Organic Analysis Flashcards
What is a mass spectrometer used for?
To measure the relative mass of isotopes
Describe electron impact ionisation
High energy e- fired from an electron gun at a sample to knock off an e-
X(g)—-> X+(g) + e-
When does fragmentation occur?
When covalently bonded molecules have their bonds broken during electron impact ionisation and these molecules break into fragments
Does fragmentation occur to all molecules?
Nope
What happens to the molecules that don’t get fragmented?
What do these give?
They remain intact and gain a positive charge
The peak with the highest m/z value
This will tell us the Mr
Describe electrospray ionisation
Substance dissolved in volatile substance and injected through hypodermic needle
Tip of needle has a high voltage
Each particle ionised by gaining a proton
Generic equation for electrospray ionisation
Does fragmentation occur?
X(g) + H+ —-> XH+(g)
No
When you find the m/z value of a molecule after electrospray ionisation, what must you do and why?
Minus 1
To account for added H+
With electrospray ionisation what does the Mr=?
The peak with the greatest Mr value minus one
With both techniques there will be a very small peak at a m/z value of 1 greater than the molecular ion peak. What causes this and what do we do with it?
Some molecules contain either C13 or H2
We ignore it
Do isotopes have exactly whole number atomic masses?
What is the exception?
No
Carbon-12 by definition
Define molecular ion
A molecule with 1 electron removed
Why is high resolution mass spectrometry more useful?
Some molecules have the same Mr to one decimal place so more precision is necessary
How does IR spectrometry work?
Causes bonds to vibrate
The stronger they are the faster they vibrate
What is the shape of the O-H wavelength peak?
Always broad
What bond must you always clarify for IR?
O-H
Whether it’s acid or alcohol
Where is the fingerprint region?
Below 1500
Do you acknowledge peaks below 1500?
Why?
No
This is the fingerprint region so peaks are hard to distinguish
Why is a C-O Bond hard to identify in IR spectrometry
It is in the fingerprint region
How can IR spectrometry be used to identify impurities?
Reveals absorptions that should not be there
How is IR used to identify greenhouse gases
The more effective the greenhouse gas the more IR radiation it absorbs
What peaks should you ignore around 2850-3300?
C-H peaks as they are in almost every organic molecule
Does O-H and C=O peaks always indicate carboxylic acids?
No
How to differentiate between O-H (alcohol) peaks and N-H peaks?
Why would you need to?
O-H is broad
N-H appears as 2 peaks
They appear in same region
For IR what should you use if unsure?
The Mr
Which bond peak tends to be small?
C=C
DONT MISS IT
Explain how infrared spectroscopy can be used to show that an aldehyde is definitely pentanal
Use the compound’s fingerprint region and use an internet data base search to find an EXACT match
How to test if an organic product is pure
Measure melting point using melting point equipment apparatus
A sharp melting point/ a melting point that matches the data book indicates a pure substance
You are provided with a small sample of pure aspirin in a melting point tube. Describe briefly how you would determine an accurate value for the melting point of aspirin (2)
Heat using melting point apparatus
Heat slowly near melting point
