3.4 organic analysis

Question 1

How to distinguish between primary/secondary and tertiary alcohols?

Answer 1

Add acidified potassium dichromate to alcohol and gently heat in water bath
Primary and secondary go orange to green but tertiary stays orange

Question 2

How to distinguish between primary and secondary alcohols?

Answer 2

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

Question 3

how to distinguish between aldehyde and ketone?

Answer 3

use fehlings/ benedicts solution or tollens reagent

Question 4

what are fehlings/ benedicts? how to use? results?

Answer 4

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 reaction

Question 5

equation for using fehllings/ benedicts

Answer 5

2Cu^2+(ag) + 2OH^-(aq) + 2e- -> Cu2O(s) + H2O

Question 6

what is tollens reagent? how do you make it?

Answer 6

[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

Question 7

results of tollens reagent

Answer 7

aldehyde- silver mirror forms as silver ions are reduced
ketone- no reaction

Question 8

test for carboxylic acid

Answer 8

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

Question 9

test for alkenes

Answer 9

testing for unsaturation
add bromine water and shake
alkene: orange-> colourless

Question 10

How does IR spectroscopy work?

Answer 10

Beam or IT radiation passed through sample and absorbers 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

Question 11

How to work out wave number

Answer 11

1/ wavelength

Question 12

What does infrared spectrometer produce?
What can we use it for?

Answer 12

Graph showing frequencies of radiation the bonds absorb
Can use to identify functional groups- peaks show where radiation is absorbed

Question 13

What does C=O bond look like?

Answer 13

Sharp and strong peak

Question 14

What does O-H acid bind look like?

Answer 14

Broad
Due to H binding

Question 15

What does O-H bond alcohol look like?

Answer 15

Rounded
Due to H bonding

Question 16

What is the fingerprint region?

Answer 16

1000-1500 cm-1
Unique to particular compounds
Can check against data base to identify

Question 17

How can we use infrared spec to assess purity?

Answer 17

Impurities produce extra peaks in fingerprint region

Question 18

Why are CO2, H2O and CH4 greenhouse gases?

Answer 18

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

Question 19

Uses of IR?

Answer 19

Forensic science
Monitor degree of unsaturation in polymers
Quality control of perfume
Drug analysis
Test breath of drivers for alcohol

Question 20

How can we use m/z value to identify organic compounds?

Answer 20

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)

Question 21

What is high resolution mass spectrometry?

Answer 21

Measure atomic or molecular masses w extreme accuracy
Useful for identifying compounds that appear to have same mr when rounded to nearest whole number

Question 22

practicals in this topic

Answer 22

tests for functional groups

Question 23

what is the test for alcohols?

Answer 23

add sodium metal and it will effervesce