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Chemistry Y13 > Chromatography and NMR > Flashcards

Chromatography and NMR Flashcards

1
Q

what is chromatography?

A

is a technique used for separating the components of a mixture on the basis of difference in their affinities for a stationary and for a moving phase

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2
Q

why is chromatography used?

A

provides and important method of separating and identifying components in a mixture

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3
Q

in chromatography, there is always a fixed..?

A

stationary phase through which passes a moving phase containing the mixture to be separated

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4
Q

What does separation in chromatography depends on?

A

the balance between solubility in the moving phase and retention in the stationary phase

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5
Q

Chromatography can either be..?

A

Analytical or Preparative

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6
Q

what is analytical chromatography?

A

Analytical chromatography involves the analysis of small amounts of material

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7
Q

what does analytic chromatography try to identify?

A

tries to identify and measure the relative amounts of various components present in the mixture

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8
Q

How is the resulting chromatogram compared?

A

the resulting chromatogram is compared with a known ‘standards; (chromatograms of specific substances of a ,known concentration

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9
Q

What is Analytical chromatography used in?

A

used routinely in sport drug testing

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10
Q

What is preparative chromatography?

A

is a larger scale process used to separate a mixture of products at the end of a chemical reaction

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11
Q

Chromatographic separation falls into two categories, what are the two categories?

A
  • Partition chromatography and adsorption chromatography
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12
Q

What is partition chromatography?

A
  • separation depends on the balance between a solutes solubility in the moving phase and retention in the stationary phase
  • Stationary phase - Thin, non-volatile liquid film on the surface of an inner solid or fibrous matrix (e.g paper - cellulose fibres)
  • moving phase - liquid or gas
  • solute molecules equilibrate (partition between 2 phases)
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13
Q

what is Adsorption chromatography?

A
  • strongly adsorbed molecules travel more slowly in the moving phase than those that are only weakly adsorbed
  • stationary phase - solid (e.g alumina- aluminium oxide, silica - silicon dioxide)
  • moving phase - liquid or gas
  • separation occurs when solute molecules become attached to adsorption sites on the solid
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14
Q

What are different chromatographic techniques?

A
  • paper chromatography (Similar to TLC)
  • thin-layer chromatography (TLC)
  • column chromatography (CG)
  • Gas chromatography (GC)
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15
Q

what is Paper Chromatography?

A

Used in schools to separate the different dyes in an ink solution or the different food additives (colours) in sweets.

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16
Q

What is a chromatogram?

A

a chromatogram is a pattern of separated substances obtained by chromatography

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17
Q

How can the number of substances be identified?

A

the number of substances present in a mixture is given by the number of spots on the chromatogram

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18
Q

How can compounds be identified?

A

Using their Rf values (under standard conditions)

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19
Q

How are mixture patterns compared with?

A

Known standards on the same chromatogram

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20
Q

How can Rf values be calculated?

A

Distance moved by substance/ distance moved by solvent front

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21
Q

Why is the Rf value different for the same substances?

A

The Rf value for a substance is different in different solvents

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22
Q

When is complete separation only possible?

A

sometimes complete separation is only possible by rotating the paper through 90°, changing the solvent and repeating the process (2D- paper chromatography)

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23
Q

What is Thin-layer Chromatography?

A

a plate is coated with a thin layer of a solid and a solvent moves up the plate

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24
Q

when is TLC used?

A

used routinely in laboratories by bench-top scientists -

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25
Q

Explain how TLC differs from Paper chromatography?

A

similar technique to paper chromatography but the stationary adsorbent phase is usually a thin layer of silica or alumina coated on an aluminium sheet (plate). And an organic solvent e.g ethanol is usually used (moving phase)

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26
Q

In TLC, the mixtures are spotted on…?

A

a pencil baseline and the resulting chromatogram can be used to identify substances using Rf values

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27
Q

TLC chromatograms are usually viewed under..? which reveals..?

A

Ultra-violet light and colourless chemicals are revealed using a UV lamp

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28
Q

TLC is used to monitor?

A

the course of organic reactions

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29
Q

What is Column Chromatography ?

A

in CC, a column is packed with a solid and a solvent moves down the column

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30
Q

when is this used?

A

used routinely by top bench scientists

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31
Q

Explain how Column chromatography is carried out?

