6: Organic Analysis
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What is potassium dichromate used for?
An oxidising agent
What colour is potassium dichromate?
Orange
What conditions should you use when oxidising a primary alcohol?
Distillation
What conditions should you use when oxidising a secondary alcohol?
Acidified potassium dichromate with sulfuric acid
Describe the colour change when acidified potassium dichromate gets reduced.
Orange to green
What colour are dichromate ions?
Orange
Give an ionic equation for the reduction of potassium dichromate and production of Cr3+ ions.
Cr2O7 2- + 6e- + 14H+ -> 2Cr 3+ + 7H2O
What colour are chromium 3+ ions?
Green
What can primary alcohols be reduced to?
Aldehydes. With further oxidation, carboxylic acids are produced
What can secondary alcohols be oxidised into?
Ketones
Explain the test for the presence of primary, secondary and tertiary alcohols.
Add 10 drops of alcohol to 2cm3 of potassium dichromate Warm in a hot water bath If primary/secondary alcohols are present, the orange solution changes colour to green. If tertiary alcohols are present then there is no colour change.
When heating alcohols, why do we use a water bath instead of a bunsen burner?
Alcohol is flammable
If we see a positive result for primary and secondary alcohols, what can we do to see whether the alcohol is primary? (2 methods)
Oxidise under reflux Test for carboxylic acid If the test is positive then it is a primary alcohol Oxidise under distillation conditions Test for aldehyde If the test is positive then it is a primary alcohol
What conditions are needed to convert an aldehyde to carboxylic acid?
Oxidation under reflux
What conditions are needed to convert a primary alcohol to aldehyde?
Oxidation under distillation conditions
Explain the test for aldehydes using Fehling’s solution.
Add 2cm of Fehlings Add 5 drops of sample If aldehydes are present then there will be a colour change from blue to brick red
Give the ionic equation for the reduction of Cu2+ ions to Cu2O
2Cu2+ + 2OH- + 2e- -> Cu2O + H2O
State the complex ion present in Tollens’
[Ag(NH3)2]+
What happens to silver metal ions in Tollens’ when added to an aldehyde?
They are reduced
What will you observe if you add Tollens’ reagent to an aldehyde?
Silver mirror formed
Why must aldehydes and ketones always be heated in a water bath?
They are flammable
Describe the test for carboxylic acid.
• Add 2cm3 of test sample • Add a spatula of sodium carbonate • If bubbles are produced, bubble through limewater • If the gas is CO2 then the limewater turns cloudy. • This means that a carboxylic acid is present.
Describe the test for alkenes.
Add 2cm3 of sample solution Add 2cm3 of orange bromine water Shake Solution changes from orange to colourless if alkenes are present
What does the molecular ion peak mean on a mass spectra?
The Mr of a compound.