Module 6.3 Flashcards
What is qualitative analysis
It is an observable change and does not involve observations using numerical values .
how to identify the functional group unsaturation (alkenes )
-Reagents ; Bromine water
Observations ; Add a few drops of bromine water to the sample and shake well . If the bromine water decolourises then the compound was unsaturated .
how to identify the functional group for haloalkanes
-Reagents ; aqueous silver nitrate and ethanol .
Observations ; If a precipitate forms then the halogen can be inferred . White precipitate is silver chloride and implies a chloroalkane .
-Cream ppt is a silver bromide and implies a bromoalkane .
-Yellow ppt , is silver iodide and implies an iadoalkane .
how to identify the functional group of a carbonyl .
-Reagents ; Brady’s reagent (2,4DNP)
Observations ; a yellow or orange ppt is formed when a ketone or aldehyde is present .
how to identify the functional group of aldehydes
-Reagents ; Fehling’s solution , Tollens reagent , acidified potassium dichromate .
Observations ; With Fehling solution the blue solution forms a dark red pptt . With Tollens reagent , a silver mirror is produced , With acidified potassium dichromate ,t he orange solution turns green .
How to identify aliphatic carboxylic acids
-Reagents ; Universal indicator or pH probe , reactive metal , e.g magnesium , metal carbonate e.g calcium carbonate .
Observations ; check that the pH is that for a weak acid , no effervescence with the carbonate ion .
How to identify alcohols
-Reagents ; Acidified potassium dichromate
Observations ; Colour change from orange to green , as first an aldehyde is formed and then a carboxylic acid . Secondary alcohols will be oxidised to a ketone but not a carboxylic acid and will also produce this colour change .
-Tertiary alcohols cannot be oxidised in this way and there is not an observable change .
What is a TMS
it is an internal standard for both carbon and proton NMR .
What is a chemical shift
it is the scale that compared the frequency of NMR absorption with the frequency of the reference peak of TMS .
What is a deuterium
it is an isotope of hydrogen and does not produce a signal in the proton NMR spectrum .
What is a nuclear magnetic resonance
nuclear magnetic resonance is a non - destructive , analytical technique . The spectrum can confirm the formula mass of an organic molecule and gives clue tot he structure of the chemical .
How can nmr work for 1H and carbon 13
As they have a property known as a SPIN .
-In an NMR spectrometer , organic compounds containing these isotopes are put in a large magnetic field .
-The direction of spin , of the nucleons aligns with the direction of the magnetic field .
-Energy int he form of radio waves is used to force the spins to flip and change direction .
-The energy required to change the alignment depends on the environment that the atom is in and can be compared to a standard .
how does figure 2 on page 187 show
-Atoms with spin will align themselves to a magnetic field , will align themselves to a magnetic field .
-Energy can be used to flip the alignment this information can be used to produce an NMR spectrum .
What is the internal standard of NMR
Internal standard of NMR spectrometer to compare the values from the sample . In both proton and carbon , NMR , tetramethyl silane is used . This chemical is also known as TMS , and has the formula (CH3)4si .
-The spectrometer makes a relative scale (chemical shift ) for the x-axis of the spectrum . The TMS reference peak is given the value 0 ppm and all other peaks are placed on the x axis compared to it .
Why is TMS a good internal standard (1)
-It contains both carbon and hydrogen so can be used for carbon and proton NMR .
-It produces one sharp signal as hydrogen atoms are in a single environment .
- As there is only one carbon atom , this will also produce one signal in carbon-13 NMR .
Why is TMS a good internal standard (2)
-It is non-toxic
-It has a low boiling point and is volatile so it can be easily removed from the sample although the sample is rarely recovered .
-It is inert so unlikely to react with the chemical that is being investigated
What is a chemical shift
check figure 4 and figure 5 on page 188
Chemical shift is the x-axis variable on an NMR spectrum . It is given the symbol delta and is measured in parts per million .
-TMS , which is the internal standard is given 0 ppm on the far right of the axis . In proton NMR , the values are from 0 to about 15 ppm , but int he carbon NMR 13c the dcal eis a bout form 0 to 220
the more similar the protons and carbon atoms are to the symmetrical non-polar TMS atoms , what happens ?
The lower the chemical shift values . So , a proton , in ethane would have a lower chemical shift , but as proton on the end of the alcohol group in ethanol would have a very high chemical shift .