A
  • the stationary adsorbent phase is usually finely-divided silica gel or alumina house in a vertical glass tube (column)
  • the moving phase (usually the organic solvent) is called the eluent
  • the mixture solution (compounds dissolved in an organic solvent) is added to the top of the column followed by enough fresh solvent to wash the components down the column
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32
Q

which substances take the longest time to flow through the column?

A

the most strongly adsorbed substances take the longest time to flow through the column (longer retention time)

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33
Q

what is the retention time?

A

the retention time is the time each component remains in the column

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34
Q

the more polar a substance, the..?

A

the more polar the molecule in the mixture, the greater the retention time

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35
Q

How is the solution collected when it exits the column?

A
  • as the solution exits the column it is normally collected as a series of fractions in small beakers
  • the fractions that you want (e.g the main product) can then be processes and analysed further
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36
Q

How do you get the compound that you want?

A

to get the compunds that you want you need to evaporate the solvent and recrystalise

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37
Q

what is column chromatography a good method for?

A

column chromatography is a great method for separating and purifying a mixture of organic products after a chemical reaction (preparative chromatography)

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38
Q

How can the process be speeded up?

A

by forcing the solvent through the system under pressure (flash chromatography)

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39
Q

what does separation by column chromatography depend on?

A

depends on the balance between solubility in the moving phase and retention in the stationary phase

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40
Q

What is Gas Chromatography?

A

in GC, a column is packed with a solid (or a solid coated by a liquid), and a gas is passed through a column under pressure at high temperatures

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41
Q

What is this technique used by?

A

This is a powerful technique used by skilled analytical chemists and chemistry technicians

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42
Q

What can GC be used to separate?

A

mixtures of volatile liquids

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43
Q

Explain how Gas chromatography is carried out?

A
  • uses carrier gas (helium) as the moving phase
  • the stationary phase is often an inert powder coated with a film of non-volatile liquid
  • the powder is then packed into a long narrow column
  • a vapourised mixture sample is injected at the entrance to the column and carried down the column by the carrier gas (the moving phase)
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44
Q

Each component has a..?

A

characteristics retention time

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45
Q

what is a major importance in the separation process?

A

the solubility of the different mixture components in the non-volatile liquid film (stationary phase) is of major importance in the separation process

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46
Q

what components present in the mixture will have a longer retention time?

A

a component that is highly soluble will have in the non-volatile liquid film (stationary phase) takes longer to leave the column (longer retention time)

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47
Q

why does Gas chromatography produce a peak on a graph?

A

a detector monitors as they exit the column - so that each substance appears as a peak on a cart

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48
Q

what does the number of peaks give?

A

the number of peaks gives the number of substances present in the mixture

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49
Q

the chart provides the peak retention time to enable..?

A

Identification

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50
Q

under standard conditions, each substance will have..? And we can use this to..?

A

a unique retention time - so you can use the retention time to identify the components of the mixture

51
Q

what are retention times compared with..?

A

standards to identify substances

52
Q

the peak area is proportional to..?

A

the amount of substance present

53
Q

What can Gas chromatography measure?

A

can measure and separate tiny quantities of closely related compounds

54
Q

what is GC usually connected to?

A

a mass spectrometer

55
Q

why is it usually connected to a mass spectrometer?

A

mass spec is used to analyse each component separated by gas chromatography

56
Q

what is this combined technique called?

A

GCMS

57
Q

Explain what this enables?

A

this enables positive identification of substances through their Mr. substances with similar GC retention times can be distinguished by GCMS (they will have different Mr. values

58
Q

NMR Spectroscopy

A
59
Q

What does NMR stand for?

A

Nuclear magnetic Resonance (NMR) spectroscopy

60
Q

what is NMR used for?

A

is used to determine the structure of a molecule. It provides information on the different environments of atoms in molecules.

61
Q

what are the two main types of NMR?

A
  • Carbon 13 NMR (13C NMR)
  • Proton NMR (1H NMR)
62
Q

what does carbon 13 NMR tell you?

A

tells us how carbon atoms in a molecule are arranged

63
Q

what does proton NMR tell you?

A

tell us us how hydrogen atoms in a molecule are arranged

64
Q

How is a weak magnetic field made?

A
  • if an atomic nucleus has an odd number of nucleons (protons and neutrons), then it has a nuclear spin
  • the nuclear spin creates a weak magnetic field
65
Q

Explain how NMR uses this magnetic field?

