Alcohol + Functional Gp Tests

Question 1

Test for degrees of alcohol

Answer 1

• alcohol mix with acidified potassium dichromate
• warm in hot water bath
• primary= green
  Secondary = green
  Tertiary = stay orange

Question 2

How to distinguish primary and secondary alcohol ? (Extraction)

Answer 2

• add alcohol with potassium dichromate and put it in distillation apparatus
• heat the flask, alcohol will oxidised and product distilled off immediately to collect it

Question 3

Test for aldehyde

Answer 3

• Fehling’s
• Benedict
  -tollen

Question 4

Fehling or Benedict test

Answer 4

• add fehling or Benedict into aldehyde or ketone
• put in hot water bath to warm it
• colour change from blue to brick red = aldehyde

Question 5

Tollens test

Answer 5

• add silver nitrate, dilute sodium hydroxide together = form light brown precipitate
• add few drops of ammonia until all brown dissolves
• add aldehyde or ketone in the tube and put in hot water path
  Results: silver mirror formed on the tube wall = aldehyde

Question 6

Test for carboxylic acid

Answer 6

• add solid sodium carbonate = results in fizzy and bubble gas
• link tube with lime water
• turn cloudy = carbon dioxide present = there is carboxylic acid

Question 7

Describe the process of obtaining ethanal and how to minimise loss of ethanal (5)

Answer 7

• mixture of liquid heated to b.p in a round bottom flask
• with still head containing thermometer
• water cooled condenser connected with suitable cooled collecting vessels
• collect sample at b.p of ethanal
• minimise loss of ethanal by using cooled collection vessels to reduce evaporation of it

Question 8

Describe what happens when a reaction mixture is refluxed and why it is necessary for complete oxidation to ethanoic acid from ethanol (3)

Answer 8

1) a mixture of liquid heated to b.p for a prolonged time
2) vapour is formed which escapes from the liquid mixture, will condense back into liquids and drop back into mixture
3) any ethanal or ethanol initially evaporates can be oxidised fully

Question 9

Based on structure and bonding, explain why it is possible to separate ethanal in distillation (2)

Answer 9

1) there is hydrogen bonding in ethanol or ethanoic acid but no h bonding in enthanal
2) IMF in ethanal will be weaker than in alcohol or acids so easier to break by only using distillation

Question 10

Test to distinguish MgCl and BaCl

Answer 10

Sodium sulfate
- MgCl will make MgSO4= soluble in water = no ppt form
- but BaCl will make BaSO4 = highly insoluble= form white ppt