Organic analysis

What can you use to distinguish between primary/secondary and tertiary alcohols?

Acidified potassium dichromate (K₂Cr₂O₇).

True or false? Acidified potassium dichromate oxidises tertiary alcohols but not primary/secondary alcohols.

False. ACIDIFIED POTASSIUM DICHROMATE OXIDISES PRIMARY/SECONDARY ALCOHOLS BUT NOT TERTIARY ALCOHOLS.

What is the colour change when acidified potassium dichromate is reduced?

Orange to green.

What test would you do to determine whether an alcohol is primary or secondary once you have determined with acidified potassium dichromate that it’s not tertiary?

Tollen’s reagent/Fehling’s solution.

What is the positive result for Fehling’s solution?

From blue solution to formation of brick red precipitate.

What is the positive result for Tollen’s reagent?

Formation of silver coating.

How do you make Tollen’s reagent?

1) Get some silver nitrate solution.

2) Add a few drops of sodium hydroxide to the silver nitrate solution (forms pale brown precipitate).

3) Add a few drops of dilute ammonia until the pale brown precipitate dissolves (this makes the Tollen’s reagent).

What reagent do you use to test for alkenes?

Bromine water.

What is the positive result for using bromine water to test for alkenes?

Bromine water is decolourised.

What do you use to test for carboxylic acids?

A carbonate (contains CO₃^2-) and limewater.

What is the positive result for testing for carboxylic acids with carbonates? What causes this results?

When gas produced is bubbled through lime water it turns cloudy. Carbon dioxide gas being produced causes lime water to go cloudy.

True or false? Any acid reacting with a carbonate will liberate carbon dioxide from the carbonate.

True.

What is the purpose of mass spectrometry?

To find the relative molecular mass of a compound.

What does m/z mean on a mass spectrum?

Mass to charge ratio, the relative mass divided by the charge.

Why are most m/z values the same as the relative mass?

Because the majority of particles in the mass spectrometer have a 1+ charge.

What is the molecular ion peak?

The peak on a mass spectrum that shows the relative molecular mass of the molecule being analysed.

When is high resolution mass spectrometry most useful?

When compounds with the same relative molecular mass rounded to a whole number are being analysed.

To how many decimal places does a high resolution mass spectrometer usually go?

To four decimal places.

How does IR spectroscopy work?

It uses increases the vibrational energy of covalent bonds in a molecule with infrared radiation, different covalent bonds absorbs different frequencies of infrared radiation.

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

• The atoms that are either side of the bond (the atoms that the bond is between).

• The position of the bond in the molecule.

How do peaks appear on an IR spectrum?

As dips at certain wavenumbers.

What is the fingerprint region of an IR spectrum?

A section of an IR spectrum that is unique to every molecule.

Between which wavenumbers of an IR spectrum does the fingerprint region lie?

Between 500cm^-1 and 1500cm^-1

What do extra peaks in the fingerprint region imply?

There are impurities in the sample.

What type of radiation do greenhouse gases in the atmosphere absorb?

Infrared radiation.

Give three examples of greenhouse gases.

• Carbon dioxide (CO₂)

• Water vapour (H₂O)

• Methane (CH₄)

What exactly in greenhouse gases absorb infrared radiation?

Covalent bonds.