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A Level Chemistry > Module 6: Chapter 29 - Chromatography and Spectroscopy > Flashcards

Module 6: Chapter 29 - Chromatography and Spectroscopy Flashcards

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1
Q

What is chromatography?

A

Chromatography is used to separate individual components from a mixture of substances

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2
Q

What is a chromatogram?

A

A visual record showing the result of separation of the components of a mixture by chromatography

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3
Q

What do all forms of chromatography contain?

A
  • Stationary Phase
  • Mobile Phase
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4
Q

What is the stationary phase in chromatography?

A

The stationary phase does not move and is normally a solid or a liquid supported on a solid

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5
Q

What is the mobile phase in chromatography?

A

The mobile phase does move, and is normally a liquid or a gas

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6
Q

What are examples when chromatography can be used?

A

In the analysis of:
* Drugs
* Plastics
* Flavourings
* Air samples

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7
Q

What are 2 types of chromatography?

A
  • Thin layer chromatography (TLC)
  • Gas chromatography
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8
Q

What are the benefits of thin layer chromatography?

A

It is quick and inexpensive way to determine how many components are in a mixture

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9
Q

Explain how thin layer chromatography works:

A
  • TLC uses a TLC plate, which is usually a plastic or glass sheet coated with a thin layer of a solid adsorbent substance - usually silica
  • In TLC, the adsobent is the stationary phase. The different components in the mixture have different affinities for the adsorbent and bind with differing strengths to its surface.
  • Separation is achieved by the relative adsorptions of surfaces with the stationary phase
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10
Q

What is an adsorbent substance?

A

A solid substance that tends to adsorb another substance to its surface

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11
Q

How do you interpret a TLC plate?

A

You calculate the retention factor (Rf) and compare it to known values recorded using the same solvent system and absorbent

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12
Q

What is the equation for retention factor (Rf)?

A

Rf = (Distance moved by component) / (Distance moved by the solvent front)

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13
Q

What do you use to draw the base line in TLC?

A

A pencil as it will not move along as a mobile phase, rather stay stationary

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14
Q

When is gas chromatography useful?

A

It is useful for separating and identifying voltaile organic compounds present in a mixture

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15
Q

What is the stationary phase in TLC chromatography?

A

The adsorbent on the TLC plate

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16
Q

What is the mobile phase in TLC chromatography?

A

The organic solvent

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17
Q

What is the stationary phase in gas chromatography?

A

A high boiling liquid adsorbed onto an intert solid support

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18
Q

What is the mobile phase in gas chromatography?

A

An inert carrier gas such as helium or neon

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18
Q

Explain how gas chromatography works

A
  • A small amount of the volatile mixture is injected into the apparatus, called a gas chromatograph. The mobile carrier gas then carries the components in the sample through the capillary column which contains the liquid stationary phase absorbed onto the solid support.
  • The components slow down as they interact with the liquid stationary phase inside the column. The more soluble the component is in the liquid stationary phase, the slower it moves through the capillary column.
  • The components are separated depending on their solubility in the liquid stationary phase. Therefore, they reach the recorder at different times. The compound retained in the column for the shortest amount of time has the lowest retention time and is detected first
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19
Q

What is the retention time?

A

The time taken for each component in the mixture to travel through the coluumn in gas chromatography

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20
Q

How do you interpret a gas chromatogram?

A
  • Retention times can be used to identify the components present in the sample by comparing these to retention times for known components
  • Peak integrations (the areas under each peak) can be used to determine the concentrations of components in the sample
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21
Q

How can peak integrations be used to determine the concentration of components in a sample?

In gas chromatography

A
  1. Prepare standard solutions of known concentrations of the compound being investigated
  2. Obtain gas chromatograms for each standard solution
  3. Plot a calibration curve of peak area against concentration.
  4. Obtain a gas chromatogram of the substance being investigated (under the same conditions)
  5. Use a calibration curve to measure the concentration of the compound
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22
Q

What is external calibration?

A

A method used to convert a peak integration (area) into a concentration

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23
Q

What is the chemical test for an alkene functional group?

A
  • Add bromine water dropwise
  • If positive, the bromine water will be decolourised from orange to colourless
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24
Q

What is the chemical test for a haloalkane functional group?

A
  • Add silver nitrate and ethanol and warm to 50 degrees celcius in a water bath
  • If positive, precipitates should form (white for chloroalkane, cream for bromoalkane, yellow for iodoalkane)
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25
Q

What is the chemical test for a carbonyl functional group?

A
  • Add 2,4-dinitrophenylhydrazine (2,4-DNP)
  • If positive, an orange precipitate should form
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26
Q

What is the chemical test for an aldehyde functional group?

