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A-Level Organic Chemistry AQA (Paper 2) > Structure Determination > Flashcards

Structure Determination Flashcards

1
Q

What does NMR stand for?

A

•Nuclear magnetic resonance spectrometry.

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2
Q

What is NMR used for in organic chemistry?

A

•A technique to find structures of complex molecules.

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3
Q

How does NMR work? (3)

A
  • A magnetic field is applied to a sample, which is surrounded by a source of radio waves and a radio receiver.
  • Generates an energy change in the nuclei of atoms in the sample that can be detected.
  • Electromagnetic energy is emitted, which can then be interpreted by a computer.
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4
Q

What happens when a nucleus with a spin is placed in a magnetic field? (4)

A
  • The small magnetic field generated by the nuclear spin can be aligned with or against the main magnetic field.
  • There is a small difference in energy between these two alignments that corresponds to the energy of radiowaves.
  • If radio waves are passed through the substance, some frequencies of radiowaves will be absorbed to flip the nucleus from one spin direction to the other.
  • Resulting spectrum gives valuable information about the compound.
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5
Q

Why is carbon-13 used for NMR instead of carbon-12? (2)

A
  • 12C has no nuclear spin, 13C does have one.

* Only 1% of carbon atoms are carbon-13, modern instruments are sensitive enough to obtain a carbon-13 spectrum.

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6
Q

Why don’t all carbon-13 atoms in a molecule resonate at exactly the same magnetic field strength? (3)

A
  • Carbon atoms in different functional groups feel the magnetic field differently.
  • All nuclei are shielded from the external magnetic field by the electrons that surround them.
  • Nuclei with more electrons around them are better shielded.
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7
Q

The greater the electron density around a carbon-13 atom… (3)

A
  • The smaller the magnetic field felt by the nucleus and the lower the frequency at which it resonates.
  • Less electronegative groups attached, more shielding.
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8
Q

What does the NMR instrument produce?

A

•A graph of energy absorbed (from radio signal) vertically against a quantity called chemical shift (𝛿) (related to resonant frequency) horizontally.

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9
Q

What is chemical shift (𝛿) measured in? (3)

A
  • Parts per million (ppm), from a defined zero related tetramethylsilane.
  • CNMR values of 𝛿 range from 0 to 200 ppm.
  • HNMR values of 𝛿 range from 0 to 10 ppm.
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10
Q

What is chemical shift related to? (2)

A
  • The difference in frequency between the resonating nucleus and that of TMS (tetramethylsilane/Si(CH3)4).
  • The chemical shift of hydrogen and carbon atoms in the compound.
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11
Q

What are MRI (magnetic resonating imaging) machines?

A

•A type of NMR machine used in medicine.

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12
Q

What does NMR tell us?

A

•A nucleus is surrounded by different bonds and atoms (different chemical environments).

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13
Q

What are equivalent nuclei?

A

•Nuclei in the same environment.

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14
Q

Carbon atoms in different environments… (3)

A
  • Will give different environments e.g. ethanol has two peaks, one for each carbon.
  • The carbon atoms are in different environments - one is further from the oxygen atom than the other.
  • The oxygen atom, being electronegative, draw electrons away from the carbon atom to which it is directly bonded.
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15
Q

What does NMR not tell you?

A

•How many carbons are present in a molecule.

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16
Q

The more electronegative groups attached to carbon… (2)

A
  • The higher the chemical shift - the electronegative atom (NOF) draws electrons away from the carbon.
  • The carbon is deshielded and feels a greater magnetic field and so resonates at a higher frequency and has a greater 𝛿 value.
  • Decrease in electron density.
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17
Q

How do you work out the number of chemical environments? (2)

A
  • Look for symmetry.

* Look at each carbon in each direction and the groups and side chains bonded.

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18
Q

Why is tetramethylsilane used to calibrate the spectrum? (5)

A
  • Its signal is away to the right from all the others - doesn’t cause confusion with organic compounds.
  • Only gives one signal - carbons are all the same.
  • It is non-toxic - it’s edible.
  • It is inert - doesn’t react.
  • Has a low boiling point (26°C), so it is easily removed and so can move on to the next compound.
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19
Q

What is proton NMR?

A

•1H nucleus is being examined.

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20
Q

Why is it easier to get an NMR spectrum for 1H than for 13C?

A

•Nearly all hydrogen atoms are 1H.

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21
Q

How does HNMR work? (2)

A
  • Hydrogen atoms attached to different functional groups feel the magnetic field differently because all nuclei are shielded from the external magnetic field by the electrons that surround them.
  • Nuclei with more electrons are better shielded.
22
Q

The greater the electron density around a hydrogen atom…

A

•The smaller the chemical shift.

23
Q

What do the values of chemical shift for HNMR range between?

A

•0 and 10 ppm.

24
Q

If all the hydrogen nuclei in an organic compound are in identical environments…

A

•You only get one chemical shift value e.g. CH4.

25
Q

The further away a hydrogen atom is from an electronegative atom (NOF)…

A

•The smaller its chemical shift e.g. in CH3CH2OH (ethanol), there are three chemical values of chemical shift.

26
Q

In 1HNMR, the areas under the peaks are proportional to…

A

•The number of hydrogen atoms of each type.

