3.3.6 Organic Analysis

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

Shake with bromine water.

Bromine water decolourises. (Orange → colourless)

How do you test for haloalkanes and what is the result?

Warm NaOH (aq), acidify with NHO₃ then add AgNO₃
Silver precipitate.

• Chlorine = white

• Bromine = cream

• Iodine = yellow

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

Acidified potassium dichromate (VI) and heat.

Colour changes from orange → green for primary and secondary alcohols. No change for tertiary alcohols.

How do you test for aldehydes and what is the result? (1)

Warm sample with Fehling’s

blue → brick red

How do you test for aldehydes and what is the result? (2)

Warm with Tollens’ reagent
colourless → silver mirror (Ag precipitate forms)

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

Add Na₂CO₃ (aq)
CO₂ given off so there is effervescence. Can test to see if it’s CO₂ by bubbling through limewater → turns milky/cloudy

What is mass spectrometry?

Used to find the relative molecular masses of organic compounds.

How does mass spectrometry work?

1. Compound is dissolved in solution

2. Then ionised by a high voltage supply to 1+ ions.

3. Accelerated by a negatively charged plate.

4. Becomes a bream of ionised molecules.

5. Reaches detector then causes a current to flow.

6. Time of flight used to work out m/z value and plot graph

What does the x axis effectively show on the mass spectrum and why?

Shows m/z value (mass divided by ionic charge).
Since most ions are 1+, the effectively shows Mr.

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

Molecular ion is shown as the last peak. But spectrometer will also display the peaks of fragments of smaller molecules.
Due to isotopes of atoms, different peaks may be seen.

What is high resolution mass spectrometry?

Mass spectrometers which give Mr to 4 d.p are called high resolution.

What does high res mass spec allow you to do?

Differentiate between compounds that have the same Mr (to a whole number), but are made up of different atoms and therefore have different values of Mr to 4 d.p.

Why do atoms and chemical bonds absorb infrared radiation?

Constantly vibrating, so can absorb infrared radiation that is the same frequency as their frequency of vibration.

What effect does a stronger bond have on the frequency of vibration?

Vibrates faster (with higher frequency)

What effect do heavier atoms have on the frequency of vibration?

Vibrate slower (with slower frequency)

How does infrared spectroscopy work?

Every bond has a unique vibration frequency in the infrared region of the EM spectrum.
Bonds absorb radiation that has the same frequency as their frequency of vibration.

Infrared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compound’s functional group.

What is the fingerprint region?

Area of the infrared spectrum below 1500 cm^-1