Deducing Structures from Mass Spectra

Question 1

what would the m/z values for CH3+, OH+ and CO+ be

Answer 1

• CH3+ = 15
• OH+ = 17
• CO+ = 28

Question 2

what would the m/z values be for CH3CH2+, CHO+, CH3CH2CH2+, CH3CHCH3+ and CH3CO+

Answer 2

• the first two = 29

- the last three = 43

Question 3

the compounds A and B have the molecular formula C3H6O, with A’s spectrometry having peaks at m/z = 15, 29 and 58 while B’s has 15, 43 and 58. using this knowledge and the m/z values of the previous ions, what would the name of molecule A be

Answer 3

• C3H6O+ has the RAM of 58 so it doesnt help
• theres a peak at 15 for both so it doesnt help either
• the peak at 29 could either be CH3CH2+ or CHO+
• 58-29 = 29, but because there r no other ions or combination of ions = 29, its simply must be those two
• so the structure is CH3CH2CHO, meaning its an aldehyde (propanal)

Question 4

what would molecule B be

Answer 4

• peak at 58 doesnt help
• the peak at 43 and 15 add up to 58 so there is an ion for each
• the only ion for 15 is CH3+
• out of the three ions for 43, only CH3CO+ has two carbon atoms which would match up with CH3+ to correctly have 3 carbon atoms, not 4
• therefore the structure is CH3COCH3, making it a ketone (propanone)

Question 5

what would be the equation for the formation of the ions that form molecule A

Answer 5

(CH3CH3CHO)+ = CH3CH2+ + CHO. or with opposite signs

Question 6

what would be the equation for the formation of the ions that form molecule B

Answer 6

(CH3CHCH3)+ = CH3CO+ + CH3.

Question 7

what is the relative molecular mass of C3H8O

Answer 7

60

Question 8

compound C has peaks at 60, 15 and 31 whereas D has peaks at 60, 15 and 45. with the ion CH2OH+ = 31 and COOH+ or CH3CHOH+ = 45, what is the structural formula of C

Answer 8

• they both have peaks at 60 so it doesnt distinguish
• they both have peaks at 15 so same thing but we know CH3+ is there
• the peak at 31 means CH2OH+ ion is present too
• this leaves an uncharged CH2 group to fill in the gap and one C and 2 H are missing
• making CH3CH2CH2OH (propan-1-ol)

Question 9

what is the structural formula of D

Answer 9

• the peak at 15 means CH3+ is there
• peak at 45 mean one of the two ions for that m/z value are present
• however it cant be COOH+ as there are 2 oxygens
• so it has to be CH3CHOH+ combined with CH3+
• giving CH3CH(OH)CH3 (propan-2-ol)