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Chemistry > Chapter 29 - Chromatography and Spectroscopy > Flashcards

Chapter 29 - Chromatography and Spectroscopy Flashcards

1
Q

Why is chromatography used ?

A

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

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2
Q

What are the two phases involved in chromatography ?

A
  • Stationary phase
  • Mobile phase
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3
Q

What is the stationary phase ?

A

It does not move and is normally a solid or a liquid that is supported on a solid

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4
Q

What is the mobile phase ?

A

It does move and is normally a liquid or a gas

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5
Q

Where can chromatography be used ?

A

Can be used in the analysis of drugs, plastics, flavourings, air samples and in forensic sciences

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6
Q

What is TLC ?

A

It is a quick and inexpensive analytical technique that indicates how many components there are in a mixture

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7
Q

What does TLC use ?

A

It uses a TLC plate which is usually a plastic sheet or glass, coated with a thin layer of a solid adsorbent substance

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8
Q

In TLC what is normally the adsorbent substance ?

A

Silica

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9
Q

In TLC what is the stationary phase ?

A

The adsorbent substance is normally the stationary phase

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10
Q

What is adsorption ?

A

It is the process by which the solid silica holds the different substances in the mixture to its surface

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11
Q

How is separation achieved in TLC ?

A
  • By the relative adsorptions of substances with the stationary phase
  • Each component in the mixture has different affinities for the absorbent and binds with differing strengths to its surface
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12
Q

How are Thin Layer Chromatograms analysed ?

A

By calculating the value for the retention factor for each component

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13
Q

What is the Rf value ?

A

Distance moved by the component / distance moved by the solvent front

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14
Q

How can we know what a substance is given we know its experimental Rf value ?

A
  • Comparing the Rf values with known values recorded using the same solvent fronts
  • By running a TLC sample of the pure compound and comparing them to see what is visible
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15
Q

When is a gas chromatogram useful ?

A

When separating and identifying volatile organic compounds present in a mixture

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16
Q

What is the stationary phase in gas chromatography ?

A

High boiling point liquid absorbed onto an inert solid support

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17
Q

What is the mobile phase in gas chromatography ?

A

Inert carrier gas such as helium or neon

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18
Q

What happens in gas chromatography ?

A
  • A small amount of the volatile mixture is injected into the apparatus
  • The mobile carrier gas carries the components in the sample through the capillary column which contains the liquid stationary phase
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19
Q

Why do components slow down in gas chromatography ?

A

They slow down as they interact with the liquid stationary phase inside the column

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20
Q

What is the relationship between the solubility of the component and how quick it moves ?

A

The more soluble the component is in the liquid stationary phase, the slower it moves through the capillary column

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21
Q

What are the components separated depending on ?

A

Their solubility in the liquid stationary phase

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22
Q

What is retention time ?

A

The time taken for each component to travel through the column

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23
Q

What is each component displayed as on a gas chromatogram ?

A

As a peak

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24
Q

What info can be obtained from a gas chromatogram ?

A
  • Identify the component present by comparing the retention time to known times
  • Peak integrations can be used to determine the concentrations of the component in the sample
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25
Q

What is the test for alkenes ?

A
  • Add bromine water drop wise
  • Bromine water decolourises from orange to colourless
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26
Q

What is the test for haloalkanes ?

A
  • Add AgNO3 and ethanol and warm to 50 degrees
  • White, cream and yellow precipitates (Cl, Br, I)
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27
Q

What is the test for carbonyls ?

A
  • Add 2,4-DNP
  • Should see an orange precipitate
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28
Q

What is the test for aldehydes ?

A
  • Add tollens reagent and warm
  • Silver mirror should be seen
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29
Q

What is the test for primary and secondary alcohols ?

A
  • Add acidified potassium dichromate and warm in a water bath
  • Colour change from orange to green
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30
Q

What is the test for carboxylic acids ?

A
  • Add aqueous sodium carbonate
  • Should see effervescence
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31
Q

What is the test for carboxylic acids ?

A
  • Add aqueous sodium carbonate
  • Should see effervescence
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32
Q

What does NMR spectroscopy use a combination of ?

A

A very strong magnetic field and radio frequency radiation

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33
Q

How can the energy of absorption be measured ?

A

Using a NMR spectrum

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34
Q

What is nuclear spin ?

A

The spin on the nucleus, just like how electrons have spin

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35
Q

When is nuclear spin significant ?

A

When there is an odd number of nucleons

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36
Q

What compounds is NMR relevant for ?

A
  • H-1
  • C-13
37
Q

What can NMR spectroscopy be used for ?

A

To detect isotopes of other elements with odd numbers of nucleons

38
Q

What is proton NMR ?

A

Refers to the NMR of H-1 as it is just one single proton in the nucleus

39
Q

How many spin states does the nucleus have ?

A

Two spin states

40
Q

What do the different spin states of the nucleus have ?

A

They have different energies

41
Q

How can the nucleus absorb energy ?

A

With the right combination of a strong magnetic field and radio frequency radiation

42
Q

What happens when the nucleus absorbs energy ?

A

It rapidly flips between the two spin states

43
Q

What is resonance ?

A

When the nucleus flips between its two spin states

44
Q

Give an example of where NMR is used every day

A

MRI scans in hospitals

45
Q

What do the electrons surrounding the nucleus do ?

A

They shift the energy and radio frequency needed for NMR to take place

46
Q

What does chemical shift refer to ?

A

It refers to the frequency shift that takes place due to electrons surrounding the nucleus

47
Q

What is TMS ?