What solvents are used in ,NMR spectroscopy
Most NMR spectroscopy analysis is achieved by first dissolving the molecule in a solvent .
-As this technique is mainly used to investigate organic compounds , organic solvents like benzene trichloromethane are needed . However , these chemicals contain carbon and hydrogen and they would produce signals that would swamp the spectrum and make it unreadable .
Although hydrogen has three isotopoes , only H1 , has the property of spin and is active in NMR .
-So , we use organic solvents that contain hydrogen isotopes that are not NMR active .
when is the deuterium isotope used
it is used H2 , and is shown by using a D in the formula . As deuterium has a greater atomic mass compared to the more abundant H1 isotope those deuterated solvents are often called heavy . Therefore trichloromethane CHCL3 (Also known as chloroform ) would be heavy trichloromethane CDCL3 .
figure 6 page 188
NMR samples are prepared using a deuterated solvent and a tiny amount of TMS . Both the solvents and TMS , can be evaporated off after the analysis and this recovers the originals ample . Sample recovery is o0nly done if the yield of product was very small or further analysis is required .
How is carbon 13 identified in nmr spectroscopy
Carbon has three isotopes but on 13C has the property of spin and can be detececged in an NzmR spectrometer . The NMR spectrum , gives information about the number of carbon environments but not the ratio of atoms in each environment .
How is TMS used
TMS is used as thr internal standard and the chemical shift has large range 0-220 ppm . There is one signal as there is only one atom found .
How are carbon atoms detected in the NMR spectrometer
In a sample , carbon atoms m in the same environment are known as equivalents . They are bonded to the same atoms and feel the same magnetic field in the NMR spectrometer .
How do you interpret carbon 13 nmr spectra
On thr back of your data sheet in your examination , you are given a chart That shows you the range of chemical shifts for carbon atoms in different environments . This can be used to give clues to the structure of the organic chemical .
INTERPRET PROPAN-1-OL (1)
- propan-1-ol has the structural formula CH3CH2CH2OH
It contains three carbon atoms all in different environments . ACCOUTNING FOR THREE LEAJS .
Interpreting propan-1-OL number (2)
The carbon atom. Attached to alcohol functional group will have the largest chemical shifts for carbon, as it is the most different from the internal standard TMS . Using the chart on the data sheet we can see this carbon atom should have a shift of between 50 and 70 ppm . Looking at the spectrum we can see the actual value is 64 ppm and within thus range .
Interpreting propan-1xOL part three
The other two carbon carbon environments are more similar to the carbon atom in TMS and will have a shift of between 10 and t5 ppm . Looking at the spectrum they have a value of 2u ppm for the for the CH2 and q5 ppm for the CH3 carbon . This is due to the CH3 groups giving signals at lower frequency than ch2 groups .
Interpreting propan 2ol nmr spectrum
Propan2ol has the structural formula CH3 CHOHCh3. Although , it contains three carbon atoms , due to the plane of symmetry , it only has two carbon environments This means there will be two peaks , as there are two equivalent carbon toms l. These are drawn in a 2:1 ratio that shows peak height corresponding to the carbon atoms .
CHECK FIGURE 4 of 190.
How do you determine the structure of a compound with carbon 13
The carbon 13 slecfrum along with a molecular formula can be used to determine the structure of compound . It is important to analyse the spectrum closely , considering the number of carbon environment and using data sheet to suggest the functional group that the carbon atoms may be joined to .
Do the worked example of page 191
In the question we were told that the compounds has a carbonyl group . Using the molecular formula we also know there are three carbon atoms so it could be propanol , or propanone . The spectrum , shows three signals senowing all the carbon atoms are in different environments . So thr chemical must be propane.
Worked example 4 191…
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Equivalent protons
Are hydrogen atoms bonded to the same atoms that therefore experience the same magnetic field in the NMR spectrometer
The most common hydrogen isotope that can be detected in proton nmr spectroscopy is what ?
1H , it has the property of spin and. Can be detected in an NMR machine . The NMR spectrum gives information about thr number of proton environments and the area under each peak is the ratio in of options in each environment;5 .
What is TMS
It is used as the internal standard as this molecule has 12 equivalent protons , and gives an intense single peak , whuch can be assigned as 0ppm . The range of chemical shifts for protons NMR is much smaller than for carbonNMR .
