Chemistry - 3.3.6: Organic Analysis

How do you test for alkenes?

Add bromine water - colour change from orange to colourless

How do you test for halogenoalkanes?

Add NaOH and warm, add nitric acid and silver nitrate solution - precipitate of AgX should form

How do you test for primary/secondary alcohols?

Add acidified potassium dichromate - colour change from orange to green

How do you test for aldehydes?

Warm with Tollens' reagent - silver mirror forms OR warm with Fehling's solution - colour change from blue to red

How do you test for carboxylic acids?

Add NaHCO3 so CO2 forms and test gas produced by bubbling it through limewater - turns cloudy

What is on a mass spectrum?

Relative abundance against m/z ratio

Molecular ion

A molecule of the sample which has been ionised but has not broken up during its flight through the instrument

What is the biggest peak on a mass spectrum?

The molecular ion

Why is there a peak one mass unit to the right of the molecular ion?

Because some ions contain the carbon-13 isotope

On a mass spectra, why are there other ions with smaller Mr?

Fragmentation - the molecular ion breaks up as some of the bonds break during ionisation

What is the difference between high resolution mass spectrometry and normal mass spectrometry and why is this useful?

HR mass spectrometry measures masses to 3 d.p. or 4 d.p. - means you can distinguish between compounds with similar Mr as a whole number

Do stronger bonds vibrate faster or slower?

Faster

Do heavier atoms vibrate more or less?

More

Why does infrared spectroscopy work?

When you shine beam of IR through a sample, bonds in sample can absorb energy from IR and vibrate more - bonds can only absorb radiation that has same frequency as natural frequency of bond so radiation that emerges misses frequencies corresponding to bonds in the sample

What happens in an infrared spectrometer? (4)

1. Beam of infrared radiation containing a spread of frequencies is passed through a sample
2. Radiation that emerges is missing frequencies that correspond to the type of bonds in a sample
3. Instrument plots graph of intensity of radiation emerging from sample (transmittance)
4. Frequency is expressed as a wavenumber (cm^-1)

What is a dip in an infrared spectra called?

A peak - each one represents a particular bond

Fingerprint region

The area of an IR spectrum below 1500cm^-1, caused by complex vibrations of the whole molecule and is characteristic of a particular molecule

How is IR used to identify a molecule?

Computer is used to match unique fingerprint region of a sample with those on a database of compounds - exact match confirms identification

Why can't mass spectrometry be used to distinguish between isomers?

They have the same Mr

Why do molecules absorb infrared radiation?

Their bonds vibrate at the same frequency as infrared radiation