3.3.6 Organic analysis

How do you test for alkenes & what is the result?

Shake with bromine water → the result is bromine water is decolourised (orange to colourless)

How do you test for halogenoalkanes & what is the result?

• Add 5 drops of halogenoalkane to 1cm³ of NaOH solution in a test tube & warm at 60°C for a few minutes

• Add 2cm³ of dilute nitric acid & 1cm³ of silver nitrate solution

• If a halogenoalkane is present, a precipitate should form:

  • AgCl = white

  • AgBr = cream

  • AgI = yellow

How do you test for alcohols & what is the result?

Add acidified potassium dichromate (K₂Cr₂O₇):

• colour change from orange to green for primary & secondary alcohols

• no colour change for tertiary alcohols

How do you test for aldehydes & what is the result?

• Warm with Fehling’s solution: colour change from blue solution to brick red precipitate

• Warm with Tollens’ reagent: colour change from colourless to silver mirror

How do you test for carboxylic acids & what is the result?

Add sodium hydrogencarbonate → if carboxylic acid is present, carbon dioxide gas is produced (effervescence)

What is mass spectrometry & how does it work?

Used for find the Mr of organic compounds:

1. compound is dissolved in solution

2. ionised by a high voltage supply (+1 ions)

3. accelerated by a negatively charged plate

4. reaches detector, picks up an electron & produces an electric current

   
   

(TOF used to work out m/z value & plot graph)

What does the x axis show on a mass spectrum & what does this effectively show & why?

• Shows m/z value

• Since most ions are +1, this effectively shows Mr

Why are there multiple peaks when molecules are put into a mass spectrometer?

• The molecular ions are shown as a peak, but it will also fragment into smaller molecules, so these peaks are shown as well

• Also due to isotopes, different peaks may be seen

What is high resolution mass spectrometry?

Gives the Mr to 3.d.p. or 4.d.p. (compared to standard low resolution, which is to nearest whole number)

Why is high resolution mass spectrometry useful?

Allows you to identify compounds that appear to have the same Mr, when they’re rounded to the nearest whole number

How does infrared spectroscopy differ from mass spectrometry?

• It uses the fact that bonds in different functional groups absorb different frequencies of infrared light

• Can also be used to assess how pure a compound is → impurities are indicated by extra peaks in fingerprint region

How does infrared spectroscopy work?

• A beam of IR radiation is passed through a sample of a chemical

• The IR radiation is absorbed by the covalent bonds in the molecules → increases vibrational energy

What do the peaks on an infrared spectrum show?

The frequencies where radiation has been absorbed

What is the fingerprint region?

• Lies between 500cm^-1 to 1550cm^-1

• Unique to a particular compound

How is the fingerprint region used to identify compounds?

The fingerprint region’s pattern is matched to a database on a computer to identify the compound accurately

What is the relationship between infrared absorption & global warming?

• Molecules of greenhouse gases (e.g. CO₂, methane & water vapour) have bonds that are really good at absorbing infrared energy

• If the amounts of these gases in the atmosphere increase, more IR radiation is absorbed → leads to global warming