Organic analysis

What is acidified potassium dichromate K₂Cr₂O₇ used for? and how does it work

• Used to distinguish between primary, secondary and tertiary alcohols

• Oxidises primary and secondary alcohols

How can you identify primary and secondary alcohols?

• They both give the same colour change

• Collect the products produced from the oxidation of both alcohols via distillation

• Test the products for aldehyde (primary alc) and ketone (secondary alc)

How does Fehling’s solution work?

• Its an oxidising agent

• Wille oxidise aldehydes not ketones

How do use Fehling’s solution?

• Put Fehling’s solution in test tubes with the substances you want to test

• Put it in a warm water bath

Results from Fehlings test

• Brick red precipitate for aldehydes (Cu₂O)

• Ketones remain blue

How do you make tollens reagent?

• Silver nitrate solution(colourless)

• Add few drops of NaOH

• Pale brown precipitate forms

• Add a few drops of dilute ammonia until precipitate dissolves

How do you use tollens reagent

• Add ketones/aldehydes to tollens reagent

• And place in hot water bath

Why don’t you use a Bunsen burner to heat up aldehydes and ketones?

• Both are flammable

Results for tollens reagent

• Tollens reduced to silver which coats the inside of the flask for aldehydes

• No silver precipitate formed for ketones

Test for alkenes

• Add bromine water to alkene in a test tube

• Swirl it

• If alkene is present solution will become colourless from brown

Test for carboxyclic acids

• React the acid with a carbonate (sodium carbonate)

• If acid is present it will begin to fizz

• Will produce carbon dioxide and water

• Carbon dioxide will react with limewater turning it cloudy

What is the problem with testing for carboxylic acids?

• All acids not just carboxylic acids will react the same way with a carbonate

What is mass spectrometry used for?

• To find the relative molecular mass (Mr) of a compound

What does the M+ peak show?

• Known as the molecular ion peak

• Molecular ion peak is the same relative molecular mass of the molecule

What is high resolution mass spectrometry used for?

• Useful when identifying different molecules with the same Mr rounded to the nearest whole number

How do high resolution mass spectrometers measure the relative mass?

• To several decimals

• E.g. Instead of H=1 it will use H= 1.0078

How does infrared spectroscopy work?

• Uses infrared radiation to increase the vibrational energy of covalent bonds in a sample

What does the frequency of infrared radiation absorbed by a covalent bond depend on?

• The atoms that are either side of the bond

• Position of the bond in the molecule

What is the fingerprint region used for?

• Allows you to identify specific molecules

• As it is different for all molecules

How can you use the fingerprint reigon?

• Compare the fingerprint region generated against a known library of spectra to identify the molecule

What do extra peaks in the fingerprint region indicate?

• Impurities in the sample

What do greenhouse gases in the atmosphere do?

• Absorb infrared radiation form the sun

Examples of greenhouse gases

• Co2

• water vapour

• Methane (CH4)

How does the greenhouse effect occur?

• Electromagnetic radiation from the sun reaches the earth and is absorbed by the land and the sea

• Some of this radiation is re-emitted as infra red

• Greenhouse gases absorb this radiation and re-emit this back towards earth-Greenhouse effect

• (The covalent bonds in the greenhouse gases absorb the radiation)

Human activities such as burning fossil fuels has increased the levels of….

• Greenhouse gases in the atmosphere

• Causing the gradual warming of the earth-global warming