Chapter 29 - Chromatography and Spectroscopy Flashcards
Why is chromatography used ?
It is used to separate individual components from a mixture of substances
What are the two phases involved in chromatography ?
- Stationary phase
- Mobile phase
What is the stationary phase ?
It does not move and is normally a solid or a liquid that is supported on a solid
What is the mobile phase ?
It does move and is normally a liquid or a gas
Where can chromatography be used ?
Can be used in the analysis of drugs, plastics, flavourings, air samples and in forensic sciences
What is TLC ?
It is a quick and inexpensive analytical technique that indicates how many components there are in a mixture
What does TLC use ?
It uses a TLC plate which is usually a plastic sheet or glass, coated with a thin layer of a solid adsorbent substance
In TLC what is normally the adsorbent substance ?
Silica
In TLC what is the stationary phase ?
The adsorbent substance is normally the stationary phase
What is adsorption ?
It is the process by which the solid silica holds the different substances in the mixture to its surface
How is separation achieved in TLC ?
- 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
How are Thin Layer Chromatograms analysed ?
By calculating the value for the retention factor for each component
What is the Rf value ?
Distance moved by the component / distance moved by the solvent front
How can we know what a substance is given we know its experimental Rf value ?
- 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
When is a gas chromatogram useful ?
When separating and identifying volatile organic compounds present in a mixture
What is the stationary phase in gas chromatography ?
High boiling point liquid absorbed onto an inert solid support
What is the mobile phase in gas chromatography ?
Inert carrier gas such as helium or neon
What happens in gas chromatography ?
- 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
Why do components slow down in gas chromatography ?
They slow down as they interact with the liquid stationary phase inside the column
What is the relationship between the solubility of the component and how quick it moves ?
The more soluble the component is in the liquid stationary phase, the slower it moves through the capillary column
What are the components separated depending on ?
Their solubility in the liquid stationary phase
What is retention time ?
The time taken for each component to travel through the column
What is each component displayed as on a gas chromatogram ?
As a peak
What info can be obtained from a gas chromatogram ?
- 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
What is the test for alkenes ?
- Add bromine water drop wise
- Bromine water decolourises from orange to colourless
What is the test for haloalkanes ?
- Add AgNO3 and ethanol and warm to 50 degrees
- White, cream and yellow precipitates (Cl, Br, I)
What is the test for carbonyls ?
- Add 2,4-DNP
- Should see an orange precipitate
What is the test for aldehydes ?
- Add tollens reagent and warm
- Silver mirror should be seen
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
What is the test for carboxylic acids ?
- Add aqueous sodium carbonate
- Should see effervescence
What is the test for carboxylic acids ?
- Add aqueous sodium carbonate
- Should see effervescence
What does NMR spectroscopy use a combination of ?
A very strong magnetic field and radio frequency radiation
How can the energy of absorption be measured ?
Using a NMR spectrum
What is nuclear spin ?
The spin on the nucleus, just like how electrons have spin
When is nuclear spin significant ?
When there is an odd number of nucleons
What compounds is NMR relevant for ?
- H-1
- C-13
What can NMR spectroscopy be used for ?
To detect isotopes of other elements with odd numbers of nucleons
What is proton NMR ?
Refers to the NMR of H-1 as it is just one single proton in the nucleus
How many spin states does the nucleus have ?
Two spin states
What do the different spin states of the nucleus have ?
They have different energies
How can the nucleus absorb energy ?
With the right combination of a strong magnetic field and radio frequency radiation
What happens when the nucleus absorbs energy ?
It rapidly flips between the two spin states
What is resonance ?
When the nucleus flips between its two spin states
Give an example of where NMR is used every day
MRI scans in hospitals
What do the electrons surrounding the nucleus do ?
They shift the energy and radio frequency needed for NMR to take place
What does chemical shift refer to ?
It refers to the frequency shift that takes place due to electrons surrounding the nucleus
What is TMS ?
- Tetramethylsilane
- It is used as the standard reference chemical against which all chemical shifts are measured
What is the chemical shift value of TMS ?
Has a chemical shift value of 0
What is the amount of chemical shift determined by ?
- The chemical environment
- Especially the presence of nearby electronegative atoms
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
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
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
What happens to the tube in NMR ?
It is spun to even out any imperfections in the magnetic field within the sample
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
How are absorptions shown in NMR ?
They are displayed on a computer screen
How can you recover the sample after NMR ?
Evaporate the solvent
What is a deuterated solvent ?
A solvent that involves deuterium H-2
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
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
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
When can chemical shifts be outside the normal ranges ?
Depending on the solvent, concentration and substituents
What is the chemical environment of a carbon atom determined by ?
It is determined by the position of the atom within the molecule
What is necessary for carbon atoms to have different environments ?
They must be bonded to different atoms or groups of atoms
What do carbon atoms that have different environments also do ?
They will absorb at different chemical shifts
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
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)
When can the chemical shift values differ in proton NMR ?
If the solvent, concentration and substituents are different
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
What will protons do if they are equivalent ?
They will absorb at the same chemical shift, increasing the size of the peak
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
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
What is the integration trace ?
The area under each peak
How is the integration trace shown ?
- As an extra line on the spectrum
- As a printed number of the relative peak areas
What is spin-spin coupling ?
The splitting of a main peak into sub-peaks
What is the n+1 rule ?
The number of sub peaks is one greater than the number of adjacent protons causing the splitting
What can you tell from the spin spin splitting ?
You are seeing the number of hydrogen atoms on the immediately adjacent carbon atom
What is a singlet ?
- Relative peak area is 1
- Single spike
- No H on adjacent atoms
What is a doublet ?
- Relative peak areas are 1:1
- Two spikes
- Adjacent CH
What is a triplet ?
- Relative peak areas are 1:2:1
- Three spikes, middle one twice as large as outer two
- Adjacent CH2
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
What is a heptet ?
- CH(CH3)2
- Seven spikes
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
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
What is proton exchange ?
It is a technique that has been devised to identify OH and NH protons
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
What is the role of D2O in proton exchange ?
It exchanges and replaces the OH and NH protons in the sample with deuterium atoms
Does D2O appear on the spectrum ?
No it does not
What does the disappearance of D2O on the spectrum also mean ?
OH and NH do not appear on the spectrum