A
  • Add Tollen’s reagent and warm
  • If positive, a silver mirror should form
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27
Q

What is the chemical test for a primary/secondary alcohol or an aldehyde functional group?

A
  • Add acidified potassium dichromate (VI) and warm in a water bath
  • If positive, the colour should change from orange to green
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28
Q

What is the chemical test for a carboxylic acid functional group?

A
  • Add aqueous sodium carbonate
  • If positive, you should observe effervescence
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29
Q

What does acidified potassium dichromate (VI) test for?

A
  • Primary Alcohols
  • Secondary Alcohols
  • Aldehydes

NOT Tertiary Alcohols

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30
Q

What is nuclear spin?

A

Like electrons, the nucleus of an atom has a property called spin. However, this is only significant if there is an odd number of nucleons

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31
Q

What can NMR spectroscopy be used to analyse?

A

Isotopes of atoms with an odd number of nucleons, i.e Carbon-13, Hydrogen-1, Fluorine-19, or Phosphorus-31

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32
Q

What is resonance of a nucleus?

A

With the right combination of a strong magnetic field and radio frequency radiation, the nucleus can absorb energy and rapidly flip between the two spin states. This is known as resonance

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33
Q

What is TMS?

A

Tetramethylsilane

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34
Q

What is chemical shift, δ, in NMR spectroscopy?

A

In all organic molecules, every carbon and hydrogen atom is bonded to other atoms. All atoms have electrons surrounding the nucleus. Therefore, the energy and radio frequency required for nuclear magnetic resonance to take place is shifted. This frequency shift is measured on a scale called chemical shift (δ). TMS (Tetramethylsilane) is used as the standard refernce for chemical shift and is given a value of 0ppm

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35
Q

What is the standard for chemical shift, δ?

A

TMS (Tetramethylsilane) is used as the standard refernce for chemical shift and is what all chemical shifts are measured against. It is given a value of 0 ppm

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36
Q

What are the units for chemical shift, δ?

A

Parts per Million (ppm)

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37
Q

What is the amount of chemical shift determined by?

A

The chemical environment around the atom being analysed, especially if there is a presence of highly electronegative atoms

38
Q

How can NMR spectograms be interpreted?

A

As the chemical environment influences the energy and frequency required for NMR to occur, the carbon/hydrogen arrangement in a molecule can be mapped out by comparing it to data in a table for chemical shifts relative to NMR. This allows the structure to be mapped out without needing to carry out conventional chemical tests and without destroying the organic compound under test

39
Q

Explain how NMR spectroscopy works:

A
  • In an NMR spectometer, the sample is dissolved in a solvent and placed in a narrow NMR sample tube, together with a small amount of TMS
  • The tube is placed inside the spectrometer, where it is spun to even out any imperfections in the magnetic field within the sample
  • The spectrometer is zeroed against the TMS standard and then the sample is given a pulse of radiation containing a range of radio frequencies whilst maintaining a constant magnetic field
  • Any absoptions of energy resulting from resonance are detected
40
Q

What solvent is used in NMR spectroscopy?

A

A deuterated solvent, this is a solvent in which the Hydrogen-1 atoms have been replaced by Hydrogen-2 atoms (Deuterium, D). Such as Deuterated trichloromethane CDCl₃

41
Q

Why is a deuterated solvent used in NMR spectroscopy?

A

Deuterium produces no NMR signal in the frequency ranges used in Hydrogen-1 and Carbon-13 NMR spectroscopy as it has an even number of nucleons. This is important as most other solvents constain carbon and hydrogen atoms which will produce peaks

42
Q

How can deuterated trichloromethane (CDCl₃) be used as a solvent in NMR despite containing a carbon atom?

A

Despite containing a carbon atom, it can still be used as a solvent as the carbon-13 peak from this molecule will be filtered out by a computer. The important thing is that it is deuterated.

43
Q

What are the 2 types of NMR spectroscopy?

A
  • Carbon-13 spectroscopy
  • Proton NMR spectroscopy
44
Q

What pieces of information about the molecule does Carbon-13 NMR spectroscopy reveal?

A
  • The number of different carbon environments - from the number of peaks
  • The types of carbon environments - from the chemical shifts
45
Q

What is the chemical shift range of Carbon-13 NMR?

A

0-220ppm

46
Q

What are the 4 broad types of carbon atom that obsorb over different chemical shift ranges in Carbon-13 NMR?

