Module 6.3

Question 1

What is qualitative analysis

Answer 1

It is an observable change and does not involve observations using numerical values .

Question 2

how to identify the functional group unsaturation (alkenes )

Answer 2

-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 .

Question 3

how to identify the functional group for haloalkanes

Answer 3

-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 .

Question 4

how to identify the functional group of a carbonyl .

Answer 4

-Reagents ; Brady’s reagent (2,4DNP)
Observations ; a yellow or orange ppt is formed when a ketone or aldehyde is present .

Question 5

how to identify the functional group of aldehydes

Answer 5

-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 .

Question 6

How to identify aliphatic carboxylic acids

Answer 6

-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 .

Question 7

How to identify alcohols

Answer 7

-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 .

Question 8

What is a TMS

Answer 8

it is an internal standard for both carbon and proton NMR .

Question 9

What is a chemical shift

Answer 9

it is the scale that compared the frequency of NMR absorption with the frequency of the reference peak of TMS .

Question 10

What is a deuterium

Answer 10

it is an isotope of hydrogen and does not produce a signal in the proton NMR spectrum .

Question 11

What is a nuclear magnetic resonance

Answer 11

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 .

Question 12

How can nmr work for 1H and carbon 13

Answer 12

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 .

Question 13

how does figure 2 on page 187 show

Answer 13

-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 .

Question 14

What is the internal standard of NMR

Answer 14

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 .

Question 15

Why is TMS a good internal standard (1)

Answer 15

-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 .

Question 16

Why is TMS a good internal standard (2)

Answer 16

-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

Question 17

What is a chemical shift
check figure 4 and figure 5 on page 188

Answer 17

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

Question 18

the more similar the protons and carbon atoms are to the symmetrical non-polar TMS atoms , what happens ?

Answer 18

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 .

Question 19

What solvents are used in ,NMR spectroscopy

Answer 19

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 .

Answer 20

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 .

Question 21

when is the deuterium isotope used

Answer 21

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 .

Question 22

figure 6 page 188

Answer 22

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 .

Question 23

How is carbon 13 identified in nmr spectroscopy

Answer 23

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 .

Question 24

How is TMS used

Answer 24

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 .

Question 25

How are carbon atoms detected in the NMR spectrometer

Answer 25

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 .

Question 26

How do you interpret carbon 13 nmr spectra

Answer 26

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 .

Question 27

INTERPRET PROPAN-1-OL (1)

Answer 27

- propan-1-ol has the structural formula CH3CH2CH2OH It contains three carbon atoms all in different environments . ACCOUTNING FOR THREE LEAJS .

Question 28

Interpreting propan-1-OL number (2)

Answer 28

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 .

Question 29

Interpreting propan-1xOL part three

Answer 29

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 .

Question 30

Interpreting propan 2ol nmr spectrum

Answer 30

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.

Question 31

How do you determine the structure of a compound with carbon 13

Answer 31

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 .

Question 32

Do the worked example of page 191

Answer 32

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.

Question 33

Worked example 4 191…

Answer 33

…

Question 34

Equivalent protons

Answer 34

Are hydrogen atoms bonded to the same atoms that therefore experience the same magnetic field in the NMR spectrometer

Question 35

The most common hydrogen isotope that can be detected in proton nmr spectroscopy is what ?

Answer 35

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 .

Question 36

What is TMS

Answer 36

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 .

Question 37

Difference between 13C NMR and proton NMR spectroscopy

Answer 37

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 .

Question 38

How to do the proton NMR for ethanal

Answer 38

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.

Question 39

Check the integration on page 194

Answer 39

…

Question 40

Learning tip for proton nmr

Answer 40

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 .

Question 41

What is Spin spin coupling

Answer 41

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 .

Question 42

What happens when an organic chemical is put into the NMR spectrometer (magnetic field )

Answer 42

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 .

Question 43

How can the splitting pattern be predicted

Answer 43

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 ,

Question 44

The different splitting patterns are summarised in table 1

Answer 44

195

Question 45

Page 192 - example 2

Answer 45

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 .

Question 46

Check learning tip in page 195

Answer 46

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Question 47

Why is it difficult do identify -OH, amino groups , NH

Answer 47

- peaks can be appear over a wide range of different chemicals shifts values , depending on the solvent used and the concentrations of the sample ,

Question 48

Why is it difficult do identify -OH, amino groups , NH 2

Answer 48

Signals are often broad Usually no splitting patterns

Question 49

Why is it difficult do identify -OH, amino groups , NH 3

Answer 49

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.

Question 50

How can we then filter out absorptions due to OH and NH protons in

Answer 50

Deuterium oxide D2O

Question 51

Use of D20 (1)and how is it different to H2o

Answer 51

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 .

Question 52

Water and heavy water have different physical properties give example

Answer 52

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.

Question 53

D20 is used by following these stages (2)

Answer 53

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 .

Question 54

D20 is used by following gene stages (3)

Answer 54

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

Answer 55

p

Question 57

What happens to when D is added to ethanol CH3CH2OH deteriorate exchanges with the OH proton

Answer 57

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 .

Question 58

Splitting from OH and NH protons are nit split . An OH or NH peak usually shows as a singlet , whuch may be broad .n

Answer 58

… 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 .

Question 59

Are protons on adjacent carbon atoms split by OH and NH

Answer 59

No just ignore the OhpH and NH

Question 60

How is ethanol split CH3

Answer 60

Check bullet split into a triplet

Question 61

How is CH2 split in ethanol

Answer 61

Split into a quarter by the adjacent CH3 , IGNORE OH NIT DPLIT INTO A SINGAL.

Question 62

Do worked example on page 198

Answer 62

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Question 63

Mass spectrometry

Answer 63

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 .

Question 64

Mass spectrometry (2)

Answer 64

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 .

Question 65

Infrared Spectrsocpy

Answer 65

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 .

Question 66

What does nmr spectroscopy give information about

Answer 66

The number snd types of carbon environments ina. Molecule

Question 67

What does proton nmr spectroscopy give information about

Answer 67

About the number of each type of proton environment and the number of protons on adjacent carbon atoms .

Question 68

Number of peaks in Carbon 13 NMR

Answer 68

Number of types of C

Question 69

Chemical shift of carbon 13

Answer 69

S of range of 0-220

Question 70

Number of peaks in proton nmr

Answer 70

Number of types of H

Question 71

Peak area for carbon 13 NMR

Answer 71

Relative number of carbon atoms of each type

Question 72

Peak area for proton NMR

Answer 72

Relative number of protons of each type

Question 73

Carbon -13 nmr spin spin coupling

Answer 73

N/A

Question 74

Spin spin coupling proton nmr

Answer 74

Number of protons on adjacent carbon atoms

Question 75

Worked example 200-201

Answer 75

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Question 76

What is the stationary phase in chromatography

Answer 76

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

Question 77

What is the mobile phase in. Chromatography

Answer 77

Moves in a definite direction . This could be an inert gas as. I. GC or a liquid solvent as in TLC .

Question 78

What decided how far the substance travels up the chromatography paper

Answer 78

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 ,

Question 79

One way thin layer chromatography TLC

Answer 79

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 ,

Question 80

How to work out rf value

Answer 80

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 ,

Question 81

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

Answer 81

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 ,

Question 82

Learning tip about RF values

Answer 82

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

Question 83

What is the gas chromatography

Answer 83

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 .

Question 84

What is the stationary phase in gas chromatography

Answer 84

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 .

Question 85

Results for GC (1)

Answer 85

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 ,

Question 86

Results for GC(2)

Answer 86

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 ,