Organic Analysis

Question 1

Test for alkenes

Answer 1

Reagent: bromine water
Result: orange colour becomes colourless

Question 2

Test for aldehydes

Answer 2

Reagent: Fehling’s solution
Results: blue solution to red precipitate

Reagent: Tollen’s reagent
Results: silver mirror formed

Question 3

Test for carboxylic acid

Answer 3

Reagent: sodium carbonate
Result: effervescence of CO2 formed

Question 4

Test for primary and secondary alcohols and aldehyde

Answer 4

Reagent: sodium dichromate and sulfuric acid
Result: orange to green colour

Question 5

Tollens reagent

Answer 5

Reagent: formed by mixing aqueous ammonia and silver chloride

Conditions: heat gently

Reaction: aldehydes only are oxidised by tollens reagent into carboxylic acid -> the silver (I) ions are reduced to silver

Observation: a silver mirror forms coating the inside of the test tube, ketones result in no change

CH3CHO + 2Ag+ + H2O -> CH3COOH + 2Ag + 2H+

Question 6

Fehlings solution

Answer 6

Reagent: fehlings solution containing blue Cu2+ ions

Conditions: heat gently

Reaction: aldehydes only are oxidised by the solution into a carboxylic acid -> the Cu2+ ions are reduced to Copper (I) oxide

Observation: blue Cu2+ ions in solution change to a red precipitate of Cu2O -> ketones don’t react

CH3CHO + 2Cu2+ + 2H2O -> CH3COOH + Cu2O + 4H+

Question 7

Equation for carboxylic acid test

Answer 7

2CH3CO2H + Na2CO3 -> 2CH3CO2Na+ + H2O + CO2