3.6: Organic Analysis

What is the test to identify an alkene?

• Shake with bromine water

• If alkene is present, solution turns from orange to colourless

What is the test to identify a halogenoalkane

• Add aqueous sodium hydroxide and warm gently

• Acidify with nitric acid

• Add silver nitrate

• If halogenoalkane present, precipitate forms (Cl=white, Br=cream, I=yellow)

What is the test to identify an alcohol?

• Add acidified potassium dichromate solution

• If primary/secondary alcohol present, solution turns from orange to green

• No colour change if tertiary alcohol present

What is the test to identify an aldehyde?

• Warm with Tollen’s reagent/Fehling’s solution

• If aldehyde present, either silver mirror forms or blue solution turns to a red precipitate

What is the test to identify a carboxylic acid?

• Add sodium hydrogen carbonate

• If carboxylic acid present, carbon dioxide gas formed

What is mass spectrometry used for?

Identifying organic compounds by determining their molecular formula

How can the Mr be deduced from mass spectroscopy?

• The highest molecular ion peak represents the intact ion and is therefore the Mr of the molecule

Why is high resolution mass spectrometry beneficial?

• It can measure to 4 decimal places making it easier to distinguish molecules with a similar Mr

What is the process of infrared spectroscopy?

• A spectrophotometer irradiates the sample with infrared electromagnetic waves

• It detects the intensity of the wavelength of IR radiation which goes through the sample

• Covalent bonds absorb the infrared radiation, causing them to vibrate

• The molecules will only vibrate at a specific frequency

• Depending on the rest of the molecule, each vibration will absorb specific wavelengths of IR radiation which are also shown as the reciprocal of the wavelength

What is the resonance frequency?

The specific frequency at which the molecules will vibrate to stimulate larger vibrations

Why can infrared spectroscopy often not be used alone?

Some absorption bands overlap eachother

What is the fingerprint region?

The region of the infrared spectroscopy below 1500 that is unique to each molecule

How does the absorption of infrared in carbon dioxide, methane and water vapour link to global warming?

• Molecules of greenhouse gases in the atmosphere absorb the infrared radiation emitted by the sun

• They re emit some of it back towards the Earth keeping it warm (the greenhouse effect)

• Burning fossil fuels and leaving rubbish to rot in landfills has caused a rise in greenhouse gas concentration which means more heat is being trapped and the Earth is getting warmer