Difference between 13C NMR and proton NMR spectroscopy
Proton NMR spectro is much smaller sample size can be used to compare 13 C NMR spectroscopy. There will be a signal for each environment that a hydrogen atom is found in . The area under each signal gives the ratio of atoms in each environment . This is often show as an integration trace added by the NMR machine directly in the spectrum .
How to do the proton NMR for ethanal
Although there are four hydrogen atoms , there are actually only two proton environments . So the proton NMR spectrum will contain two signals . One will be close. To TMS from the CH3 protons and one signal will have a hugger chemicalshuft from the CHO proton.
Check the integration on page 194
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Learning tip for proton nmr
On the back of the data sheet , is examination we are given a chart whuch shows you yhe range of chemical shifts for for hydrogen atoms in different envurienmtns . This can be used to give clues to the structure if the organic chemicals .
What is Spin spin coupling
The High resolution NMR spectra it is possible to see that some signals are actually made from a cluster of peaks . This is known as splitting pattern caused by spin spin coupling of neighbouring protons .
What happens when an organic chemical is put into the NMR spectrometer (magnetic field )
It experiences a large magnetic field . Hwiever , thr protons on the neighbouring carbon atoms also affect the magnetic field . This affects the alignment of the protons and causes a point in the signal . Different number of adjacent proteins will cause different number of splits .
How can the splitting pattern be predicted
Number of peaks in the splitting pattern = number of protons on adjacent carbon atoms in different +q.
Often elxresed as the n +1 rule where n is the number of equivalent adjacent hydrogen atoms on a neighbouring carbon ,
The different splitting patterns are summarised in table 1
195
Page 192 - example 2
CHECK FIGURE 4
SINGLET is produced by -OCH3 as there are no. Eight nroung carbon atoms and hence no adjacent protons so n =0 and n+1 =1 hence a singlet .
Check how the quartet and sunbelt are produced .
Check learning tip in page 195
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Why is it difficult do identify -OH, amino groups , NH
- peaks can be appear over a wide range of different chemicals shifts values , depending on the solvent used and the concentrations of the sample ,
Why is it difficult do identify -OH, amino groups , NH 2
Signals are often broad
Usually no splitting patterns
Why is it difficult do identify -OH, amino groups , NH 3
These factors not only make it difficult to assign absorption to an OH or NH proton m but may also lead to confusion with other peaks .
Consequently an -OH or -NH absorption may mistakenly be assigned to another type of proton on the basis of an unpredictable chemical shift values.
How can we then filter out absorptions due to OH and NH protons in
Deuterium oxide D2O
Use of D20 (1)and how is it different to H2o
In thr previous topic we have met the use of CDCL3 , as a solvent in NMR spectroscopy.
Deuterium oxide is the same chemical compound as water, H2O put the isotope deteriorate come up to HOD, in the place of hate, the common isotope of of hydrogen . This is why the common name for deuterium oxide is heavy water .
Water and heavy water have different physical properties give example
D2O has an Mr value of 20.0 and has a density kf 1.1@gcm-3 so it is truly heavier than normal m water . Do you two are not at 3.8° and boils 101.4°. Most importantly, for animal spectroscopy, did you cheerio nucleus does not produce a single signal in a proton. I don’t remember that’s not number of the plant is needed for an animal.
D20 is used by following these stages (2)
First a proton NMR spectrum is run
- A small amount of D20 is added to the sample solution snd the mixture is shaken vigorish,y as. It is immiscbinel .
D20 is used by following gene stages (3)
Then a second proton NMR spectrum is run an any peak de to the ob or no disappears thus works bceasye the deuterium exchanges with the h present in oh and nh. proton disapEAR
p
What happens to when D is added to ethanol CH3CH2OH deteriorate exchanges with the OH proton
What happens when D2O is added to ethanol to ethanol the CH3CH2OH deuterium exchanges with the -OH proton
Check 197 for becomes CH3ch2OD , as there is no OH proton there will obviously be no OH signal .
Check figure 1 on page 197 .
Splitting from OH and NH protons are nit split . An OH or NH peak usually shows as a singlet , whuch may be broad .n
… it is difficult to get solvents that are absolutely dry - traces of water in thr solvent form hydrogen bonds with OH and NH protons in the compound being analysed this results in the broadening of OH and NH signals .
