RQP 6: Tests for organic compounds

Question 1

What is the active species in Tollens reagent

Answer 1

[Ag(NH3)2]

Question 2

How do you test for an alkene

Answer 2

Add bromine water- alkenes decolourise bromine water

Question 3

What are the two reagents which test for an aldeyde

Answer 3

Tollens reagent and Fehlings reagent

Question 4

Describe the key points in testing for an aldehyde with Tollens reagent

Answer 4

• Reagent: Tollens reagent which is formed by mixing aqeous ammonia and silver nitrate. The active substance is the complex ion [Ag(NH3)2].
• Conditions: Heat gently
• Reaction: Aldehydes only are oxidised by Tollens reaget into a carboxylic acid. The silver (I) ions are reduced to silver atoms.
• Observation: A silver mirror forms coating the inside of the test tube.

Question 5

Describe the key points in testing for an aldehyde with Fehlings reagent

Answer 5

• Reagent: Fehlings solution which contains Cu 2+ ions
• Conditions: Heat gently
• Reaction: Aldehydes only are oxidised by Fehlings solution into carboxylic acid. The copper (II) ions are reduced to copper (I) oxide
• Observation: The blue Cu 2+ ions change to a red precipitate of Cu2O

Question 6

How do you test for a carboxylic acid

Answer 6

Add sodium carbonate- effervescence indicates an acid is present

Question 7

How do you test for a halogenoalkane

Answer 7

Add acidified silver nitrate and water- the silver halide precipitates form

Question 8

Which halogenoalkane has the fastest rate of substitution

Answer 8

Iodine becuase the C–I bond is weakest

Question 9

If solutions of 1-haloalkanes with each of the different halogens are all added to water and acidified silver nitrate, which silver halide precipiate forms first

Answer 9

Silver iodide