Module 6: Chapter 29 - Chromatography and Spectroscopy Flashcards
1
Q
What is chromatography?
A
Chromatography is used to separate individual components from a mixture of substances
2
Q
What is a chromatogram?
A
A visual record showing the result of separation of the components of a mixture by chromatography
3
Q
What do all forms of chromatography contain?
A
- Stationary Phase
- Mobile Phase
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
5
Q
What is the mobile phase in chromatography?
A
The mobile phase does move, and is normally a liquid or a gas
6
Q
What are examples when chromatography can be used?
A
In the analysis of:
* Drugs
* Plastics
* Flavourings
* Air samples
7
Q
What are 2 types of chromatography?
A
- Thin layer chromatography (TLC)
- Gas chromatography
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
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
10
Q
What is an adsorbent substance?
A
A solid substance that tends to adsorb another substance to its surface
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
12
Q
What is the equation for retention factor (Rf)?
A
Rf = (Distance moved by component) / (Distance moved by the solvent front)
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
14
Q
When is gas chromatography useful?
A
It is useful for separating and identifying voltaile organic compounds present in a mixture
15
Q
What is the stationary phase in TLC chromatography?
A
The adsorbent on the TLC plate
16
Q
What is the mobile phase in TLC chromatography?
A
The organic solvent
17
Q
What is the stationary phase in gas chromatography?
A
A high boiling liquid adsorbed onto an intert solid support
18
Q
What is the mobile phase in gas chromatography?
A
An inert carrier gas such as helium or neon
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
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
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
21
Q
How can peak integrations be used to determine the concentration of components in a sample?
In gas chromatography
A
- Prepare standard solutions of known concentrations of the compound being investigated
- Obtain gas chromatograms for each standard solution
- Plot a calibration curve of peak area against concentration.
- Obtain a gas chromatogram of the substance being investigated (under the same conditions)
- Use a calibration curve to measure the concentration of the compound
22
Q
What is external calibration?
A
A method used to convert a peak integration (area) into a concentration
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
24
What is the chemical test for a haloalkane functional group?
* 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)
25
What is the chemical test for a carbonyl functional group?
* Add 2,4-dinitrophenylhydrazine (**2,4-DNP**)
* If positive, an orange precipitate should form
26
What is the chemical test for an aldehyde functional group?
* Add Tollen's reagent and warm
* If positive, a silver mirror should form
27
What is the chemical test for a primary/secondary alcohol or an aldehyde functional group?
* Add acidified potassium dichromate (VI) and warm in a water bath
* If positive, the colour should change from orange to green
28
What is the chemical test for a carboxylic acid functional group?
* Add aqueous sodium carbonate
* If positive, you should observe effervescence
29
What does acidified potassium dichromate (VI) test for?
* Primary Alcohols
* Secondary Alcohols
* Aldehydes
| NOT Tertiary Alcohols
30
What is nuclear spin?
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
31
What can NMR spectroscopy be used to analyse?
Isotopes of atoms with an odd number of nucleons, i.e Carbon-13, Hydrogen-1, Fluorine-19, or Phosphorus-31
32
What is resonance of a nucleus?
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
33
What is TMS?
Tetramethylsilane
34
What is chemical shift, δ, in NMR spectroscopy?
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
35
What is the standard for chemical shift, δ?
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
36
What are the units for chemical shift, δ?
Parts per Million (ppm)
37
What is the amount of chemical shift determined by?
The chemical environment around the atom being analysed, especially if there is a presence of highly electronegative atoms
38
How can NMR spectograms be interpreted?
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
Explain how NMR spectroscopy works:
* 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
What solvent is used in NMR spectroscopy?
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
Why is a deuterated solvent used in NMR spectroscopy?
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
How can deuterated trichloromethane (CDCl₃) be used as a solvent in NMR despite containing a carbon atom?
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
What are the 2 types of NMR spectroscopy?
* Carbon-13 spectroscopy
* Proton NMR spectroscopy
44
What pieces of information about the molecule does Carbon-13 NMR spectroscopy reveal?
* The **number** of different carbon environments - from the number of peaks
* The **types** of carbon environments - from the chemical shifts
45
What is the chemical shift range of Carbon-13 NMR?
0-220ppm
46
What are the 4 **broad** types of carbon atom that obsorb over different chemical shift ranges in Carbon-13 NMR?