Are protons on adjacent carbon atoms split by OH and NH
No just ignore the OhpH and NH
How is ethanol split CH3
Check bullet split into a triplet
How is CH2 split in ethanol
Split into a quarter by the adjacent CH3 ,
IGNORE OH NIT DPLIT INTO A SINGAL.
Do worked example on page 198
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Mass spectrometry
Chemical analysis of an unknown compound can provide the percentage by mass of each element . The empirical formulas, can be determined from the results . A mass spectrum gives the relative molecular mass of a compound . Usgung the empirical formula , the molecular formula can then be found .
Mass spectrometry (2)
Unfortunately, the same Mr value could apply to more than one compound with the same molecular formula for example CH3COOH and HCOOHCH3 both have an Mr vale of 60.0
FRAGEMNTATUON patterns also give clues about thr carbon Skelton in the molecules kf a compounds .
Infrared Spectrsocpy
Gives information about the binds present ina. Molecule and the likely functional groups being presenf while being a non destructive technique ,
Different members of a homologous series have the same functional group - for example different alcohols have C-O and O-H absorptions in their IR spectra .
What does nmr spectroscopy give information about
The number snd types of carbon environments ina. Molecule
What does proton nmr spectroscopy give information about
About the number of each type of proton environment and the number of protons on adjacent carbon atoms .
Number of peaks in Carbon 13 NMR
Number of types of C
Chemical shift of carbon 13
S of range of 0-220
Number of peaks in proton nmr
Number of types of H
Peak area for carbon 13 NMR
Relative number of carbon atoms of each type
Peak area for proton NMR
Relative number of protons of each type
Carbon -13 nmr spin spin coupling
N/A
Spin spin coupling proton nmr
Number of protons on adjacent carbon atoms
Worked example 200-201
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What is the stationary phase in chromatography
Fixed in place .
This could be liquid
In gC systems , where the mixture components have different attractions to each phase . Or it foouke be a solid m as in. Thin layer chromatography , where the components of the mixtures are adsorbed by different amounts .l
What is the mobile phase in. Chromatography
Moves in a definite direction . This could be an inert gas as. I. GC or a liquid solvent as in TLC .
What decided how far the substance travels up the chromatography paper
The separation occurs as each chemical in a mixture has different attractions to each phase ,.
-A substance that is very attracted mobile phase but not to the stationary phase will travel a long way up fhe chromatography but a chemical that is feet attracted or reg staruibaey phase will not travel far ,
One way thin layer chromatography TLC
TLC is a quick and easy technique thst can be used to check the purity of a sample or to determine the extent of a chemical reaction .
Styatiowndy pahse is an inert material usually glass covered with an absorbent chemical such as aluminium oxide . The mobile pahse is usually an organic solvent , which will move in one direction , beritclsly up thr plate snd develop the chromatograph ,
How to work out rf value
An rf value is a quantity that shows how dad a chemical has moved up a chromatography compared to thr solvent front . It can be expressed as
RF : distance moved by component / distance moved by solvent ,
Data tables list Rf values for a wide variety of chemicals and this allows the indentificsrion of substances in a sample . Values are different if the pahsssnadenxhanged so if is importantly to what
Use the correct tackle for the mobile and stationary phase in your experiment . Alternatively control spots m can be run on the same TLC plate and a direct comparison can be made ,
Learning tip about RF values
Similar compounds will have similar Rf values and also chemicals that arte very attracted to the mobile phase may not be separated using this method
What is the gas chromatography
The GC technique is used to separate volatile components in a mixture . The gas chromatograph is often connected to a mass spectrometer so that the separate parts of the mixture can undergo further analysis .
What is the stationary phase in gas chromatography
The stationary pahse is solid or liquid coating on a coiled tube , this coating is usually an organic hydrocarbon , with a high boiling point . The mobile phases is an unreactive carrier gas , such helium or nitrogen .
Results for GC (1)
GC will produce a chart of absorption against time . The x axis is the retention time of a chemical or thr time t takes for a component to pass from the colours intel to the detector . This value can be compared to known values to identify the component ..
As with TLC retention times , for the same chemical , it will be different when different phases are used. So it is imoorrsnt to compare with the correct data table ,
Results for GC(2)
The area under the absorption peaks is proportional of the concentration of each component . The greater the area under the peak , the larger the amount f that chemical in the original mixture ,
