6.3 Analysis Flashcards
Rf value
Comparison between how far a component has moved compared to the solvent in thin layer chromatography
Retention time
Time taken for a component to travel from the inlet to the detector in a gas chromatohraph
Why do different substances travel different amounts in paper chromatography?
- The more polar a compound is the greater affinity it has to the polar mobile phase
- moves further up the paper with it
- the less polar compounds aren’t as attracted and so don’t move as far
What are the components to thin-layer chromatography?
- The stationary phase is a thin inert material, coated in an aDsorbent (taken onto surface) chemical
- the mobile phase is an organic solvent
What are the components to gas chromatography?
Stationary phase is an inert solid or liquid coating on the inside of a thin long coiled tube
-the mobile phase is an unreactive gas (He or N)
How are the results of a gas chromatogram displayed?
x-axis shows retention time
y-axis shows abundance
What is qualtitative analysis?
Observable changes to a reaction/solution, observations not including the use of numerical values
How do you test for Alkenes?
Add a few drops of bromine water, shake, if the solution decolourises then it is an alkene
How do you test for a Haloalkane?
Add aqueous silver nitrate and ethanol, precipitate forms to show presence of halogen:
white precip-chloroalkane
cream precip-bromoalkane
yellow precip-iodoalkane
How do you test for a Carbonyl?
Add brady’s reagent (2-4,DNP), it will turn form a yellow/orange precipitate if the solution contains an aldhyde or ketone
How do you test for an Aldehyde?
Add fehlings solution: dark blue solution produces red precipitate
Tollens reagent: silver mirror forms
What is Tollen’s reagent and how does it work?
It is a solution of silver nitrate and ammonia, the silver nitrate is reduced as the aldehyde is oxidised to a carboxlyic acid, this reduction releases the silver ions which colourise the solution
How do you test for a Carboxylic acid?
Add Universal Indicator, pH colour of a weak acid will show
Adding a metal or metal carbonate will cause effervescence
What is TMS?
An internal standard that generates a reference peak for both Carbon and Proton NMR
What is ‘chemical shift’?
Scale comparing frequency of NMR absorption of compound with the frequency of the absorption of the reference peak
Why is TMS useful/good?
- produces one sharp signal as hydrogen atoms are in a single environment and only one carbon environment as there is only one carbon atom
- It is non toxic
- low boiling point so is easily removed from sample
- is inert so doesn’t interfere with test chemicals
- has both carbon and hydrogen
What can you deduce from a Carbon NMR graph
The shift of peaks can be compared to a data sheet to show what functional group it is, the height of peaks shows how many carbons there are of that environment/ functional group
What are equivalent protons?
Hydrogen atoms bonded to the same atom, they therefore experience the same magnetic field in an NMR spectrometer and so show the same reading
What can you deduce from a Proton NMR graph?
Shift compared to data sheet shows which atom it is bonded to,
Integration trace shows number of hydrogen atoms in particular environment
Split patterns: no. of split peaks -1 = no. of hydrogens on adjacent carbons
How do you identify if a peak is formed by an -OH or -NH group in proton NMR?
Run a NMR test, then add D2O and shake vigorsously, run another NMR test and compare the resulting graphs, the peaks on the first graph that are -OH or -NH groups will have disappeared on the second graph
4 factors to consider when making synthetic routes
No solvent
Non hazardous starting material
Fewer reaction steps
High yield
What do we use when filtering something under reduced pressure?
A Buchner or Hirsh funnel on top of a filter flask
Why is it ideal to use no solvent in a synthetic route?
The products do not have to be separated from the solvent at the end of the reaction
Why is it ideal to use a non-hazardous starting material when designing a synthetic route?
Limits the danger to the chemist and the safety precautions necessary
Why is it ideal to use few reaction steps in a synthetic route?
Saves money on equipment and time as well as producing fewer by-products
Why is it ideal to have a high atomic yield in synthetic routes?
Saves money as largest amount of starting material is made into product
What does Rf stand for?
Retention factor
What is the coating in the long coiled tube in gas chromatography usually?
A hydrocarbon with a high boiling point as we tend to heat the mixture in gas chromatography
How can we analyse gasses after they have been separated in gas chromatography?
We can attach the chromatograph to a mass spectrometer
What does the area under a gas chromatogram represent?
The amount of the component that is present
What must you do to the compounds you are investigating in NMR spectroscopy?
You must dissolve them
If they are organic you should dissolve them in an organic solvent
Why cant we use most organic solvents as solvents in C NMR?
Because most organic solvents contain C13 atoms which would make the spectrums unreadable
What molecule doesnt affect spin?
Deuterium
What is a common solvent used in NMR?
heavy trichloromethane, CDCl3
Bad thing about oxygen and nitrogen peaks on NMR?
- Can appear almost anywhere on spectrum
- BROAD peaks which can be mistake for other peaks
What can we use D2O for?
replaces H in compounds containing -OH or -NH so that they do not appear in spectrum
How do we actually use D2O?
1) run original proton NMR spectrum]
2) Shake sample VIGOROUSLY with D2O as it is immiscible with CDCl3
3) Run the NMR spectrum again and any peaks due to -OH or -NH will have been removed
What splitting pattern do -OH and -NH groups give and why?
Singlets
-the hydrogens on the OH or NH are too far away from other hydrogens to cause splitting patterns
Why are the OH and NH peaks broad?
We cannot remove all traces of water from our solvent
Hydrogens bonds form between water and OH or NH and this broadens the peaks
