organic analysis

Question 1

how do you test for alkenes

Answer 1

shake with bromine water

result = bromine water decolourised from orange to colourless

Question 2

how do you test for haloalkanes

Answer 2

add NaOH (aq) and warm, acidify with HNO3, add AgNO3

result: precipitate of AgX

Cl=white
Br=cream
I=yellow

Question 3

how do you test for alcohols

Answer 3

add acidified K2Cr2O7 and heat

colour change from orange to green for primary and secondary alcohols

Question 4

how do you test for aldehydes

Answer 4

1. warm with Fehlings
2. warm with Tollens

Question 5

how do you test for carboxylic acids

Answer 5

add Na2CO3

result CO2 given off = effervescence

Question 6

what is mass spectrometry

Answer 6

used to find the relative molecular masses of organic compounds

compound is dissolved in solution, ionised by a high voltage supply (to mostly 1
+ ions), accelerated by a negatively charged plate, becomes a beam of ionised molecules, reach detector and cause a current to flow. time of flight used to work out m/z value and plot graph

Question 7

what does the x axis show on a mass spectrum

Answer 7

show m/z value (mass / ionic charge)
since most ions are +1 this effectively shows Mr

Question 8

why are there multiple peaks when molecules are put into a mass spectrometer

Answer 8

molecular ion is shown as a peak, but will also fragment into smaller molecules, so these peaks are shown as well

also, due to isotopes of atoms, different peaks may be seen

Question 9

how does gas chromatography work and what does it tell you

Answer 9

stream of gas carries a mixture of vapours through a column packed with solids, different compounds move through at different speeds, so they are separated

the amount of each compound can then be measured

Question 10

what does GCMS stand for

Answer 10

gas chromatography linked to mass spectrometry

Question 11

what might GCMS be used for

Answer 11

powerful chemical analysis - forensic work, measuring water pollution, drug testing on athletes, racehorses

Question 12

what is high resolution mass spectrometry

Answer 12

gives Mr to 3 dp or 4 dp

Question 13

what does high resolution mass spectrometry allow

Answer 13

distinguish between compounds that have same Mr to nearest whole number, but are made up of different atoms and therefore have different values of Mr to 3dp

Question 14

why do atoms and chemical bonds adsorb infrared radiation

Answer 14

constantly vibrating - they can absorb infrared radiation that is the same frequency as their frequency of vibration

Question 15

what effect does a stronger bond have on the frequency of vibration

Answer 15

vibrate faster (with higher frequency)

Question 16

what effect do heavier atoms have on the frequency of vibrations

Answer 16

vibrate slower (lower frequency)

Question 17

how does infrared spectroscopy work

Answer 17

every bond has a unique vibration frequency in the infrared region of the EM spectrum

bonds adsorb radiation that has same frequency as their frequency of vibration

infrared radiation emerged from a sample is missing the frequencies that have been absorbed - this information can be used to identify the compound’s functional group

Question 18

what happens inside an infrared spectrometer

Answer 18

beam of infrared radiation with a range of frequencies is passed through the sample

radiation that emerges is missing frequencies that have been absorbed by the bonds in the sample

graph is plotted of intensity against frequency of radiation

Question 19

what do the troughs on an infrared spectrum show

Answer 19

frequencies where radiation has been absorbed - match to table to find out which bonds they represent

Question 20

what is the fingerprint region

Answer 20

area of the infrared spectrum below wavelength of 1500 cm-1

many peaks, caused by complex vibrations of the whole molecule. unique to every compound, so can be used to identify compounds