A
  • Carbon bonded to carbon
  • Carbon bonded to an electronegative atom, e.g, O, N, Cl, Br
  • Carbon as part of C=C or aromatic ring
  • Carbon as part of C=O
47
Q

Explain the chemical environments of a carbon atom in NMR spectroscopy:

A
  • If the carbon atoms are bonded to different atoms or groups of atoms, they will have different chemical environments and will absorb at different chemical shifts, they are non-equivalent
  • If 2 carbon atoms are positioned symmetrically within a molecule, then they are equivalent and have the same chemical environment. Therefore they will contribute to the same peak
48
Q

Identify the chemical environments for carbon in propanal and describe what the NMR spectra would look like:

A
49
Q

Identify the chemical environments for carbon in propanone and describe what the NMR spectra would look like:

A
50
Q

Why does propanal have 3 chemical regions for carbon despite carbons 2 and 3 falling within the same broad C-C environment?

A

Carbon 2 is nearer to the oxygen atom connected to carbon 1, therefore it is likely to be shifted more than carbon atom 3

51
Q

How do you interpret a carbon NMR spectrum?

A
  1. Draw out all the different isomers from the molecular formula
  2. Identify the expected number of carbon chemical environments
  3. Determine which isomer matches the spectrum
52
Q

The 2 spectra below are 2 isomers of C₄H₁₀O (that are alcohols), identify which isomer corresponds to each spectrum and assign each peak to a specific chemical environment

A
53
Q

Predict the number of peaks in its carbon-13 NMR spectrum and the approximate chemical shift for each peak

A

5 peaks
1: δ = 0-50ppm
2,3,4,5: δ = 110-160ppm

54
Q

How do you know peaks 2 and 3 are labelled that way round?

A

Carbon environment 2 is closer than carbon environment 3 to the oxygen atom, therefore it is likely to be shifted by a higher δ value

55
Q

What 4 pieces of information does proton NMR reveal about the molecule?

A
  • The number of different proton environments - from the number of peaks
  • The types of proton environments present - from the chemical shift
  • The relative numbers of each type of proton - from integration traces or ratio numbers of the realtive peak areas
  • The number of non-equivalent protons adjacent to a given proton - from the spin-spin coupling splitting pattern
56
Q

What is the chemical shift range of Proton NMR?

A

0-12ppm

57
Q

What are the 4 broad types of hydrogen atom that absorb over different chemical shift ranges in proton NMR?

A
  • HC bonded to an alkyl chain
  • HC bonded to electronegative atom (e.g O, N, Cl, benzene ring, or C=O)
  • H bonded to C=C or aromatic ring
  • H bonded to C=O or part of COOH
58
Q

What other factors can affect the chemical shift values in NMR other than chemical environment?

A
  • Interactions with solvent
  • Concentration
  • Substituents
59
Q

Explain the chemical environments of a hydrogen atom in NMR spectroscopy:

A
  • If the hydrogen atoms are bonded to different atoms or groups of atoms, they will have different chemical environments and will absorb at different chemical shifts, they are non-equivalent
  • If 2 hydrogen atoms are equivalent and have the same chemical environment, they will contribute to the same peak and increase the size of the peak
60
Q

Identify the number of chemical environments for hydrogen in butanoic acid

Explain your answer

A

4 chemical environments
The 2 CH₂ groups are connected to different groups on each side and therefore the protons are in different chemical environments

61
Q

Identify the number of chemical environments for hydrogen in butanedioic acid

A

2 chemical environments
There is a plane of symmetry and therefore the protons on each CH₂ group are equivalent and the protons in the COOH groups are equivalent, therefore there are 2 chemical environments

62
Q

How can you determine the relative number of each type of proton environment in proton NMR?

A

Using the area under each peak. This is shown on the spectrogram as integration traces which are shown either as an extra line or as a printed number of the relative peak areas

63
Q

What is spin-spin coupling?

A

In an proton NMR spectrum, the interaction between spin states of non-equivalent (in different chemical environments) hydrogen nuclei that results in the splitting of a signal

64
Q

What is the n+1 rule?

A

For a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in the splitting pattern is equal to n+1

65
Q

What does spin-spin coupling allow you to determine?

A

The total number of hydrogen atoms on the immediately adjacent carbon atoms

66
Q

In a proton NMR spectrum, what is this splitting pattern called and how many hydrogen atoms are there on adjacent carbon atoms?

A
  • Singlet
  • 0 adjacent hydrogens
67
Q

In a proton NMR spectrum, what is this splitting pattern called and how many hydrogen atoms are there on adjacent carbon atoms?

A
  • Doublet
  • 1 adjacent hydrogen
68
Q

In a proton NMR spectrum, what is this splitting pattern called and how many hydrogen atoms are there on adjacent carbon atoms?

A
  • Triplet
  • 2 adjacent hydrogens
69
Q

In a proton NMR spectrum, what is this splitting pattern called and how many hydrogen atoms are there on adjacent carbon atoms?

