6. Organic Analysis
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What is the test for an alkene?
Add bromine water
What is the result for when bromine water is added to an alkene?
Colour change from orange to colourless
What is the test for an aldehyde?
Tollen’s reagent/Fehling’s solution
What is the test for a carboxylic acid?
Add sodium hydrogencarbonate
What is the result for when sodium hydrogencarbonate is added to a carboxylic acid?
Effervesces
What happens when a compound is analysed in a mass spectrometer?
• gaseous molecules bombarded with high speed electrons from an electrospray • these knock out electrons from some molecules, creating +ve ions which travel to detector plates • mass spectra then generated
Where do the high speed electrons come from in mass spectrometry?
An electrospray
Which peak on a mass spectra represents the molecular ion?
The peak with the largest m/z ratio
Why is the molecular ion the peak with the largest m/z ratio?
Because the molecule hasn’t been fragmented into smaller pieces
Why do larger molecules tend to have more peaks on a mass spectra?
They can be fragmented in many different ways
Why is it not possible to deduce the exact molecular formula of a compound using mass spectrometry?
Different compounds may have the same relative formula
What does C₄H₁₀ and C₃H₆O both having a peak of 58 on the mass spectrum mean for the substances’ molecular formula?
It is unknown which molecular formula is that of the substance
How can the exact molecular formula of a substance be determined using mass spectrometry?
If the mass spectrum is examined under high resolution
What do high resolution mass spectrometers do?
Measure the m/z values to enough precision to find the molecular formula
What can high resolution spectrometers not identify? Why is this?
They cannot identify compounds - often there is more than one compound with the same molecular formula
What can high resolution spectrometers determine?
Molecular formula
What happens when covalent bonds are subjected to infra-red radiation with the same frequency as their characteristic frequency?
The bonds will bend or stretch more and the radiation of that frequency will be absorbed
What are covalent bonds constantly rotating, bending and stretching with?
Characteristic frequencies
What frequencies does each of covalent bond absorb?
Only absorbs one or two distinct frequencies
What will happen to the frequencies in infrared spectroscopy that are not absorbed?
They pass through unaffected
What would the spectra formed from infrared spectroscopy be like if there was no sample?
A horizontal line straight across
What regions can be analysed on an infrared spectrum?
• region above 1500 cm⁻¹ * region below 1500cm⁻¹
On an infrared spectrum, what is the region below 1500cm⁻¹ known as?
The fingerprint region
Where is the fingerprint region on an infrared spectrum?
Below 1500cm⁻¹