Organic 10 (preparation of benzoic acid)

Question 1

alcohol used

Answer 1

phenyl methanol (benzyl alcohol)

Question 2

what type of conditions are you reducing the alcohol in

Answer 2

alkaline conditions

Question 3

what provides the alkaline conditions

Answer 3

sodium carbonate Na2CO3

Question 4

which is the limiting reactant

Answer 4

phenyl methanol (benzyl alcohol)

Question 5

which is the reactant in excess

Answer 5

potassium manganate (VII)

Question 6

what does an excess of potassium manganate (VII) ensure

Answer 6

it ensures the complete oxidation to benzoic acid, not to benzaldehyde only

Question 7

what is the potassium manganate (VII) reduced to?

Answer 7

Mn (+4)

Question 8

appearance of Mn (+4)

Answer 8

brown precipitate

Question 9

why is sodium carbonate added?

Answer 9

as the rate of reaction will increase in alkaline solution

Question 10

what is then added?

Answer 10

hydrochloric acid

Question 11

3 reasons why HCl is added

Answer 11

(i) to precipitate the crystals
(ii) to neutralise the sodium carbonate used and the potassium hydroxide produced
(iii) protonates the benzoate anion to form benzoic acid as the solution cools

Question 12

how can you prevent the contamination of the crystals by the brown precipitate of MnO2?

Answer 12

by adding sodium sulphite as it reduced Mn(+4) to Mn(+2)

Question 13

what does sodium sulphite look like

Answer 13

colourless in solution

Question 14

what does phenyl methanol (benzyl alcohol) look like?

Answer 14

colourless oily liquid

Question 15

sodium sulfite formula

Answer 15

Na2SO3

Question 16

what does this experiment require?

Answer 16

heat in a water bath

Question 17

when does crystallisation occur?

Answer 17

when cooled

Question 18

after crystals are formed what is done?

Answer 18

they are isolated and purified

Question 19

how to find the purity of the crystals

Answer 19

by melting point determination

Question 20

if complete oxidation does not occur, how would one notice and why?

Answer 20

the smell of almonds will be noticed as the alcohol will only be oxidised to phenyl methanal (benzaldehyde)

Question 21

if after 20 minutes there is still a smell of almonds what should you do?

Answer 21

heat some more to complete the oxidation process

Question 22

where and how do you add the HCl and why?

Answer 22

drop by drop in a fume cupboard as HCl gas is given off

Question 23

what test do you perform on the crystals and why and what is the result

Answer 23

dip a glass rod into them and touch with blue litmus paper, it should turn red in the presence of benzoic acid crystals

Question 24

if litmus paper doesn’t turn red

Answer 24

aldehyde may be present instead

Question 25

what happened when all the MnO2 dissolved

Answer 25

the conical flask was placed on ice and the benzoic acid was crystallised

Question 26

after recrystallisation occurs and why

Answer 26

crystals filtered ad washed with iced water to maximise the yield

Question 27

what would washing the crystals in warm water do?

Answer 27

would dissolve the crystals and reduce the yield

Question 28

the slower the cooling

Answer 28

the bigger the crystals

Question 29

the faster the cooling

Answer 29

the smaller the crystals

Question 30

where are crystals dried 2

Answer 30

in a desiccator or warm oven

Question 31

how do you find the actual yield of the crystals

Answer 31

weigh them

Question 32

what forms after benzaldehyde

Answer 32

salt of a carboxylic acid

Question 33

salt of carboxylic acid formed

Answer 33

sodium benzoate

Question 34

draw sodium benzoate

Answer 34

benzene ring with C=O -O-Na+

Question 35

percentage yield

Answer 35

actual yield over theoretical yield x100