A

NMR detects how these weak magnetic fields are affected by a larger external magnetic field

66
Q

why does both Hydrogen and Carbon have a nuclear spin?

A
  • hydrogen has one proton
  • Carbon 13 has
67
Q

what percentage of Carbons are 13C?

A

1% of carbons are 13C

68
Q

In which direction does the nuclei spin in?
What is it affected by

A

spins in random directions, However, when an external magnetic field is applied. They Align in 2 directions

69
Q

what are the two directions they spin in?

A
  • nuclei either spin in the direction of the external magnetic field or Against it.
70
Q

Those that spin in the direction of the magnetic field spin, Have..?

A

lower energy

71
Q

Why can NMR be used to identify molecules with a nuclear spin?

A
  • NMR fires out radio waves
  • at a specific frequency the nuclei that are aligned with the magnetic field absorb the energy and flip to a higher energy level
  • those with a higher energy can also drop to a lower energy level and emit radio waves
  • initially, there is more nuclei aligned with the magnetic field, so overall more energy is absorbed than emitted
  • NMR measures the amount of energy absorbed
72
Q

why is there more energy absorbed than emitted?

A

initially, there is more nuclei aligned with the magnetic field, so overall more energy is absorbed than emitted

73
Q

Rule: The energy absorbed by the nuclei is dependent on…?

A

the environment it is in.!!!

74
Q

How can a nucleus be shielded?
What affects the shielding of amount of electron shielding?

A
  • a nucleus can be shielded from the effects of an external magnetic field by it’s surrounding electrons
  • any atoms or groups of atoms that are around/adjacent to the nucleus will affect the amount of electron shielding
75
Q

Give an example of a type of atom that will affect the electron shielding?

A
  • e.g An electronegative atom such as oxygen near the carbon atom will reduce the electron shielding on the carbon atom.
  • this is because electrons are pulled further away from the carbon atom. the carbon atom thus is shielded less.
76
Q

Overall, why is the effect of the external magnetic field different on each nucleus?

A

depending on the environment within the molecule.
- The magnetic field will be felt by the nuclei differently depending on the environment it is in.

77
Q

Therefore, NMR picks up the..?

A

difference in absorption of energy between environments, as they absorb different amounts of energy and various frequencies.

78
Q

for an atom to be in the same environment, it must be bonded to…?

A

an atom or group of atoms that are identical

79
Q

What chemical is used as a standard, when looking at chemical shift in NMR spectra?

A

(TMS) - Tetramethylsilane

80
Q

Draw the structure of TMS

A
81
Q

Why do we use a standard?

A

As nuclei absorb different amounts of energy at different frequencies, it is difficult to measure the magnitude of those without a reference or standard chemical to measure it against.

82
Q

Why is TMS specifically used as a standard?

A
  • TMS has 12 hydrogens all in identical environments, so produces a large single peak well away from sample peaks.
  • it is also inert, Non-texic and volatile so easy to remove from your sample.
83
Q

What is the chemical shift?

A

the difference between the TMS peak and the peaks produced by the substance under test is called the chemical shift (δ)

84
Q

what is chemical shift measure in?

A

chemical shift is measure in Parts per million (ppm) relative to a standard (TMS)

85
Q

what do we assign the peak TMS has?

A

δ = 0

86
Q

Why will we often see a peak produced as δ=0 on a spectra?

A

because TMS is added to the test compund for calibration purposes

87
Q

13C NMR spectroscopy

A
88
Q

what does 13C NMR spectroscopy tell us?

A

Tells us how many different carbon environments there are in a sample being tested.

89
Q

what does the number of peaks on a Carbon 13 NMR spectrum tell you?

A

the number of carbon environments there are present in a particular molecule being tested.

90
Q

describe the chemical shift and electron shielding for a normal C-H bond?

A

electron shielding is more, lower chemical shift (will be closer to δ=0

91
Q

Describe the chemical shift for A carbon with an electronegative atom like O, N adjacently attached?

A
  • electron shielding is lower and chemical shift is higher (will be further away from δ = 0)
92
Q

How can we identify chemical shifts in cyclical compounds?

A

look for symmetry, use line of symmetry

93
Q

Describe how many chemical environments there are in Cyclohexane-1,3-diol ?
What would this look like on a graph?

