Organic Analysis

Question 1

Give a test that can be used to identify an aldehyde?

Answer 1

• use Tollen’s reagent, and warm gently
• if an aldehyde is present, silver mirror will form

Question 2

Give a test to identify the presence of primary and secondary alcohols

Answer 2

• add acidified potassium dichromate
• turns from orange to green if present

Question 3

Why will tertiary alcohols not turn orange in the presence of potassium dichromate?

Answer 3

As it is not able to be oxidised

Question 4

What does Fehling’s reagent test for? How is it carried out?

Answer 4

• the presence of aldehydes
• add Fehling’s reagent and warm gently
• if present, solution will turn from blue to brick red

Question 5

What is a test for alkenes?

Answer 5

• add bromine water
• if present, orange turns to colourless

Question 6

What is a test for a carboxylic acid?

Answer 6

• add sodium carbonate
• CO2 is produced
• gas is collected and tested with lime water
• will turn cloudy if CO2 (positive for COOH)

Question 7

How does IR radiation show the functional groups present in a compound?

Answer 7

• IR radiation is passed through a sample
• different types of bond absorb different amounts of IR radiation
• these varying amounts of absorbance are measured and recorded and allow for certain bonds/functional groups to be identified

Question 8

How is the test for an aldehyde carried out using Tollens’ reagent?

Answer 8

• Mix aqueous silver nitrate with aqueous sodium hydroxide.
• add concentrated ammonia solution until ppt. dissolves
• add few drops of solution, heat
• silver mirror will form

Question 9

Describe the test and positive result for a haloalkane

Answer 9

warm with silver nitrate solution in ethanol
Cl- white ppt
Br- cream ppt
I- yellow ppt

Question 10

Describe a test for any alcohol

Answer 10

add Mg, effervescence will form

Question 11

Why do aldehydes give a positive result for Tollens’? For Fehling’s?

Answer 11

• as the reduce Ag+ to Ag, as they themselves are oxidised
• Cu2+ is reduced to Cu+, aldehydes are oxidised