Organic analysis Flashcards
What is infrared spectroscopy?
It is an organic analysis method used to identify functional groups in a sample
What happens to a pair of atoms in a covalent bond?
They are always vibrating
What is unique about each bond?
Each bond has its own vibrational frequency
What happens if the infrared radiation frequency corresponds to the bond frequency?
The radiation is absorbed and the vibrating frequency increases
What happens to bonds and atoms when they absorb infrared radiation?
In different molecules, stronger bonds vibrate faster and heavier atoms vibrate slower
How does infrared spectroscopy work?
- A beam of IR radiation containing various frequencies is passed through a sample.
- The missing radiation corresponds to the types of bonds in the sample.
- The detector plots a graph of the intensity of the radiation against the frequency
What is the fingerprint region?
The area below 1500cm-1 which is unique to a particular compound
How else can infrared spectroscopy be used?
It can be used to identify the purity of a compound because impurities create extra peaks in the fingerprint region
How is infrared radiation linked to global warming?
The bonds of the molecules of carbon dioxide, methane and water vapour absorb IR radiation and cause the greenhouse effect.
Burning fossil fuels etc has caused a rise in the greenhouse gas concentration.
This results in global warming.
Describe a test that could be performed to distinguish between primary, secondary and tertiary alcohols.
Add 10 drops of the alcohol to 2 cm^3 of acidified potassium dichromate solution in a test tube.
Warm the mixture gently in a hot water bath
Observe colour change
-orange to green for primary and secondary alcohols
-no visible change for tertiary alcohols
Describe a test to distinguish between primary and secondary alcohols
If the alcohol is oxidised under reflux and tests positive for a carboxylic acid, then it is a primary alcohol.
If the alcohol is oxidised under distillation conditions and tests positive for an aldehyde, then it is a primary alcohol.
If the alcohol is oxidised under reflux/distillation conditions and tests positive for a ketone, then it is a secondary alcohol.
Describe a test that could be performed using Fehling’s or Benedict’s solution to test for an aldehyde or a ketone
Add 2 cm^3 of Fehling’s/Benedict’s solution to a test tube
Add 5 drops of the aldehyde or ketone to the test tube
Warm the mixture in a hot water bath
Observations- aldehyde-brick red precipitate
-Ketone- no visible change
Describe a method to make Tollen’s reagent
Put 2 cm^3 of 0.10 mol dm^-3 silver nitrate solution in a test tube
Add a few drops of dilute sodium hydroxide solution and a light brown precipitate should be formed
Add drops of dilute ammonia solution until the brown precipitate dissolves
What are the observations made when testing for an aldehyde or ketone using Tollen’s reagent?
Aldehyde- silver mirror
Ketone- no visible change
Describe a test that could be performed to test for carboxylic acids.
Add 2 cm^3 of the solution that you want to test into a test tube
Add 1 small spatula of solid sodium carbonate or 2 cm^3 of sodium carbonate solution
If the solution begins to fizz, bubble the gas through limewater in a second test tube
A white precipitate is formed confirming the presence of carbon dioxide
