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Chemistry > 29) Chromatography and spectroscopy > Flashcards

29) Chromatography and spectroscopy Flashcards

1
Q

What is the purpose of chromatography?

A

to separate individual components from a mixture of substances

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2
Q

Define stationary phase

A

phase that does not move in chromatography

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3
Q

Define mobile phase

A

phase that moves in chromatography

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4
Q

Define thin layer chromatography (TLC)

A

quick and inexpensive analytical technique that indicated how many components are in a mixture

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5
Q

What is the stationary phase in TLC?

A

TLC plate (plastic or glass) coated with a thin layer of solid adsorbent substance (e.g. silica)

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6
Q

Define adsorption

A

the process that occurs when a gas or liquid or solute is held to the surface of a solid

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7
Q

How is separation achieved in TLC?

A

by relative adsorptions of substances with the stationary phase (different components in the mixture have different affinities and bind with different strengths to the surface)

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8
Q

Describe a 5 step method for TLC

A

1) base line drawn across the plate 1cm from the edge in pencil
2) small amount of sample solution spotted using a capillary tube onto the base line
3) prepare a chromatography tank with a small beaker and watch glass, solvent depth 5mm, then place the TLC plate inside ensuring the solvent doesn’t cover the spot
4) leave undisturbed until the solvent has risen to about 1cm below the top of the plate, remove + mark solvent front with pencil. allow to dry
5) circle any visible spots with pencil. You may need to use a UV lamp or spray with a chemical/locating agent e.g. iodine

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9
Q

How can each component of TLC be identified?

A

by comparing its Rf value with known values recorded using the same solvent system and stationary phase
OR TLC of a sample could be run against pure/known values

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10
Q

retention factor, Rf = ?

A

distance moved by component / distance moved by solvent front

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11
Q

What is the purpose of gas chromatography?

A

separating and identifying volatile organic compounds in a mixture

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12
Q

What is the stationary phase in gas chromatography?

A

high boiling liquid adsorbed onto an inert solid support

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13
Q

What is the mobile phase in gas chromatography?

A

inert carrier gas e.g. helium or neon

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14
Q

Describe a 3 step method for gas chromatography

A

1) small amount of volatile mixture is injected into the gas chromatograph + is carried by the mobile phase through the capillary column
2) components slow down as they interact with the stationary phase. the more soluble the component is in the stationary phase, the slower it moves through the column. therefore, compounds are separated depending on their solubility in the liquid stationary phase
3) components reach the detector at different times. the compound retained in the column for the shortest time has the lowest retention time and is detected first

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15
Q

Define retention time

A

the time taken for each component to travel through the column (from the column inlet to the detector)

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16
Q

Each component is detected as a _ on the gas chromatogram

A

peak

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17
Q

How can compounds be identified in GC?

A

retention times can be compared to known components

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18
Q

Define peak integrations

A

the areas under each peak

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19
Q

How can you determine the concentrations of components in the sample, in GC? Give a 4 step method

A

compare peak integrations to values obtained from the standard solutions of each component:

1) prepare standard solutions of known concentrations of the compound being investigated
2) obtain gas chromatograms for each
3) plot a calibration curve of peak area against concentration = external calibration
4) obtain a gas chromatogram of the compound being investigated under the same conditions + use the calibration curve to measure the concentration of the compound

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20
Q

Give the chemical test and observation for an alkene

A

+ bromine water dropwise

orange -> colourless

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21
Q

Give the chemical test and observation for a haloalkane

A 
\+ silver nitrate, ethanol
water bath at 50 degrees C
Cl -> white ppt
Br -> cream ppt
I -> yellow ppt
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22
Q

Give the chemical test and observation for a carbonyl

A

+ 2,4-DNP

orange ppt

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23
Q

Give the chemical test and observation for an aldehyde

A

+ Tollens’ reagent and warm

silver mirror

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24
Q

Give the chemical test and observation for an alcohol (primary/secondary) or aldehyde

A

+ acidified potassium dichromate (VI) and warm in a water bath
orange -> green

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25
Q

Give the chemical test and observation for a carboxylic acid

A

+ aqueous sodium carbonate

effervescence

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26
Q

Define nuclear magnetic resonance (NMR) spectroscopy

A

technique using a very strong magnetic field and radio frequency radiation so that the nuclei of some atoms absorb radiation. the energy for the absorption can be measured and recorded as a NMR spectrum

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27
Q

Name two types of NMR

A

proton NMR and carbon-13 NMR

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28
Q

When is nuclear spin significant?

A

when there is an odd number of nucleons (protons and neutrons)

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29
Q

Describe resonance and how it can occur

A

when the nucleus rapidly flips between two spin states
the nucleus has 2 different spin states which have different energies. with the right combination of a strong magnetic field and radio frequency radiation the nucleus can absorb energy

30
Q

What uses the same technology as an NMR

A

a MRI

31
Q

What do the e- surrounding the nucleus do for NMR to take place?

A

shift the energy and radio frequency

32
Q

What can frequency shift be measured on?

A

chemical shift δ, in ppm

a scale that compares the frequency of an NMR absorption with the frequency of the reference TMS at δ = 0ppm

33
Q

What is tetramethylsilane (TMS), (CH3)4Si used as?

A

the standard reference chemical against which all chemical shifts are measured

34
Q

What is the amount of chemical shift determined by and why?

