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 .