Lecture 1 Flashcards
(17 cards)
What do spectroscopy techniquies help us to do?
determine the structure of molecules
What are the different spectroscopic techniques and what do we primarily use them for?
mass spec for molecular formula
•Then use IR sometimes to justify different functionalities
•Sometimes use UV VIS
•But primarily use NMR. Much preferred for solving structures
Why do we need to be able to solve structures of molecules?
- in research lab of making molecules, we need to characterise every molecule we make.
- We need To confirm we’ve made what we aimed for
- We can use spectroscopic techniques to see if all data matches what We’ve just made Or
- We know what we’ve started the reaction with and what reaction we do but need to see if we can detect that new functional group we just added? (Usually done with IR and Backed up with simple NMR)
What phase does the sample have to be before placing in a mass spectrometer?
Gas.
Mass spec only allows us to analyse gas species.
This is done by putting the sample in a vacuum and heating it up really quickly
Mass spec: What happens after the sample is in the gas phase?
We need to turn it into a molecular ion as we can only analyse molecules when its got a positive cahrge on it.
We cannot analyse neutral molecules
Why cant we analyse neutral molecules in mass spec?
a Charged ion flying through a magnetic field has its trajectory bent due to magnetic field.
A neutral molecule would not have its path affected by the magnetic field
We would not be able to determine the molar mass as this is given to us by the machine dictated by the amount of deflection
What does the mass spec machine tell us?
It gives us a molar mass, or the molecular ion.
We can convert the molar mass into a molecular formula
What else can we analyse from mass spec data?
We can analyse the fragments and reconstruct which functional groups are present in the molecule
What is ionising?
Turning neutral molecule into molecular ion
How is ionising done?
- done by firing a beam of electrons at the sample. This high energy beam knocks out an electron.
- This is notated as M+ but in mass spectrometry it’s called M+(dot) to remind ourselves that there is an unpaired electron
- The unpaired electron is a valence shell electron so it’s not worth a negative charge to the molecule.
- It’s an electron the molecule originally owned. So it doesn’t have a negative charge to counteract the positive charge. Overall the molecule has a +
What is a radical cation?
molecule which is positively charged but does not have a negative charge to counteract it.
these are quite reactive and are full of energy
How does the highly energetic molecular ion get rid of its energy?
- by bumping into other molecules but
- in a gas phase in a vacuum, there are hardly any other molecules around to bump into, so instead, it breaks up.
- To break bonds and release other fragments which also use high energy. This is known as fragmentation
- Certain bonds are weaker than others especially those joined onto a benzene ring.
- These break easily inside a mass spectrometer
How do we know the number of ions inside the mass spectrometer?
- The more ions there are, the more intense that peak is.
What is one of the things that drives mass spec?
- the stability of the radical cations.
- given a choice a molecule will break apart to give more stable products.
What is the relative stability of cations?
- Methyl cations are not stable at all,
- primary alkyl cations are slightly more table.
- Secondary more stable,
- tertiary carbocations are relatively stable.
What two bits of info does seeing really intense peaks in a mass spectrum?
- there are a bunch of ions present,
- those Ions are more stable.
