Organic Chemistry Test Tube Reactions

Question 1

Test for an alcohol

Answer 1

Acidified potassium dichromate
It oxidises primary and secondary alcohols but not tertiary ones, as it is a mild oxidising agent.
Colour change for orange to green if a primary or secondary alcohol is present.
Remains orange if only tertiary alcohols are present.

Question 2

Test to distinguish between a primary and secondary alcohol

Answer 2

FIRST do acidified potassium dichromate to oxidise the alcohol
Primary alcohols oxidise into aldehydes, and secondary alcohols oxidise into ketones.
Can use either Fehling’s or Tollen’s reagent.

Question 3

Name the tests to distinguish between aldehydes and ketones

Answer 3

Fehling’s or Tollen’s solution

Question 4

Describe the Fehling’s test

Answer 4

Fehling’s solution oxidises aldehyde but not ketones
If an aldehyde is present, the colour will change from blue to brick red (Cu2+ ions are oxidised to copper oxide)
If only ketone is present, the colour will remain blue

Question 5

Describe how to make tollens solution

Answer 5

Add silver nitrate (colourless)
Add sodium hydroxide (pale brown precipitate)
Add dilute ammonia
When precipitate dissolves, this is tollens solution

Question 6

Descirbe the tollen’s test

Answer 6

Add tollen’s solution to sample
Place in a warm water bath
If aldehyde is present then tollen’s is reduced to form a silver mirror (precipitate)
No silver precipitate forms in ketones

Question 7

Test for an alkene

Answer 7

Add bromine water
Shake
If alkene is present them bromine water reacts with the C=C bond to form a dibromoalkane (mechanism in alkenes topic)
Colour change from orange to colourless if alkene is present

Question 8

Test for a carboxylic acid

Answer 8

Add aqueous sodium carbonate
Produces carbon dioxide gas if a carboxylic acid is present (acid reacts with carbonate)
Effervescence observed if a carboxylic acid is present
Could test gas produces using limewater, so if CO2 is produced then limewater turns cloudy, meaning a carboxylic acid was present in the sample.

Question 9

Test for a halogenoalkane

Answer 9

Heat under reflux with sodium hydroxide
Acidify with dilute nitric acid
Then add silver nitrate
If a chloroalkane is present, a white precipitate forms that is soluble in excess ammonia
If a bromoalkane is present, a cream precipitate forms that is insoluble in excess dilute ammonia but soluble in excess concentrated ammonia
If an iodoalkane is present, a yellow precipitate forms that is insoluble in all excess ammonia