27
Q

In the spectrum of methanol (CH3OH) for 1HNMR, what are the peaks like?

A

•The CH3 peak is three times the area of the OH peak (3:1 ratio).

28
Q

What does the chemical shift value at which the peak representing each type of proton appears, tell you about its…

A

•Environment - the type of functional group of which it is a part, hydrogen atoms in any functional group have a particular chemical shift value.

29
Q

When is TMS added for NMR spectroscopy?

A

•Before their spectra are run, and gives a peak at a chemical shift value of exactly 0 ppm, 𝛿 = 0, to calibrate the spectrum.

30
Q

What are integer traces?

A

•Areas under peaks that tell you relative intensities of hydrogen (how many hydrogen atoms, but not the actual number).

31
Q

What is spin-coupling?

A

•When you zoom in on most NMR peaks and they are split into particular patterns.

32
Q

Why does spin-coupling occur in 1HNMR? (2)

A
  • The applied magnetic field felt by any hydrogen atom is affected by the magnetic field of the hydrogen atoms on the neighbouring carbon atoms.
  • This spin-spin splitting gives information about the neighbouring hydrogen atoms, used to work out the structure of the molecule.
33
Q

What does 1HNMR tell us? (4)

A
  • How many types of 1H atoms are present (equivalent carbons).
  • Information about the groups containing 1H atoms (chemical shift).
  • What the 1H atoms are next to (spin coupling).
  • How many 1H atoms are present (relative intensities).
34
Q

What is the n+1 rule for 1HNMR? (2)

A
  • n Hydrogens on an adjacent carbon atom will split a peak into n + 1 smaller peaks (high resolution NMR).
  • The number of adjacent hydrogens split the peak into one more peaks than there are adjacent hydrogens.
35
Q

If there is one hydrogen atom on an adjacent carbon…

A

•It will split the NMR signal of a particular hydrogen into two peaks of each of the same height (doublet)

36
Q

If there are two hydrogen atoms on an adjacent carbon…

A

•It will split the NMR signal of a particular hydrogen into three peaks with the height ratio 1:2:1 (triplet).

37
Q

If there are three hydrogen atoms on an adjacent carbon…

A

•It will split the NMR signal of a particular hydrogen into four peaks with the height ratio 1:3:3:1 (quartet).

38
Q

What does high resolution NMR tell us about? (2)

A
  • Protons on adjacent carbons to the nuclei being looked at.
  • Effect of spin-coupling is to split a single peak into a cluster of peaks, each hydrogen behaves like a magnet.
39
Q

When does no spin-coupling occur? (4)

A
  • With Hs on oxygen e.g. -OH, C=O and Hs on nitrogen.
  • They have no impact on other groups.
  • Produces a single peak (singlet).
  • When two identical chemical environments are next to each other (equivalent hydrogens).
40
Q

What are equivalent atoms?

A

•Atoms in the same chemical environment.

41
Q

What is 1HNMR spectra usually run in? (2)

A
  • Solution - solvent must not contain any hydrogen atoms - the signal from the hydrogen atoms in the solution would affect the spectrum (polar solvent)
  • The signals from the hydrogen in the sample would be swamped because there are more hydrogens in solution.
42
Q

What do the 1HNMR solvents usually contain?

A

•Deuterium (D or 2H, an isotope of hydrogen) atoms instead of 1H atoms as S atoms have no nuclear spin.

43
Q

What are the common solvents often used for 1HNMR? (4)

A
  • Tetrachloromethane, CCl4.
  • Deuterotrichloromethane, CDCl3.
  • Deuterium oxide, D2O.
  • Perdeuterobenzene, C6D6.
44
Q

In HNMR, if there is a triplet and quartet with the relative intensity 2:3, what could this be?

A

•Ethyl group/CH2CH3, triplet and quartet don’t have to be next to each other in the spectrum, atom joined to CH2 must have any hydrogens unless they don’t couple e.g. OH or an equivalent CH2.

45
Q

In HNMR, if there is a triplet and a triplet with the relative intensity 2:2, what could this be?

A

•-CH2-CH2-, triplet and triplet don’t need to be next to each other in the spectrum, the two CH2 groups must be inequivalent and the side atoms joined must not have any hydrogens on them.

46
Q

In HNMR, if there is a doublet and a quartet with the relative intensity 3:1, what could this be?

A

•-CH3-CH-, doublet and quartet don’t need to be next to each other in the spectrum and atoms joined to CH cannot have any hydrogens or is not coupled e.g. OH.

47
Q

In HNMR, if there is a singlet with the relative intensity 3, what could this be?

A

•Methyl group/CH3-, the atom joined cannot have any hydrogens on it.

48
Q

In HNMR, if there is a singlet with the relative intensity 1, what could this be?

A

•Hydroxy/-OH, for an alcohol or carboxylic acid - hydrogen doesn’t couple to any other atoms.

49
Q

When the integration ratios aren’t 2, 3, 9, 1 and numbers like 4 and 6, what could this suggest?

A

•Equivalent hydrogens (symmetry) e.g. (CH2CH3)2 could be 4:6.

50
Q

What is the molecular formula of tetramethylsilane?

A

•C4H12Si.

A-Level Organic Chemistry AQA (Paper 2) (17 decks)

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