A
  • Quartet
  • 3 adjacent hydrogens
70
Q

In a proton NMR spectrum, what is this splitting pattern called and how many hydrogen atoms are there on adjacent carbon atoms?

A
  • Multiplets
  • > 3 adjacent hydrogens (4 or more)
71
Q

What is the relative peak areas within a singlet?

A

1

72
Q

What is the relative peak areas within a doublet?

A

1:1

73
Q

What is the relative peak areas within a triplet?

A

1:2:1

74
Q

What is the relative peak areas within a quartet?

A

1:3:3:1

75
Q

What determines the relative peak areas within spin-spin coupling sub-peaks?

A

Pascals triangle

76
Q

What would the NMR splitting pattern of an aromatic proton look like?

A

It would form 1 or more multiplets

77
Q

What is important to remember about spin-spin coupling?

A

It always occurs in pairs as each proton splits the signal of another. Therefore you can pair up splitting patterns to determine structural features

78
Q

Explain how -OH and -NH groups are handled in proton NMR spectroscopy

A

A method called proton exchange is used to identify -OH and -NH protons:
1. A proton NMR is run as normal
2. A small amount of Deuterium oxide, D₂O, is added, the mixture is shaken and then the NMR spectrum is run again

The deuterium exchanges and replaces the OH and NH protons in the sample with deuterium atoms. Therefore, the second spectrum does not contain the peak of OH or NH as they are now OD and ND

79
Q

What are the problems with -OH and -NH groups in proton NMR spectroscopy?

A
  • -OH and -NH groups may be involved in hydrogen bonding and the NMR peaks are often broad and are of variable chemical shift (-OH and -NH peaks can occur at almost any chemical shift).
  • The broadening of peaks also mean that OH and NH protons are not usually involved with spin-spin coupling.
    Therefore, it can make assigning -OH and -NH protons difficult
80
Q

How do you interpret a proton NMR spectrum?

A
  1. Analyse the types of proton environments present (using chemical shifts)
  2. Analyse the number of each type of proton environment present (using integration traces)
  3. Analyse the splitting patterns to find information about adjacent protons
  4. Combine these pieces of data and any other data provided to suggest a structure for the molecule
81
Q

Determine the structure of this molecule

A
82
Q

Determine the structure of this molecule

A
83
Q

Predict the proton NMR spectra of 2-hydroxypropanoic acid

A
84
Q

Predict the carbon-13 NMR spectra of 2-hydroxypropanoic acid

A
85
Q

What is the typical sequence for identification of a molecule?

A

1. Elemental analysis - use of percentage composition by mass to determine the empirical formula of a compound
2. Mass Spectra - Use of the molecular ion peak to determine the molecular mass and formula, use of fragment ions to dtermine parts of the molecule
3. Infrared Spectra - Use of absorption peaks to inentify bonds and functional groups present
4. NMR Spectra - To determine number and types of chemical environments of carbon and hydrogen atoms present from chemical shifts. Surrounding structure of hydrogen atoms from splitting patterns

86
Q

The unknown compound gave the percentage composition by mass: C, 73.17%; H, 7.32%; O, 19.51%. Determine the 2 possible displayed formulae for this compound

A
87
Q

Why is TMS (Tetramethylsilane) used as the standard in Spectroscopy?

A
  • It only gives one signal (that is further right than most signals from common organic compounds)
  • Is non-toxic
  • It is inert and has a low boiling point so can easily be removed from the sample afterwards
88
Q

How does the distance from a strongly electronegative atom affect the chemical shift?

A

The closer the atom is to the electronegative atom, the greater the chemical shift

89
Q

What does the number of signals in NMR mean for carbon-13 and Hydrogen-1?

A

Same for both:
* One signal for each set of equivalent Hydrogen-1 or Carbon-13 atoms

90
Q

What does the position of signals in NMR mean for carbon-13 and Hydrogen-1?

A

Same for both:
* The position of the signals indicate the chemical environment of the Hydrogen-1 or Carbon-13 atoms

91
Q

What does the relative size of signals in NMR mean for carbon-13 and Hydrogen-1?

A

Different for each:
* In Hydrogen-1 NMR, the relative area of signals is related to the relative number of Hydrogen-1 atoms
* In Carbon-13 NMR, there is NO link between area of signal to the number of Carbon-13 atoms

92
Q

What does the splitting of signals in NMR mean for carbon-13 and Hydrogen-1?

A

Different for each:
* In Hydrogen-1 NMR, the signal splitting indicates number of Hydrogen-1 atoms on adjacent atoms
* In carbon-13 NMR, there are NO splitting patterns

93
Q

What is the line drawn along the bottom of a TLC plate called?

A

The base line

A Level Chemistry (31 decks)

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