Organic Analysis

Question 1

Identification of alkene by test-tube reactions

Answer 1

Reagent: Bromine water
Result: Orange colour decolourises

Question 2

Identification of aldehyde by test-tube reactions

Answer 2

Reagent: Tollens’ reagent
Result: Silver mirror formed

OR

Reagent: Fehling’s solution
Result: Blue solution to red precipitate

Question 3

Identification of carboxylic acid by test-tube reactions

Answer 3

Reagent: Sodium carbonate
Result: Effervescence of CO2 evolved

fizzes and produce carbon dioxide
2CH3CO2H + Na2CO3 → 2CH3CO2-Na+ + H2O + CO2

Question 4

Identification of primary/secondary alcohol and aldehyde by test-tube reactions

Answer 4

Reagent: Sodium dichromate (VI) and sulfuric acid
Result: Orange to green colour change

Question 5

Identification of chloroalkane by test-tube reactions

Answer 5

Reagent: Warm with silver nitrate
Result: Slow formation of white precipitate of AgCl

Question 6

Tollens’ Reagent

Answer 6

-formed by mixing aqueous ammonia and silver nitrate

-the active substance is the complex ion of [Ag(NH3)2]+

Conditions: heat gently

Reaction: aldehydes only are oxidised by Tollens’ reagent into a carboxylic acid. The silver(I) ions are reduced to silver atoms

Observation: with aldehydes, a silver mirror forms coating the inside of the test tube. Ketones result in no change.

Question 7

Fehling’s solution

Answer 7

Conditions: heat gently

Reaction: aldehydes only are oxidised by Fehling’s Solution into a carboxylic acid

-blue copper (II) ions are reduced to copper(I) oxide

Observation:
Aldehydes: Blue Cu 2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react