* 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
Explain the chemical environments of a carbon atom in NMR spectroscopy:
* 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
Identify the chemical environments for **carbon** in propanal and describe what the NMR spectra would look like:
49
Identify the chemical environments for **carbon** in propanone and describe what the NMR spectra would look like:
50
Why does propanal have 3 chemical regions for carbon despite carbons 2 and 3 falling within the same broad C-C environment?
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
How do you interpret a carbon NMR spectrum?
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
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
53
Predict the number of peaks in its carbon-13 NMR spectrum and the **approximate** chemical shift for each peak
**5 peaks**
1: δ = 0-50ppm
2,3,4,5: δ = 110-160ppm
54
How do you know peaks 2 and 3 are labelled that way round?
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
What 4 pieces of information does proton NMR reveal about the molecule?
* 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
What is the chemical shift range of Proton NMR?
0-12ppm
57
What are the 4 **broad** types of hydrogen atom that absorb over different chemical shift ranges in proton NMR?
* **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
What other factors can affect the chemical shift values in NMR other than chemical environment?
* Interactions with solvent
* Concentration
* Substituents
59
Explain the chemical environments of a hydrogen atom in NMR spectroscopy:
* 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
Identify the number of chemical environments for hydrogen in butanoic acid
## Footnote
Explain your answer
**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
Identify the number of chemical environments for hydrogen in butanedioic acid
**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
How can you determine the relative number of each type of proton environment in proton NMR?
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
What is spin-spin coupling?
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
What is the n+1 rule?
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
What does spin-spin coupling allow you to determine?
The total number of hydrogen atoms on the immediately adjacent carbon atoms
66
In a proton NMR spectrum, what is this splitting pattern called and how many hydrogen atoms are there on adjacent carbon atoms?
* Singlet
* 0 adjacent hydrogens
67
In a proton NMR spectrum, what is this splitting pattern called and how many hydrogen atoms are there on adjacent carbon atoms?
* Doublet
* 1 adjacent hydrogen
68
In a proton NMR spectrum, what is this splitting pattern called and how many hydrogen atoms are there on adjacent carbon atoms?
* Triplet
* 2 adjacent hydrogens
69
In a proton NMR spectrum, what is this splitting pattern called and how many hydrogen atoms are there on adjacent carbon atoms?
* Quartet
* 3 adjacent hydrogens
70
In a proton NMR spectrum, what is this splitting pattern called and how many hydrogen atoms are there on adjacent carbon atoms?
* Multiplets
* > 3 adjacent hydrogens (4 or more)
71
What is the relative peak areas within a singlet?
1
72
What is the relative peak areas within a doublet?
1:1
73
What is the relative peak areas within a triplet?
1:2:1
74
What is the relative peak areas within a quartet?
1:3:3:1
75
What determines the relative peak areas within spin-spin coupling sub-peaks?
Pascals triangle
76
What would the NMR splitting pattern of an aromatic proton look like?
It would form 1 or more multiplets
77
What is important to remember about spin-spin coupling?
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
Explain how -OH and -NH groups are handled in proton NMR spectroscopy
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
What are the problems with -OH and -NH groups in proton NMR spectroscopy?
* -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
How do you interpret a proton NMR spectrum?
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
Determine the structure of this molecule
82
Determine the structure of this molecule
83
Predict the **proton** NMR spectra of 2-hydroxypropanoic acid
84
Predict the **carbon-13** NMR spectra of 2-hydroxypropanoic acid
85
What is the typical sequence for identification of a molecule?
**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
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
87
Why is TMS (Tetramethylsilane) used as the standard in Spectroscopy?
* 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
How does the distance from a strongly electronegative atom affect the chemical shift?
The closer the atom is to the electronegative atom, the greater the chemical shift
89
What does the number of signals in NMR mean for carbon-13 and Hydrogen-1?
Same for both:
* One signal for each set of equivalent Hydrogen-1 or Carbon-13 atoms
90
What does the position of signals in NMR mean for carbon-13 and Hydrogen-1?
Same for both:
* The position of the signals indicate the chemical environment of the Hydrogen-1 or Carbon-13 atoms
91
What does the relative size of signals in NMR mean for carbon-13 and Hydrogen-1?
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
What does the splitting of signals in NMR mean for carbon-13 and Hydrogen-1?
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
What is the line drawn along the bottom of a TLC plate called?
The **base** line
