13 - synthesis and analysis Flashcards
What does infrared do to molecules?
It makes the bonds bend, stretch and vibrate
What is wavenumber?
It relates to the wavelength of the infrared hitting the bonds
unit:cm^-1
What does wavenumber tell us?
Different organic molecules have different wavenumbers when they vibrate when IR hits them
What 2 properties affect the frequency of bond vibration and in what way?
Heavier atoms cause slower vibration
Stronger bonds cause faster vibration
What are the 3 greenhouse gases?
Water
Methane
Carbon dioxide
What do greenhouse gases do in terms of IR?
They absorb IR so their bonds vibrate
This causes heat to be given out to the atmosphere
Causing global warming
How do we find out what substance a sample is by using IR?
You fire IR radiation at the sample through a range of wavenumbers
A spectrophotometer will look at which wavenumbers were absorbed by the sample (not transmitted)
What is the range of wavenumbers used in IR spectroscopy?
4000 cm^-1
To
400 cm^-1
What will the spectrophotometer present to the user?
A graph showing wavenumber on the x axis
And percentage transmittance on the y axis
How can you work out from the spectroscopy graph what a substance is?
You look at where the transmittance spikes downwards because this tells you which wavenumbers have been absorbed
You can then look at the data sheet and recognise which functional groups are part of the compound because this will be the wavenumbers where transmittance spikes downwards
What do we call a downward spike in transmittance?
A peak
What are the 2 practical uses of IR spectroscopy?
Analysis of vehicle emissions
Breath test for ethanol
What are the 2 uses of mass spectrometry?
To identify the molecular ion
To determine molar mass
Give the 5 steps of mass spectrometry
Injection of the sample of molecules as a gas
Ionisation by a electron gun which knocks off electrons from outer shells
Acceleration of positive ions towards electromagnet
Deflection by the electromagnet to change direction of ions
Detection of the different ions and their abundances
How does the mass spectrometer differentiate between different ions?
Lighter ions get deflected more than the heavier ions
What 2 things does the mass spectrometer measure?
Abundance of ions
m / z ratio of ions
What is m / z ratio?
This means mass to charge ratio although the charge is always 1+
So m/z is just the mass of the ion
What do you look for on a mass spectrum (the output graph)
The peak which is furthest to the right is the one we are always looking for
What is the only exception to looking for the “furthest right” peak?
Sometimes there will a tiny peak which is 1 greater than what are looking for
This is due to carbon-13 isotopes
You have to ignore this tiny peak if it appears
What is fragmentation?
The molecular ion in the mass spectrometer is unstable as it passes through and can break into different fragments
Which parts of molecular ion break up?
The break can happen anywhere
What happens to the positive charge on the molecular ion when it undergoes fragmentation?
The +1 positive charge goes onto either but only one of the fragments
What decides which fragment the charge goes to?
It goes to either
It’s random
How can you detect fragments in mass spectrometry?
The charged fragment will appear on the mass spectrum
