Topic 7.1 Analytic Techniques Flashcards
Mass spec of an element
The mass spectrum of an element often appears very simple, with a very small number of vertical lines, called peaks, each one representing an isotope of the element.
Mass spec of a compound
The mass spectrum of inorganic compound often appears complex, with a large number of peaks
The molecular ion peak
-The peak furthest to the right on the mass spec of an organic compound.
-greatest m/z value (mass to charge ratio)/
-It is the result of an organic molecule losing an electron(an electron collides with a molecule and knocks out an electron, so forming a positive ion).
-The m/z value of this peak indicates the relative molecular mass.
The M+1 peak
The very small peak just to the right of the molecular ion peak.
This is caused by the presence of carbon in the molecule.
Smaller peaks
The peaks with smaller m/z values result form fragmentation in the mass spectrometer, rearrangement reactions and the loss of more than on electron.
(Rearrangement is often unpredictable).
Fragmentation in hydrocarbons
Fragmentation is very common and can often be used to work out the structure of an organic molecule.
Consider the breaking of a carbon-carbon bond in the molecular ion formed from a hydrocarbon. Two species are formed:
-another positive ion
-a neutral species (usually a radical)
How to deduce structures form mass spec?
- Consider possible techniques that have a molecular formula C3H8O
- Look at the unique peaks
- Which molecule(s) have a structure that can fragment to create that ion peak
Example of fragmentation from mass spec;
Butane
Butane CH3CH2CH2CH3
29 peak: [CH3CH]+
43 peak: [CH3CH2CH2]
58 peak: [CH3CH2CH2CH3]+
Example of fragmentation from mass spec;
Bromomethane
Bromomethane CH3Br
15 peak: [CH3]+
94 peak: [CH3 79^Br]+
96 peak: [CH3 81^Br]+
Infrared
Infrared is a simple, quick method that both confirms which organic functional groups are present in the test compound and those that are not.
In answers state both:
-Which peak on the spectrum you are referring to by their value
-Which feature in the structure is/is not present
Fingerprinting
-Picking out functional group signal is difficult
-This part of the spectrum is unique for every compound, and so can be used as a ‘fingerprint’
-This region can also be used to check if a compound is pure
(A distinctive characteristic or pattern indicating the presence of a certain molecule).
1oxidation ….
primary alcohols [O]–> aldehyde [O]–> carboxylic acid
secondy [O]–> ketone
tertiary can’t no space to form double bond with O