A
  • in cyclohexane-1,3-diol there are 4 different carbon environments
94
Q

How else could we identify peaks on 13C NMR?

A

we can use data sheet

95
Q

what are certain issues with using the data sheet/table?

A
  • the chemical shifts can overlap
  • δ = 190 could be carbonyl group in a ketone or aldehyde
  • δ= 30 may be caused by a C-C, C-Cl, C-Br
  • δ = 60 could be ester, alcohol, amine, or ether
96
Q

what are the steps we can follow to interpret a 13C spectra?

A
  1. count number of peaks - this tells us n of carbon environments
  2. use table of chemical shifts
97
Q

What steps would need to be taken in order to interpret 13C NMR spectra of Cyclical compounds? Use example in Chemistry text book with a formula of C6H4CL2

A
  1. the spectrum has 4 peaks
  2. all 4 peaks are between the δ = 125ppm and δ=140 ppm. Looking at the chemical shifts table, this can only be down to alkene groups or carbons in a benzene ring.
  3. the question tells you that the molecule is cyclic with 6 carbons, so these carbons must be in a benzene ring, as a 6 carbon cyclic molecule with 3 double bonds does not exist
  4. there are 3 aromatic molecules with the structure
  5. decipher using the peaks.
98
Q

1H (Proton) NMR Spectroscopy

A
99
Q

What is 1H NMR about?

A

1H NMR is all about how hydrogen nuclei react to a magnetic field.

100
Q

what is each peak on the Hydrogen NMR due to?

A

is due to one or more hydrogen nuclei (protons) in a particular environment

101
Q

what does 1H NMR Spectroscopy tell us?

A

tells us how many different hydrogen environments there are and how many hydrogens in each environment in the sample.

102
Q

when there is a larger peak, this means there are..?

A

more hydrogens

103
Q

when there is a smaller peak. this means there are…?

A

less hydrogens

104
Q

what does the relative area under each peak tell us?

A

also tells us the relative number of H atoms in each environment

105
Q

what does the numbers above each peak tell us?

A

area ration between hydrogens in each environment

106
Q

How many peaks will there be present on the 1H NMR spectrum of 1-Chloropropanone? Predict the ratio of the areas of these peaks

A
  • 2 peaks
  • ratio = 2:3
107
Q

what is a splitting pattern or spin-spin splitting?

A

peaks split into smaller peaks

108
Q

what are the split peaks called?

A

multiplets

109
Q

what does the number of splitting peaks correspond to?

A

corresponds to the number of hydrogen atoms on the adjacent carbon plus one. Called the n+ 1 rule

110
Q

what does a singlet peak tell us?

A

0 hydrogens on the adjacent carbon

111
Q

what does a doublet peak tell us?

A

1 hydrogen on the adjacent carbon

112
Q

what does a triplet peak tell us?

A

2 hydrogens on the neighbouring carbon

113
Q

what does a quartet ell us?

A

3 hydrogens on the adjacent carbon

114
Q

Integration traces

A
115
Q

what do integration traces tell us?

A

integration traces show area under the peak more clearly. this helps to work out the hydrogen ratio

116
Q

on an integration trace, the height ratio of the trace corresponds to the..?

A

area ratio

117
Q

Proton-free-solvents

A
118
Q

is a sample has to be dissolved, then a solvent is needed that doesn’t contain..?

A

protons or any 1H atoms (so proton-free)

119
Q

Why can we not use solvents that contain hydrogen?

A

because these peaks would show up on the spectrum and confuse things

120
Q

If we can not use hydrogen based solvents what can we use?

A

Deuterated solvents are often used

121
Q

what are deuterated solvents often used?

A
  • Their 1H atoms have been replaced by D or 2H.
  • Dueterium is an isotope of a hydrogen atom with 2 nucleons ( a proton and a neutron)
  • has an even number of nucleons, so not detected by NMR.
122
Q

other than deuterated solvents what can be used?

A

Alternative solvents like CCL4 (tetrachloromethane) can be used as a solvent as it doesn’t contain any 1H atoms either

123
Q

Give examples of proton free solvents that can be used?

A
  • CCL4
  • CGCL3,
  • D2O
  • C6D6
124
Q

DONE

A

Chemistry Y13 (13 decks)

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