topic 7 - analytical techniques

Question 1

what are the 5 organic liquid than can be identified (1)

Answer 1

ethanol , cyclohexane, cyclohexene, propanoic acid, 1-chlorobutane

Question 2

what is the first question to identify which organic liquid it is

Answer 2

1- is it water soluble?
yes = ethanol and propanoic acid
no = cyclohexane, cyclohexene and 1-cholorobutane

Question 3

how to identify between ethanol and propanoic acid

Answer 3

add solid sodium carbonate
and see if there is fizzing
yes = propanoic acid
no = ethanol

Question 4

how to confirm it is ethanol

Answer 4

heat with acidified potassium dichromate (or acidified potassium manganate)
orange to green
- because will undergo oxidation

Question 5

how to identify cyclohexene between cyclohexane, cyclohexene and 1-cholorobutane

Answer 5

first add bromine water if it goes from orange to colourless ( undergoes electrophilic addition ) then it is cyclohexene

Question 6

how to identify between cyclohexane and 1-cholorobutane

Answer 6

• heat with aqueous sodium hydroxide
• excess nitric acid to neutralize sodium hydroxide
  add silver nitrate
• if there is white precipitate then it is 1-chlorobutane
  -if not then it is cyclohexane

Question 7

what are the 5 organic liquid than can be identified (1)

Answer 7

• propan-1-ol
• propan-2-ol
• propanal
• propanone
• propanoic acid

Question 8

what is the first step identifying between propan-1-ol, propan-2-ol,propanal, propanone and propanoic acid

Answer 8

see which ones will undergo oxidation
- heat with acidified potasium dichromate
- orange to green colour change = propan-1-ol, propan-2-ol and propanal
- no = propanone and propanoic acid

Question 9

how to identify the aldehyde between propan-1-ol, propan-2-ol and propanal

Answer 9

• heat with tollens reagant
• if there is a silver mirror then it is propanal
  no= propan-1-ol, propan-2-ol

Question 10

how to identify between propan-1-ol and propan-2-ol

Answer 10

• heat both with pottasium dichromate
• propan-1-ol –> carboxylic acid
• propan-2-ol –> ketone
  add sodium carbonate
  if there is fizzing then it is propan-1-ol, no fizzing = propan-2-ol

Question 11

how to identify between propanone and propanoic acid

Answer 11

add sodium carbonate
fizzing = propanoic acid
no fizzing = propanone

Question 12

what is the use of fragmentation

Answer 12

• gives important information about an unknown organic molecule
• compound ‘fragments’ during ionisation process to give multiple peaks
• these fragments can help deduce the structure of the molecule

Question 13

how are halo-organic molecules different

Answer 13

Cl and Br containing molecules have multiple M+ peaks and fragments due to the fact that they have isotopes

Question 14

what are three ways to identify organic molecules

Answer 14

1- test tube reactions
2- fragmentation ( mass spec )
3- infrared spectroscopy

Question 15

how does infrared spectroscopy work

Answer 15

• covalent bonds absorb I.R. radiation
• different covalent bonds absorb different wavelengths of I.R.
• above 1500 cm-1 = fuctional group identification
• below 1500 cm-1 = fingerprinting

Question 16

advantages of infrared spectroscopy

Answer 16

• will tell you the type of bonds present in a molecule ( functional group )
• the absence of peaks helps eliminate other functional groups

Question 17

disadvantages of infrared spectroscopy

Answer 17

• wont tell you the size of the molecule ( mass spec needed for this )