A
  • Tetramethylsilane
  • It is used as the standard reference chemical against which all chemical shifts are measured
48
Q

What is the chemical shift value of TMS ?

A

Has a chemical shift value of 0

49
Q

What is the amount of chemical shift determined by ?

A
  • The chemical environment
  • Especially the presence of nearby electronegative atoms
50
Q

What can a different chemical environment lead to ?

A

It can lead to NMR requiring a different energy and frequency, producing absorption peaks at chemical shifts

51
Q

How is NMR spectroscopy more convenient ?

A

It allows for the molecule to be mapped without needing to carry out conventional chemical tests and destroying the compound

52
Q

In NMR, what happens to the sample ?

A

The sample is dissolved in a solvent and placed in a narrow NMR sample tube, together with a small amount of TMS

53
Q

What happens to the tube in NMR ?

A

It is spun to even out any imperfections in the magnetic field within the sample

54
Q

What happens in NMR once the spec is zeroed against TMS ?

A

It is given a pulse of radiation containing a range of radio frequencies, whilst maintaining a constant magnetic field

55
Q

How are absorptions shown in NMR ?

A

They are displayed on a computer screen

56
Q

How can you recover the sample after NMR ?

A

Evaporate the solvent

57
Q

What is a deuterated solvent ?

A

A solvent that involves deuterium H-2

58
Q

Why are deuterated solvents used ?

A

This is because they produce no NMR signal in the frequency ranges used in H-1 and C-13 NMR spectroscopy

59
Q

What does a C-13 NMR spectrum provide about a molecule?

A
  • The number of different carbon environments - from the peaks
  • The types of carbon environment present - from the chemical shift
60
Q

What are 4 main types of carbon atoms that absorb over different chemical shift ranges ?

A
  • Carbon bonded to carbon
  • Carbon bonded to an electronegative atom
  • Carbon as part of a double bond or an aromatic ring
  • Carbon as part of C=O
61
Q

When can chemical shifts be outside the normal ranges ?

A

Depending on the solvent, concentration and substituents

62
Q

What is the chemical environment of a carbon atom determined by ?

A

It is determined by the position of the atom within the molecule

63
Q

What is necessary for carbon atoms to have different environments ?

A

They must be bonded to different atoms or groups of atoms

64
Q

What do carbon atoms that have different environments also do ?

A

They will absorb at different chemical shifts

65
Q

What are two carbons that are positioned symmetrically like ?

A
  • They are equivalent and have the same chemical environment
  • They absorb radiation at the same chemical shift and contribute to the same peak
66
Q

What information does a proton NMR spec provide us with ?

A
  • The number of different proton environments (no of peaks)
  • The types of proton environments present (chemical shift)
  • Relative numbers of each type of proton (integration traces of the relative peak areas)
  • Number of non-equivalent protons adjacent to a given proton (spin-spin coupling)
67
Q

When can the chemical shift values differ in proton NMR ?

A

If the solvent, concentration and substituents are different

68
Q

What does it mean when it says carbon atoms are equivalent ?

A

The carbon atoms in the same chemical environment absorb at the same chemical shift value

69
Q

What will protons do if they are equivalent ?

A

They will absorb at the same chemical shift, increasing the size of the peak

70
Q

What does it mean when it says that protons are non-equivalent ?

A
  • When protons have different chemical environments
  • They therefore absorb at different chemical shifts
71
Q

What does the peak area show in proton NMR ?

A

The relative areas under each peak gives the ratio of the number of protons responsible for each peak

72
Q

What is the integration trace ?

A

The area under each peak

73
Q

How is the integration trace shown ?

A
  • As an extra line on the spectrum
  • As a printed number of the relative peak areas
74
Q

What is spin-spin coupling ?

A

The splitting of a main peak into sub-peaks

75
Q

What is the n+1 rule ?

A

The number of sub peaks is one greater than the number of adjacent protons causing the splitting

76
Q

What can you tell from the spin spin splitting ?

A

You are seeing the number of hydrogen atoms on the immediately adjacent carbon atom

77
Q

What is a singlet ?

A
  • Relative peak area is 1
  • Single spike
  • No H on adjacent atoms
78
Q

What is a doublet ?

A
  • Relative peak areas are 1:1
  • Two spikes
  • Adjacent CH
79
Q

What is a triplet ?

A
  • Relative peak areas are 1:2:1
  • Three spikes, middle one twice as large as outer two
  • Adjacent CH2
80
Q

What is a quartet ?

A
  • Relative peak areas are 1:3:3:1
  • 4 spikes, middle two are three times as large as outer two
  • Adjacent CH3
81
Q

What is a heptet ?

A
  • CH(CH3)2
  • Seven spikes
82
Q

What is the chemical shift like for OH and NH groups ?

A

They may be involved in hydrogen bonding so their NMR peaks are often broad and of variable chemical shift

83
Q

What does the broadening of the OH and NH groups mean ?

A

It means that OH and NH protons are not usually involved in spin-spin coupling

84
Q

What is proton exchange ?

A

It is a technique that has been devised to identify OH and NH protons

85
Q

How is proton exchange carried out ?

A
  • A proton NMR spectrum is run as normal
  • A small volume of D2O is added, the mixture is shaken and a second spectrum is run
86
Q

What is the role of D2O in proton exchange ?

A

It exchanges and replaces the OH and NH protons in the sample with deuterium atoms

87
Q

Does D2O appear on the spectrum ?

A

No it does not

88
Q

What does the disappearance of D2O on the spectrum also mean ?

A

OH and NH do not appear on the spectrum

Chemistry (29 decks)

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