Mass Spec Ochem1

Question 1

Define the y axis and x axis of a MS spectrum

Answer 1

x-axis: m/z or mass to charge ratio of the molecule assuming a +1 charge (radical)
y axis: relative intensity

Question 2

What is a McLafferty Rearrangement

Answer 2

often observed in ketones, with a prominent peak at m/z = 58

removes a hydrogen atom from the gamma carbon atom and breaks the bond b/w the alpha and beta carbon

*14 bigger than aldehyde

Question 3

what are the key fragmentation pathways of ketones

Answer 3

alpha clevage to give a resonance stable acylium ion. for methyl ketones, prominent peak is seen at m/z = 43
mc lafferty rearrangement to give m/z= 58

Question 4

alpha clevage

loss of Hydrogen

Answer 4

hydrogen radical results in a peak at M-1

Question 5

alpha clevage

loss of a methyl

Answer 5

CH3 radical results ina peak at M-15

Question 6

alpha clevage

loss of an ethyl

Answer 6

C2H5 radical results in a peak at M-29

Question 7

alpha clevage

loss of a butyl

Answer 7

C4H9 radical results in a peak at M-57

Question 8

alpha clevage

loss of a propyl

Answer 8

C3H7 radical results ina peak at M-43

Question 9

alpha clevage

loss of a pentyl

Answer 9

C5H11 radical results in a peak at M-71

Question 10

alpha clevage

Phenyl cation is often seen at…

Answer 10

m/z=77

Question 11

rearrangement (RAR)

loss of water results in

Answer 11

peak at M-18

Question 12

RAR

loss of HCl results in

Answer 12

peak at M-36

Question 13

RAR

loss of acetic acid from acetate esters results in

Answer 13

peak at M-60

Question 14

RAR

McLafferty rearrangement of methyl ketones gives a

Answer 14

peak at m/z=58

Question 15

rearrangements (RAR) occur at ________ numbers while alpha clevages occur at ________ numbers

Answer 15

RAR: even numbers
alpha clevage: odd numbers

Question 16

What information can be gathered from the M+ region?

Answer 16

• it is the right area of the spectra, or where greater x values
• M+= generally tallest, furthest bar= total molecular mass of the parent molecule = the result of stripping one electron
• can be helpful to estimate the # of carbon atoms in the molecule

Question 17

presence of bromine

Answer 17

from a M+;M+2 ratio close to 1:1 of almost equal abundance of Br-79 and Br-81

Question 18

presence of chlorine

Answer 18

from a M+;M+2 ratio close to 3:1 abundance of Cl-35 to Cl-37

Question 19

presence of iodine

Answer 19

from a peak at m/z=127 and or a gap at 127, indicating loss of Iodine radical

Question 20

presence of nitrogen

Answer 20

will have an odd numbered m/z if an odd number of nitrogen atoms are present

Question 21

Steps to identifying an unknown spectra

Answer 21

1. where is the M+ region
2. what does this tell us regarding nitrogen? (nitrogen = odd number)
3. how many carbons are there? (think about number of Cs with Hs, where C has a mm=12 and H mm=1)
4. what is the possible molecular formula?

Question 22

what is the atomic mass of carbon, hydrogen, nitrogen, and oxygen?

Answer 22

C= 12
H=1
N=14
O=16