IR/UV spectroscopy, Mass spectrometry, NMR Flashcards
Mass spectrometry is used to determine 3 things
1- overall mass of the molecule
2- determine what functional groups are present
3- how molecule is structurally constructed
What part of a molecule is most susceptible to e- loss?
1- (most important) non-bonding e- pairs (probably from the fx group)
2- where the best carbocation would form
3- where the best radical would form
Bonds break either via heterolytic or homolytic cleavage
Heterolytic cleavage- e- move towards electronegative atom
Product with positive charge will be the thing detected via mass spec, probably the carbocation
Homolytic cleaveage
Occurs between atoms with comparable electronegativity
One form is called alpha cleavage because it occurs on the alpha carbon
Product with positive change will be detected via mass spec, probably on the molecule that lost a single e- in radical formation
M+ peak
mass of the intact molecule
** alcohols don’t have M+ peak
M/Z ratio
mass/charge ratio, almost always 1
M/Z values of 31, 45, 59, etc. indicate presence of alcohol
M+1 and M+2
In some molecules, like bromine or chlorine have two different isotopes that differ by 1-3 g/mole
base peak
part present is greatest abundance
tells you about the structure or where the molecule is most likely to fragment
tells you how many Carbons are carved off of a fragment
1C
15
2C
29
3C
43
4C
57
5C
71
6C
85
7C
99
Bromine isotopes
79, 81
Chlorine isotopes
35, 37
In Alkyl Halides
- e- most likely moved from halogen
* alpha cleavage would occur via radical mechanism and C next to the halogen will always stay intact
In Ethers
- e- most likely removed from the oxygen
- bond most likely broken= one that will form the best carbocation
- base peak where bond to O breaks
- alpha cleavage would occur at any other potential cutting away from the carbon groups
- heavier group more likely to come off
In Alcohols (doesn’t work well with mass spec but will show up well with IR)
- e- most likely to be removed from -OH
- bond to -OH most likely to be broken
- most likely base peak where the carbons are left off
- Alpha cleavage will occur with additional cuts
Alcohol special notes
- OH falls off so often that never reaches the detector
- OH will steal another H from the molecule to become water when it leaves, so the carbon count will be off by one
In Amines
Characteristic N peak
Can tell when N is present because M+ peak will be odd#
Alpha cleavage dominates in aliphatic amines (C-containing)
In Ketones and Aldehydes
- e- most likely to be removed from O
- O not lost in heterolytic cleavage because it is still hanging on by one bond
- base peak most likely to occur with carbocation or heavier portion
Ketone and Aldehyde special cleavage
Called McLafferty cleavage, will occur if there is a H attached to the gamma carbon
This cleavage actually occurs between the alpha and beta carbons
Infrared Spectrometry is used to-
Determine what functional groups are present
- Does not tell you anything about structure
IR Theory
- bonds in molecules vibrate, stretch and bend
- when frequency of vibrations matches with frequency of IR radiation, radiation is absorbed and vibrations intensify
IR Theory cont.
Shorter bond= faster vibration= higher wavenumber
IR Theory cont.
> polar the bond= > intensity of absorption
*** If bond is not polar, it will will be IR silent
Short bonds vibrate faster
Bond length is determined by the # bonds and size of the molecule
General breakdown of IR chart
4000-1500, Important
1500-800 = fingerprint region/iffy at best
4000-3000
bonds to H
3000-2000
triple bonds