
What is spectroscopy?
The study of the interactions of matter and electromagnetic radiation (light)
33
What is IR (infra-red) spectroscopy used for?
To determine which functional groups are present in a molecule, rule them in or out Nothing else can be determined from IR
34
In IR spectroscopy, what is % transmittance?
The percent amount of the wavelength that reaches the detector
35
In IR spectroscopy, what is % absorbance?
The percent amount of the wavelength that is absorbed by the sample
36
What happens when a molecule is irradiated with a specific wavelength of infrared light?
The molecule's chemical bonds stretch and bend in distinct patterns, creating vibrational motions. The absorption of the light (% that doesn't reach the detector) generates the data for the IR spectrum
37
What do the peaks of an IR spectrum correspond to?
Each peak corresponds to a functional group that absorbs IR light of that wave number
38
If you have the empirical formula of a molecule, but not it's structure. You know your molecule contains an O. Why would you run an IR spectroscopy test on your sample?
To determine what specific kind of O your molecule has. It could be a hydroxyl, ether, carbonyl, etc.
39
Many compounds contain carbonyl groups. How do you use IR to distinguish between the different carbonyl-containing compounds?
You have to look for additional characteristics besides the C=O stretch
40
Should you interpret every feature of an NMR?
YES This is necessary to deduce the exact structure
41
What do 1H-NMR spectrums tell us?
The precise chemical environment of every proton (aka every H atom) in the sample molecule This information can then be used to determine the molecule's precise structure
42
What do the number of signals (peaks) of an 1H-NMR spectrum tell us?
Each peak corresponds to a unique H in the molecule This is NOT how many total H's are present in the molecule, just how many unique/equivalent H's are present
43
In NMR, what is chemical shift?
Chemical shift is the placement of the signal on the x-axis
44
What does a peak's chemical shift tell us?
Gives more information about the unique proton(s)' chemical environment Peaks further downfield (left) indicate the proton(s) are closer to some electronegative element Peaks further upfield (right) indicate the proton(s) are father away from some electronegative element
45
A group of equivalent H's are very close to an electronegative element. Where on the x-axis will their NMR signal be?
Farther downfield, to the left
46
A group of equivalent H's are very far away from an electronegative element. Where on the x-axis will their NMR signal be?
Further upfield, to the right
47
Why do protons close to electronegative elements generate peaks farther downfield (to the left)?
The electronegative atom has a deshielding effect on the proton, causing the proton's electron density to be drawn away from its nucleus. This pushes the signal downfield.
48
Why do protons further away from electronegative elements generate peaks further upfield (to the right)?
As the protons are far away from the electronegative elements, they don't suffer any deshielding effects and their electron densities are normal. Their signals don't stray, they stay where they're supposed to be
49
In NMR, what does signal area integration tell us?
The ratio of how many chemically equivalent protons generate each peak An peak integration of 2 indicates that 2, 4, 6, 8, etc equivalent H's make up the signal's group
50
In NMR, what is signal splitting?
Splitting is caused by neighboring H's, appears on the spectrum as n+1 lines making up the signal's peak
51
If a group of equivalent H's has n neighbors, how many lines will make up the group's signal?
n+1
52
What is TMS and why is it used in NMR?
TMS (tetramethylsilane) is the reference peak for NMR spectra. It places the x-axis scale at 0
53
How to determine the number of unqiue H's in a molecule
Symmetrical H's are equivalent Usually H's on the same C are equivalent
54
In what scenarios will H's on the same C NOT be equivalent?
When they have no free rotation due to the C being in either a ring or a pi bond When the H's are attached to a stereocenter
55
When solving a spectroscopy problem, what's the first thing you should do?
Determine the IHD (degrees of unsaturation)
56
What do the peaks of a 13C-NMR spectrum tell us?
How many unique/equivalent C's are present in the molecule (NOT how many total C's are present!)
57
What does IR spectroscopy probe?
Vibrational transitions
58
What does NMR probe?
Nuclear spin transitions
59
When solving an NMR problem, should your final product have any formal charges?
Hopefully no Try to avoid them at all possible, if you can
60
What 3 pieces of information does NMR give us?
Chemical shift Coupling/splitting patterns Signal integration
61
What is the fingerprint region of an IR spectrum?
The region between 1000 nm and 0 nm Every organic molecule will show some sort of absorbance in this region. As such not much can be discerned from this region
62

Orgo. I (14 decks)

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