3.3.6 - organic analysis

Question 1

how to distinguish aldehydes and ketone

Answer 1

aldehydes and ketones can be distinguished using oxidising agents

Question 2

why can aldehydes and ketones be distinguished using oxidising agents

Answer 2

aldehydes are easily oxidised but ketones aren’t

Question 3

TESTING FOR ALCOHOLS
step 1

Answer 3

1. add 10 drops of alcohol to 2cm^3 of acidified potassium dichromate solution in a test tube

Question 4

TESTING FOR ALCOHOLS
step 2

Answer 4

2. warm the mixture gently in a hot water bath

Question 5

TESTING FOR ALCOHOLS
step 3

Answer 5

3. watch for a colour change

Question 6

TESTING FOR ALCOHOLS
colour change for primary and secondary alcohols

Answer 6

potassium dichromate(VI) is reduced so colour changes from orange to green
- orange dichromate(VI) ions are reduced
- green chromium(III) ions are formed

Question 7

equation for reduction of potassium dichromate(VI)

Answer 7

(Cr2,O7) 2- + 6e- + 14H+ = (2Cr) 3+ + 7H2O

Question 8

TESTING FOR ALCOHOLS
colour change for tertiary alcohols

Answer 8

no colour change so solution stays orange

Question 9

testing for primary alcohols
(carboxylic acid)

Answer 9

if you oxidise an alcohol under reflux and it tests positive for being a carboxylic acid, then it’s a primary alcohol

Question 10

testing for primary alcohols
(aldehyde)

Answer 10

if you oxidise an alcohol under distillation conditions and it tests positive for being an aldehyde, then its a primary alcohol

Question 11

testing for secondary alcohols

Answer 11

if you oxidise an alcohol under reflux (or distillation) and it tests positive for being a ketone, then its a secondary alcohol

Question 12

what is Fehling’s solution?

Answer 12

blue solution of complex copper(II) ions dissolved in sodium hydroxide

Question 13

what is Benedict’s solution?

Answer 13

similar to Fehling’s but copper(II) ions are dissolved in sodium carbonate instead

Question 14

TESTING ALDEHYDE/KETONE WITH BENEDICT’S/FEHLING’S
stage 1

Answer 14

1. add 2cm^3 of benedict’s or Fehling’s solution to a test tube

Question 15

TESTING ALDEHYDE/KETONE WITH BENEDICT’S/FEHLING’S
stage 2

Answer 15

2. add 5 drops of the aldehyde or ketone to the test tube

Question 16

TESTING ALDEHYDE/KETONE WITH BENEDICT’S/FEHLING’S
stage 3

Answer 16

3. put the test tube in a hot water bath for a few minutes to warm it up

Question 17

TESTING ALDEHYDE/KETONE WITH BENEDICT’S/FEHLING’S
what happens if it is a aldehyde

Answer 17

benedict’s and Fehling’s solution will both reduce to a brick red Cu2,O precipitate when warmed with an aldehyde
colour change from blue to red

Question 18

TESTING ALDEHYDE/KETONE WITH BENEDICT’S/FEHLING’S
what happens if it is a ketone

Answer 18

no reaction happens with ketones so the solution stays blue

Question 19

reduction of copper ions in Fehling’s/benedict’s solution

Answer 19

2(Cu)2+ + 2(OH)- + 2e- = Cu2,O + H2,O
-copper(II) ions in Fehling’s/Benedict’s solution reduced
- electrons come from oxidation of aldehyde
- copper(I) ions form a brick red precipitate of copper(I) oxide

Question 20

what does tollens’ reagent contain

Answer 20

a [Ag(NH3)2] + complex

Question 21

TESTING ALDEHYDE/KETONE WITH TOLLENS’
stage 1

Answer 21

1. put 2cm^3 of 0.1 moldm^-3 silver nitrate solution in a test tube

Question 22

TESTING ALDEHYDE/KETONE WITH TOLLENS’
stage 2

Answer 22

2. add a few drops of dilute sodium hydroxide solution. light brown precipitate should form

Question 23

TESTING ALDEHYDE/KETONE WITH TOLLENS’
stage 3

Answer 23

3. add drops of dilute ammonia solution until the brown precipitate dissolves completely - this solution is Tollens’ reagent

Question 24

TESTING ALDEHYDE/KETONE WITH TOLLENS’
stage 4

Answer 24

4. add a few drops of the aldehyde or ketone and warm the solution in a water bath

Question 25

TESTING ALDEHYDE/KETONE WITH TOLLENS’
result for aldehydes

Answer 25

silver ions in Tollens’ reagent are reduced to silver metal when warmed with an aldehyde so a silver mirror forms

Question 26

TESTING ALDEHYDE/KETONE WITH TOLLENS’
result for ketones

Answer 26

no change

Question 27

equation for reduction of silver ions in tollens’ reagent

Answer 27

Ag+ + e- = Ag
-silver ions in tollens’ agent reduced
- electrons come from the oxidation of the aldehyde
-silver comes out of solution as solid silver (silver metal)

Question 28

TESTING FOR CARBOXYLIC ACIDS
stage 1

Answer 28

1. add 2cm^3 of the solution you want to test to a test tube

Question 29

TESTING FOR CARBOXYLIC ACIDS
stage 2

Answer 29

2. add 1 small spatula pf solid sodium carbonate (or 2cm^3 of sodium carbonate solution)

Question 30

TESTING FOR CARBOXYLIC ACIDS
stage 3

Answer 30

3. if solution begins o fizz, bubble the gas it produces through some limewater in a second test tube

Question 31

TESTING FOR CARBOXYLIC ACIDS
stage 4

Answer 31

4. if the limewater turns cloudy it is carbon dioxide

Question 32

TESTING FOR CARBOXYLIC ACIDS
stage 5

Answer 32

carbon dioxide gas should be produced if the solution was a carboxylic acid

Question 33

TESTING FOR CARBOXYLIC ACIDS
why do you have to be careful with this test

Answer 33

this test gives a positive result with any acid

Question 34

TESTING FOR ALKENES
what is the test testing for

Answer 34

unsaturated - double bonds

Question 35

TESTING FOR ALKENES
stage 1

Answer 35

1. add 2cm^3 of the solution you want to test to a test tube

Question 36

TESTING FOR ALKENES
stage 2

Answer 36

2. add 2cm^3 of bromine water to the test tube

Question 37

TESTING FOR ALKENES
stage 3

Answer 37

3. shake the test tube

Question 38

TESTING FOR ALKENES
stage 4

Answer 38

4. if alkene is present solution turns colourless, no alkenes present there is no colour change (solution stays orange)