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Chemistry > Chapter 29 - Chromatography and Spectroscopy > Flashcards

Chapter 29 - Chromatography and Spectroscopy Flashcards

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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
What is the test for alkenes ?
- Add bromine water drop wise - Bromine water decolourises from orange to colourless
26
What is the test for haloalkanes ?
- Add AgNO3 and ethanol and warm to 50 degrees - White, cream and yellow precipitates (Cl, Br, I)
27
What is the test for carbonyls ?
- Add 2,4-DNP - Should see an orange precipitate
28
What is the test for aldehydes ?
- Add tollens reagent and warm - Silver mirror should be seen
29
What is the test for primary and secondary alcohols ?
- Add acidified potassium dichromate and warm in a water bath - Colour change from orange to green
30
What is the test for carboxylic acids ?
- Add aqueous sodium carbonate - Should see effervescence
31
What is the test for carboxylic acids ?
- Add aqueous sodium carbonate - Should see effervescence
32
What does NMR spectroscopy use a combination of ?
A very strong magnetic field and radio frequency radiation
33
How can the energy of absorption be measured ?
Using a NMR spectrum
34
What is nuclear spin ?
The spin on the nucleus, just like how electrons have spin
35
When is nuclear spin significant ?
When there is an odd number of nucleons
36
What compounds is NMR relevant for ?
- H-1 - C-13
37
What can NMR spectroscopy be used for ?
To detect isotopes of other elements with odd numbers of nucleons
38
What is proton NMR ?
Refers to the NMR of H-1 as it is just one single proton in the nucleus
39
How many spin states does the nucleus have ?
Two spin states
40
What do the different spin states of the nucleus have ?
They have different energies
41
How can the nucleus absorb energy ?
With the right combination of a strong magnetic field and radio frequency radiation
42
What happens when the nucleus absorbs energy ?
It rapidly flips between the two spin states
43
What is resonance ?
When the nucleus flips between its two spin states
44
Give an example of where NMR is used every day
MRI scans in hospitals
45
What do the electrons surrounding the nucleus do ?
They shift the energy and radio frequency needed for NMR to take place
46
What does chemical shift refer to ?
It refers to the frequency shift that takes place due to electrons surrounding the nucleus
47
What is TMS ?
- Tetramethylsilane - It is used as the standard reference chemical against which all chemical shifts are measured
48
What is the chemical shift value of TMS ?
Has a chemical shift value of 0
49
What is the amount of chemical shift determined by ?
- The chemical environment - Especially the presence of nearby electronegative atoms
50
What can a different chemical environment lead to ?
It can lead to NMR requiring a different energy and frequency, producing absorption peaks at chemical shifts
51
How is NMR spectroscopy more convenient ?
It allows for the molecule to be mapped without needing to carry out conventional chemical tests and destroying the compound
52
In NMR, what happens to the sample ?
The sample is dissolved in a solvent and placed in a narrow NMR sample tube, together with a small amount of TMS
53
What happens to the tube in NMR ?
It is spun to even out any imperfections in the magnetic field within the sample
54
What happens in NMR once the spec is zeroed against TMS ?
It is given a pulse of radiation containing a range of radio frequencies, whilst maintaining a constant magnetic field
55
How are absorptions shown in NMR ?
They are displayed on a computer screen
56
How can you recover the sample after NMR ?
Evaporate the solvent
57
What is a deuterated solvent ?
A solvent that involves deuterium H-2
58
Why are deuterated solvents used ?
This is because they produce no NMR signal in the frequency ranges used in H-1 and C-13 NMR spectroscopy
59
What does a C-13 NMR spectrum provide about a molecule?
- The number of different carbon environments - from the peaks - The types of carbon environment present - from the chemical shift
60
What are 4 main types of carbon atoms that absorb over different chemical shift ranges ?
- 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
When can chemical shifts be outside the normal ranges ?
Depending on the solvent, concentration and substituents
62
What is the chemical environment of a carbon atom determined by ?
It is determined by the position of the atom within the molecule
63
What is necessary for carbon atoms to have different environments ?
They must be bonded to different atoms or groups of atoms
64
What do carbon atoms that have different environments also do ?
They will absorb at different chemical shifts
65
What are two carbons that are positioned symmetrically like ?
- They are equivalent and have the same chemical environment - They absorb radiation at the same chemical shift and contribute to the same peak
66
What information does a proton NMR spec provide us with ?
- 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
When can the chemical shift values differ in proton NMR ?
If the solvent, concentration and substituents are different
68
What does it mean when it says carbon atoms are equivalent ?
The carbon atoms in the same chemical environment absorb at the same chemical shift value
69
What will protons do if they are equivalent ?
They will absorb at the same chemical shift, increasing the size of the peak
70
What does it mean when it says that protons are non-equivalent ?
- When protons have different chemical environments - They therefore absorb at different chemical shifts
71
What does the peak area show in proton NMR ?
The relative areas under each peak gives the ratio of the number of protons responsible for each peak
72
What is the integration trace ?
The area under each peak
73
How is the integration trace shown ?
- As an extra line on the spectrum - As a printed number of the relative peak areas
74
What is spin-spin coupling ?
The splitting of a main peak into sub-peaks
75
What is the n+1 rule ?
The number of sub peaks is one greater than the number of adjacent protons causing the splitting
76
What can you tell from the spin spin splitting ?
You are seeing the number of hydrogen atoms on the immediately adjacent carbon atom
77
What is a singlet ?
- Relative peak area is 1 - Single spike - No H on adjacent atoms
78
What is a doublet ?
- Relative peak areas are 1:1 - Two spikes - Adjacent CH
79
What is a triplet ?
- Relative peak areas are 1:2:1 - Three spikes, middle one twice as large as outer two - Adjacent CH2
80
What is a quartet ?
- Relative peak areas are 1:3:3:1 - 4 spikes, middle two are three times as large as outer two - Adjacent CH3
81
What is a heptet ?
- CH(CH3)2 - Seven spikes
82
What is the chemical shift like for OH and NH groups ?
They may be involved in hydrogen bonding so their NMR peaks are often broad and of variable chemical shift
83
What does the broadening of the OH and NH groups mean ?
It means that OH and NH protons are not usually involved in spin-spin coupling
84
What is proton exchange ?
It is a technique that has been devised to identify OH and NH protons
85
How is proton exchange carried out ?
- 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
What is the role of D2O in proton exchange ?
It exchanges and replaces the OH and NH protons in the sample with deuterium atoms
87
Does D2O appear on the spectrum ?
No it does not
88
What does the disappearance of D2O on the spectrum also mean ?
OH and NH do not appear on the spectrum

Chemistry (29 decks)

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