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Lower Sixth Chemistry > NMR 7 & 19 test > Flashcards

NMR 7 & 19 test Flashcards

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1
Q

What happens when a molecule absorbs infrared radiation?

A

It makes the covalent bond vibrate more in a stretching or bending motion

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2
Q

What factors affect the amount of vibration of a bond?

A

● Bond strength
● Bond length
● Mass of each atom in the bond

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3
Q

How does infrared spectroscopy work?

A
  • Every bond has a unique vibration frequency in the infrared region of the EM spectrum
  • Bonds absorb radiation that has the same frequency as their frequency of vibration
  • Infrared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compound’s functional group
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4
Q

What do the peaks on an infrared spectrum represent?

A

Absorbance of energy from the infrared radiation

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5
Q

What are the 2 peaks that must be present to identify a substance as carboxylic acid?

A

● Very broad peak at 2500 - 3300 cm -1 → O-H group
● Sharp peak at 1680 - 1750 cm -1 → C=O group

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6
Q

What happens inside a mass spectrometer?

A

● Organic compound is vaporised and passed through the spectrometer
● Some molecules lose an electron and forms molecular ions
● Excess energy from the ionisation makes the bonds vibrate more
● Vibration causes bond to weaken
● Molecular ion splits by fragmentation

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7
Q

What is the symbol of molecular ion?

A

M +

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8
Q

Is fragmentation predictable?
Why?

A

No, because it can happen anywhere in the molecule

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9
Q

In a mass spectrometry how is a molecular ion represented?

A

It is the peak with the highest mass/charge ratio

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10
Q

The molecular mass of the molecular ions is equal to what?

A

Relative molecular mass of the compound

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11
Q

Will the molecular ion peaks of two isomers of the same compound be same or different?

A

Same

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12
Q

What is the m/z value of CH 3 + ?

A

15

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13
Q

What is the m/z value of OH - from alcohol?

A

17

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14
Q

What is the m/z value of C 2 H 5 + ?

A

29

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15
Q

What is the m/z value of C 3 H 7 + ?

A

43

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16
Q

What is the m/z value of C 4 H 9 + ?

A

57

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17
Q

What are the advantages of using mass spectrometry? (2)

A

● Cheap
● Small quantities of samples required

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18
Q

What is the main disadvantage of using mass spectrometry?

A

The sample is completely destroyed

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19
Q

How does infrared spectroscopy work?

A
  • Every bond has a unique vibration frequency in the infrared region of the EM spectrum
  • Bonds absorb radiation that has the same frequency as their frequency of vibration
  • Infrared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compound’s functional group
20
Q

What do the troughs on an infrared spectrum show?

A

The frequencies where radiation has been absorbed - match to table to find out which bonds they represent

21
Q

What is the fingerprint region?

A
  • Area of the infrared spectrum below wavenumber of 1500cm -1
  • Many peaks, caused by complex vibrations of the whole molecule. Unique to every compound, so can be used to identify compounds
22
Q

How is the fingerprint region used to identify compounds?

A

The fingerprint region’s pattern is matched to a database on a computer to identify the compound accurately

23
Q

What functional group/(s) can be identified in this spectra?

A

O-H group: Alcohol

24
Q

What functional group/(s) can be identified in this spectra?

A

C=O group: Carbonyl

25
Q

What functional group/(s) can be identified in this spectra?

A

O-H group (of carboxylic acid)
C=O group
Therefore this substance must be a carboxylic acid

26
Q

What functional group/(s) can be identified in this spectra?

A

N-H group : Amine

27
Q

What does NMR stand for?

A

Nuclear Magnetic Resonance

28
Q

What are the basic principles of NMR?

A

You can find the structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them. If radio waves of the right frequency are
absorbed, the nuclei flips from parallel to applied magnetic to field to anti-parallel. This energy change can be monitored and recorded. Uses the resonance of nuclei with spin.

29
Q

How would you carry out NMR spectroscopy?

A

Dissolve the liquid sample in suitable solvent, put in a tube along with a small amount of TMS and put the tube into an NMR machine. The sample is spun to even out any imperfections in the magnetic field and the spectrometer is zeroed against the TMS. Radiation with different radio frequencies but a constant magnetic field is applied to the sample and any absorptions (due to resonance) are detected.

30
Q

Give one use of NMR?

A

MRI scans

31
Q

What kind of nuclei does NMR work with (and examples)?

A

Those with an uneven number of nucleons,
meaning they will spin e.g. 1H, 13C

32
Q

What percentage of carbon atoms are 13C?

A

1% - but modern instruments are sensitive
enough to detect this

33
Q

What defines the resonant frequency of a 13
C atom?

A

The chemical environment that it is in; the
amount of electron shielding it has.

34
Q

What graph is produced by NMR spectroscopy ?

A

Energy absorbed against chemical shift

35
Q

What is chemical shift? What is its symbol? What are its units?

A

The resonant frequency of the nuclei, compared to that of a 1H atom in TMS.
Symbol δ
Parts per million (ppm)

36
Q

What is the range of chemical shift for 13C NMR?

A

0-200ppm

37
Q

What means 13C atoms show a different chemical shift value?

A

Having different chemical environments (but
equivalent atoms show the same peak)

38
Q

What kind of environment leads to a greater chemical shift?

A

A C atom next to more electronegative atom has a greater chemical shift.

39
Q

Summarise what these
mean for 13 C NMR:

Number of signals?
Chemical shift?
Area under peak?
Splitting?

A

Number of signals: One signal for each carbon environment (each set of inequivalent 13 C atoms)

Chemical shift: Greater from atoms closer to
electronegative atoms or C=C

Area under peak: no meaning

Splitting: there is no splitting for 13 C NMR

40
Q

Why is it easier to get a spectrum of 1
H NMR than 13C NMR?

A

Most H atoms are 1H- it is much more abundant than 13C. This means almost all H atoms have spin so show up

41
Q

What is the range of chemical shift for 1H NMR?

A

0-10ppm

42
Q

What leads to a lower chemical shift value for H NMR?

A

1H with more electrons around them i.e. further from electronegative groups/atoms

43
Q

On a low resolution spectrum, what peaks would you expect to see for H NMR?

A

One peak for each set of inequivalent H atoms
(each chemical environment shows 1 peak)

44
Q

What does the area under the peak represent (for H NMR)?

A

The area under the peak is proportional to the
number of 1H atoms represented by the peak

45
Q

What is the integration trace?

A

A stepped line that makes it easier to measure
the area under the curve (height of line = area
under that peak)

46
Q

What is TMS (name and structure)?

A

Tetramethylsilane
One silicone with 4 methyls around it

47
Q
A

Lower Sixth Chemistry (14 decks)

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