3.4 organic analysis Flashcards
How to distinguish between primary/secondary and tertiary alcohols?
Add acidified potassium dichromate (K2Cr2O7) to alcohol and gently heat in water bath
Primary and secondary go orange to green but tertiary stays orange
How to distinguish between primary and secondary alcohols?
1) oxidise alcohol under reflux and if tests positive for carboxylic acid it’s primary
2) oxidise alcohol under distillation conditions and of tests positive for aldehyde it’s primary
3) oxidise alcohol under reflux or distillation and if tests positive for ketone it’s secondary
how to distinguish between aldehyde and ketone?
use fehlings/ benedicts solution or tollens reagent
what are fehlings/ benedicts? how to use? results?
fehlings- blue solution of complexed copper (ii) ions dissolved in sodium hydroxide
benedicts- same but dissolved in sodium carbonate
add either to sample and gently heat in water bath
aldehyde- brick red precipitate of Cu2O
ketone- no visible change
equation for using fehllings/ benedicts
2Cu^2+(ag) + 2OH^-(aq) + 2e- -> Cu2O(s) + H2O
what is tollens reagent? how do you make it?
[Ag(NH3)2] +
2cm3 of 0.1 moldm-3 silver nitrate
add drops of dilute sodium hydroxide and a light brown precipitate forms
add drops of dilute ammonia until precipitate dissolves
results of tollens reagent
aldehyde- silver mirror forms as silver ions are reduced
ketone- no reaction
test for carboxylic acid
add sodium carbonate and it will effervesce. bubble the gas through limewater- it will turn cloudy if CO2 produced. but it gives positive result for all acids so must know one of the things youre testing is a carboxylic acid
test for alkenes
testing for unsaturation
add bromine water and shake
alkene: orange-> colourless
How does IR spectroscopy work?
Beam of IR radiation passed through sample and absorbed by covalent bonds which increases their vibrational energy
Bonds between diff atoms absorb diff frequencies (same as vibration) and same bond in diff places absorb diff frequencies
How to work out wave number
1/ wavelength
What does infrared spectrometer produce?
What can we use it for?
Graph showing frequencies of radiation the bonds absorb
Can use to identify functional groups- peaks show where radiation is absorbed
What does C=O bond look like?
Sharp and strong peak
What does O-H acid bond look like?
Broad
Due to H binding
What does O-H bond alcohol look like?
Rounded
Due to H bonding
What is the fingerprint region?
1000-1500 cm-1
Unique to particular compounds
Can check against data base to identify
How can we use infrared spec to assess purity?
Impurities produce extra peaks in fingerprint region
Why are CO2, H2O and CH4 greenhouse gases?
Sun emits uv/ visible radiation which is absorbed by earths surface and re emitted as IR. CO2, H2O, CH4 have bonds which absorb lots of IR
More IR absorbed leads to global warming
Uses of IR?
Forensic science
Monitor degree of unsaturation in polymers
Quality control of perfume
Drug analysis
Test breath of drivers for alcohol
How can we use m/z value to identify organic compounds?
Mr is shown by molecular ion peak
M/z value equal to mr
So can use m/z value to identify
Eg if M/z is 72, this is pentane (mr of 72)
What is high resolution mass spectrometry?
Measure atomic or molecular masses w extreme accuracy
Useful for identifying compounds that appear to have same mr when rounded to nearest whole number
practicals in this topic
tests for functional groups
what is the test for alcohols?
add sodium metal and it will effervesce