A

the chemical environment, especially the presence of nearby electronegative atoms.
depending on the chemical environment, NMR requires a different energy and frequency, producing absorption peaks at chemical shifts

35
Q

Give a 5 step method for running a NMR spectrum

A

1) sample dissolved in solvent, placed inside a narrow NMR tube with a small amount of TMS and placed inside the NMR spectrometer
2) spun to even out any imperfections inside the NMR spectrometer
3) spectrometer zeroed against TMS standard + sample given a pulse of radiation containing a range of radio frequencies whilst maintaining a constant magnetic field
4) absorptions of energy resulting from resonance are detected and displayed on a computer screen
5) after analysis the sample can be recovered by evaporation of solvent

36
Q

Why is a deuterated solvent often used in NMR?

A

common solvents contain C and H atoms which will produce a signal in the spectra
in a deuterated solvent the 1H atoms are replaced by 2H atoms (deuterium) which produce no NMR signal in the frequencies used

37
Q

What is done in response to deuterated trichloromethane (a common solvent) still producing a peak in carbon-13 spectra?

A

a computer will usually filter out the peak

38
Q

In carbon-13 NMR spectroscopy, the number of peaks indicate…?

A

number of different carbon environments

39
Q

In carbon-13 NMR spectroscopy, the chemical shift indicates…?

A

types of carbon environment present

40
Q

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

A

the position of the atom within the molecule

41
Q

Carbon atoms bonded to different atoms/groups have _ environments and will absorb at _ chemical shifts

A

different

different

42
Q

If two carbon atoms are positioned symmetrically within a molecule, then they are _ and have _ chemical environment. What implication does this have?

A

equivalent
same
They will then absorb radiation at the same chemical shift and contribute to the same peak.

43
Q

In proton NMR spectroscopy, the number of peaks indicate…?

A

number of different proton environments

44
Q

In proton NMR spectroscopy, the chemical shift indicates…?

A

types of proton environments present

45
Q

In proton NMR spectroscopy, the integration traces / ratio numbers of relative peak areas indicate…?

A

relative number of each type of proton

46
Q

In proton NMR spectroscopy, the spin-spin splitting pattern indicates…?

A

number of non-equivalent protons adjacent to a given proton

47
Q

What 3 factors may move a peak outside of the normal ranges?

A

solvent
concentration
substituents

48
Q

If two or more protons are equivalent, they will absorb at _ chemical shift, _ the size of the peak

A

the same
increasing
(look for any plane of symmetry)

49
Q

Protons of different types have _ chemical environments and are _ - they absorb at _ chemical shifts

A

different
non-equivalent
different
(look for any plane of symmetry)

50
Q

What does the NMR spectrometer measure?

A

the area under each peak as an integration trace

51
Q

What is an integration trace shown as?

A

an extra line on the spectrum or as a printed number of the relative peak areas

52
Q

A proton NMR peak can be split into sub-peaks or splitting patterns. How is this caused?

A

by the proton’s spin interacting with the spin states of nearby protons that are in different environments

53
Q

Define spin-spin coupling / spin-spin splitting

A

the splitting of a main peak into sub-peaks

the interaction between spin states of non-equivalent nuclei that results in the splitting of a signal

54
Q

The number of sub-peaks is _ than the number of adjacent protons causing the splitting.

A

one greater

55
Q

State the n+1 rule

A

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

56
Q 
If a proton has 0 protons attached to an adjacent carbon atom, state the following?
n+1
splitting pattern
relative peak areas within splitting
structural feature
A

1
singlet
1
no H on adjacent atoms

57
Q 
If a proton has 1 proton attached to an adjacent carbon atom, state the following?
n+1
splitting pattern
relative peak areas within splitting
structural feature
A

2
doublet
1:1
adjacent CH

58
Q 
If a proton has 2 protons attached to an adjacent carbon atom, state the following?
n+1
splitting pattern
relative peak areas within splitting
structural feature
A

3
triplet
1:2:1
adjacent Ch2

59
Q 
If a proton has 3 protons attached to an adjacent carbon atom, state the following?
n+1
splitting pattern
relative peak areas within splitting
structural feature
A

4
quarter
1:3:3:1
adjacent CH3

60
Q

When you are analysing spin-spin splitting, what are you seeing?

A

the number of hydrogen atoms on the adjacent carbon atom

61
Q

Describe a common splitting that results in a heptet (7) splitting pattern

A

CH(CH3)2

where a CH proton has 6 protons on the adjacent carbon atoms

62
Q

Sometimes, adjacent protons have different environments so resulting splitting would show as a…?

A

multiplet

63
Q

Why does spin-spin coupling occur in pairs?

A

because each proton splits the signal of the other

64
Q

Why is proton exchange used?

A

to identify OH and NH protons which are otherwise difficult to assign

65
Q

How is proton exchange carried out?

A

by running a second NMR spectrum with a small amount of deuterium oxide, D2O, added to the mixture (shaken) before
deuterium exchanges and replaces OH and NH protons in the sample and does not absorb in the chemical shift range so OH/NH peaks disappear

66
Q

What can you find out with elemental analysis?

A

percentage composition by mass -> empirical formula

67
Q

What can you find out with mass spectra?

A

M+ peak -> molecular mass

fragment ions -> determine parts of the molecule

68
Q

What can you calculate with an empirical formula and molecular mass?

A

molecular formula

69
Q

What can you find out with infrared spectra?

A

absorption peaks -> bonds present and functional groups

70
Q

What can you find out with NMR spectra?

A

chemical shifts of peaks -> number and types of carbon and hydrogen atoms
splitting patterns -> order of atoms within molecules

71
Q

Explain why deuterated dimethylsulfoxide, (CD3)2SO, is used as a solvent in NMR rather than (CH3)2SO

A

no peaks from interfering protons

72
Q

What information can be deduced about a compound from an integration trace in proton NMR?

A

area under graph measured

number of protons present

Chemistry (31 decks)

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