Organic Analysis

• Test for alkenes

  — add bromine water

  — goes from orange to colourless

  — done by an electrophillic addition mechanism
• test for halogenoalkanes

  — add warm aqeous sodium hydroxide

  — add dilute nitric acid

  — add aqueous silver nitrate

  — different coloured precipitates are formed
• why do we need to add sodium hydroxide when testing for haloalkanes

  — to bring about a nucleophillic substitution mechanism

  — halogenoalkane reavcts with a hydroxide ion to produce an alcohol and a halide ion

  — halide ion is then tested for
• we do we need to add nitric acids when testing for halogenoalkanes

  — added first to remove hydroxide and carbonate ions which would produce a false positive precipitate

  CO3(2-) + 2H+ ————> CO2 + H20

  OH- + H+ ————> H20
• equation for carbonate ions interfering to produce a white insoluble silver carbonate precipitate

  2Ag+ + CO3(2-) ————> AgCO3
• equation for hydroxide ions interfering to produce a white insoluble silver hydroxide precipitate

  Ag+ + OH- ————> AgOH
• test for alcohols

  — add acidified potassium dichromate

  — orange to green colour change with primary and secondary alcohols, and aldehydes
• test for aldehydes

  — Fehlings test

  — Tollen’s reagent
• Describe Fehlings test

  — warm with Fehling’s solution

  — blue solution to a brick red precipitate

  — Fehling‘s is a gentle oxidising agent, contains blue copper ions which will oxidise aldehydes but not ketones

  Cu2+ + e- ————> Cu+
• Describe Tollen’s test

  — warm with tollen’s reagent

  — silver mirror forms

  — Tollen’s reagent is a gentle oxidising agent - silver ions are reduced to a metallic silver as the aldehyde is oxidised
• test for carboxylic acids

  — add aqueous sodium hydrogen carbonate

  — effervescence/fizzing - carbon dioxide produced
• What is mass spectrometry used for?

  — can be used to find the Mr of organic compounds
• define molecular ion

  a molecule with one electron knocked off OR a molecule with a single positive charge
• what is high resolution mass spectrometry

  — measures masses to 3-4 decimal places

  — allows us to work out the molecular formula of a molecular ion
• define position isomers

  — compounds with the same molecular formula but different structures due to different positions of the same functional group on the same carbon chain
• describe infrared spectroscopy

  — stronger bonds vibrate faster at high frequencies

  — heavier atoms make the bonds vibrate more slowly and lower frequencies

  — when you shine a beam of infrared radiation, the bonds will absorb the energy and vibrate more

  — any particular bond can only absorb radiation that has the same frequency as the natural frequency
• what does frequency of infrared radiation depend on?

  — atoms that are either side of the bond

  — the position of the bond in the molecule
• what is the fingerprint region?

  — allows us to identify specific molecules

  — you can compare too a known library of spectra to identify the molecule

  — extra peaks indicate impurities
• link infrared radiation and global warming

  — electromagnetic radiation from the sun is absorbed by the earth and some is re-emitted as infrared

  — covalent bonds in greenhouse gases absorb IR because the bond is polar - greenhouse affect

  — gradual warming of earth is global warming

  — molecules which can change their polarity and vibrate can absorb IR