Mass Spectrometry in Organic Compounds

Question 1

what does each peak on the mass spectrum of an element represent

Answer 1

an isotope of the element

Question 2

where on a mass spectrum would you find the ion with the greatest mass to charge ratio of a molecule and why

Answer 2

• on the very right of the spectrum
• this is because it would be the unfragmented version of the molecule (all of it) where only one electron has been knocked off
• leading to the largest ratio you could get

Question 3

what is the highest peak for a molecule called and what it is simply

Answer 3

• the molecular ion peak

- the result of an organic molecule losing an electron in the mass spectrometer

Question 4

what would the equation of this process be for a butane molecule

Answer 4

C4H10 + e- = C4H10+ + 2e-

Question 5

how is this positive ion formed in a mass spectrometer, simply put

Answer 5

• an electron collides with a butane molecule
• it knocks out an electron from its outer shell
• this forms a positive ion from the molecule

Question 6

if there is a very small peak just to the right of the molecular ion, what would it be called

Answer 6

the M + 1 peak

Question 7

what is the M + 1 peak caused by

Answer 7

• a naturally occurring isotope of carbon
• 13-C rather than 12-C
• approximately 99% of carbon atoms are 12-C
• but if the molecule has a 13-C it would have an extra mass by 1

Question 8

what other isotope of carbon is there and what is it used for

Answer 8

• carbon-14

- it is radioactive and therefore used in radiocarbon dating (carbon-14 dating)

Question 9

why dont carbon-14 isotopes show up in mass spectra

Answer 9

• because there is simply too small of an amount of them

- so we just dont even bother with them

Question 10

what are the three reasons you could have smaller peaks in a mass spectrum than the one for the molecular ion peak

Answer 10

• fragmentation of the molecule in the mass spectrometer
• rearrangement reactions
• loss of more than one electron

Question 11

what is and isnt a usual form of fragementation

Answer 11

• the breaking of a C-C bond is usual

- the breaking of a C-H bond isnt really considered to be fragmentation we need to care about

Question 12

if a C-C bond were to broken in a molecular ion, what two types of species would you end up with

Answer 12

• a neutral species (usually a radical)

- a positive ion

Question 13

what would the equation for the fragmentation of an ethane ion be as well as the names of the species formed

Answer 13

• (CH3-CH3)+ = CH3 + CH3+
• CH3+ is a methyl cation
• CH3 is a methyl radical

Question 14

what would be the more accurate way of writing the equation for the fragmentation of an ethane ion if a radical is formed

Answer 14

• you would include the radical sign (dot) on the molecular ion and the radical
• (CH3-CH3)+. = CH3+ + CH3.