6. Organic Analysis Flashcards
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
Why is the area below 1500cm⁻¹ on an infrared spectrum called the fingerprint region?
Unique for each molecule
What are the peaks on an infrared spectrum?
If transmittance is below 50% (i.e. significant quantity at that frequency is absorbed)
What are strong, sharp peaks?
Peaks that occur at very precise frequencies
What are medium, broad peaks?
Peaks that occur over a range of frequencies
What can the peaks on an infrared spectrum be used to determine?
The different bonds present in the molecule (therefore different functional groups)
In a certain region on a mass spectrum, what do most functional groups show?
Characteristic absorptions
Can using the area above 1500cm⁻¹ on an infrared spectrum identify a molecule?
No - it cannot identify a specific molecule, only functional groups
What can be used on an infrared spectrum to identify a specific molecule?
The fingerprint region (below 1500cm⁻¹)
On which side of an infrared spectrum is the region above 1500cm⁻¹?
Left
On which side of an infrared spectrum is the region below 1500cm⁻¹?
Right
What is fingerprinting?
When the fingerprint regions of the unknown sample is compared to databases of pre-recorded infrared spectra until a match is identified
What can infrared spectroscopy be used for, as well as determining functional groups?
Identify impurities
How are impurities determined using an infrared spectrum?
The bonds present in the impurity but not in the desired product will absorb radiation, giving unexpected peaks
What peaks should a sample of ethanal have on an infrared spectrum?
Sharp absorption at around 1630-1815⁻¹ for C=O bond, as well as C-C and C-H bonds
How can you tell a sample is pure?
On the infrared spectrum, if the spectrum of the sample matches the spectrum of a sample known to be pure
What happens to radiation from the sun?
It is absorbed by the earth and emitted at a lower frequency
What frequency is radiation from the sun usually re-emitted at?
In the infrared region
Why can visible and ultra-violet light reach the earth’s surface?
Molecules like water, CO2 and methane do not absorb visible or UV light
When do molecules like water, CO2 and methane absorb radiation?
When radiation is remitted from earth, as when remitted it is in the infrared region, which the bonds can absorb
Why can molecules like water, CO2 and methane absorb infrared when it is remitted from earth?
It matches the natural frequency of vibration or rotation of bonds in these molecules
What is the ability of CO2 and water vapour to absorb radiation emitted by the earth called?
The greenhouse effect
What is causing global warming?
Increased levels of CO2 - so that more of the planet’s remitted heat to be trapped
