Organic 1 Section 3 - Unit 16: Organic Analysis

Question 1

Explain how infrared spectroscopy can be used to show that an aldehyde is definitely pentanal (2 marks)

Answer 1

• Compare spectrum of aldehyde with known spectrum of pentanal
• Exact match

Question 2

Test for primary and secondary alcohols (3)

Answer 2

• Potassium Dichromate
• Orange
• To green

Question 3

Test for tertiary alcohols (3)

Answer 3

• Potassium Dichromate
• Orange
• Remains orange

Question 4

Test for aldehydes (2)

Answer 4

• Tollens reagent
• Silver mirror precipitate
  OR
• Fehling’s reagent
• Blue to brick red

Question 5

Explain how you would distinguish between an alkane and an alkene (3)

Answer 5

• Bromine water
• Orange to colourless with alkene
• Remains orange with alkane

Question 6

Test for Carboxylic Acid (3)

Answer 6

• Add sodium hydrogen carbonate
• Then add limewater
• If the solution turns cloudy, CO2 is present (and therefore it’s a carboxylic acid)

Question 7

Explain what is meant by the fingerprint region of an infra-red spectrum and state how it is used to confirm the identity of organic molecules such as the primary, secondary and tertiary alcohols of molecular formula C4H8O (2 marks)

Answer 7

• Region 1500–400 cm–1

- Exact match to spectrum of known compound

Question 8

Suggest one reason why Tollens’ reagent is used as the oxidising agent in the specific test for aldehydes rather than the less expensive acidified potassium dichromate(VI) (1 mark)

Answer 8

• Potassium dichromate(VI) will also give a positive test with alcohols

Question 9

Outline a simple procedure to allow you to determine whether an alcohol is primary, secondary or tertiary (3 marks)

Answer 9

• Mix the alcohol with warm K2Cr2O7 (allows 3° identification by lack of reaction)
• Distillation of initial product needed for 1° / 2°
• Effect of Tollens’/Fehling’s on oxidation product to identify 1° or 2°

Question 10

Explain which one of the compounds, propan-1-ol, propanal and propanoic acid will have the highest boiling point (3 marks)

Answer 10

• Propanoic acid
• Contains hydrogen bonding
• So intermolecular forces are stronger in propanoic acid

Question 11

Describe how you would distinguish between AgBr and AgI (3 marks)

Answer 11

• Concentrated ammonia
• AgBr - cream solid dissolves
• AgI - does not dissolve