Mass spectrum MCQ's

I'd appreciate if someone would explain how to go about answering these mcq's :

There would be a major peak in the mass spectrum for butan-1-ol, CH3CH2CH2CH2OH,
but not for butan-2-ol, CH3CH2CH(OH)CH3, at m/e value
A 15
B 17
C 29
D 43

So, to begin with, how are we supposed to know how these break apart? :confused:

How many molecular ion peaks (parent ion peaks) occur in the mass spectrum of
1,2-dibromoethane, CH2BrCH2Br?
Assume the only isotopes present are 1H, 12C, 79Br and 81Br.
A 1
B 2
C 3
D 4

Why would it be 3?

Thanks! :smile:

Reply 1

NutterFrutter

Original post by A-haaa

I'd appreciate if someone would explain how to go about answering these mcq's :

There would be a major peak in the mass spectrum for butan-1-ol, CH3CH2CH2CH2OH,
but not for butan-2-ol, CH3CH2CH(OH)CH3, at m/e value
A 15
B 17
C 29
D 43

So, to begin with, how are we supposed to know how these break apart? :confused:

How many molecular ion peaks (parent ion peaks) occur in the mass spectrum of
1,2-dibromoethane, CH2BrCH2Br?
Assume the only isotopes present are 1H, 12C, 79Br and 81Br.
A 1
B 2
C 3
D 4

Why would it be 3?

Thanks! :smile:

You have two isotopes for Br, so you could have CH2Br(79)CH2Br(79), CH2Br(81)CH2Br(81) or CH2Br(79)CH2Br(81) [this one is the same as CH2Br(81)CH2Br(79)], so answer is 3.

Original post by A-haaa

I'd appreciate if someone would explain how to go about answering these mcq's :

There would be a major peak in the mass spectrum for butan-1-ol, CH3CH2CH2CH2OH,
but not for butan-2-ol, CH3CH2CH(OH)CH3, at m/e value
A 15
B 17
C 29
D 43

So, to begin with, how are we supposed to know how these break apart? :confused:

They break apart randomly, but clearly the weaker bonds are more susceptible.

C-C ................ 342 kJ/mol
C-H ................ 412 kJ/mol

So you are more likely to see fragments corresponding to cleavage of C-C bonds.

In the first molecule, CH3-CH2-CH2-CH2-OH

You will see fragments corresponding to all the bits produced when the C-C bonds break (and O-H and C-O):

[CH3-CH2-CH2-CH2-OH]⁺ molecular ion m/z = 74
[CH3-CH2-CH2-CH2-O]⁺ m/z = 73
[CH3-CH2-CH2-CH2]⁺ m/z = 57
[CH3-CH2-CH2]⁺ m/z = 43
[CH3-CH2]⁺ m/z = 29
[CH3]⁺ m/z = 15

You also get a series of fragments formed by shearing off from the other end, plus some rearrangements, but the ones above will be large(ish) peaks.

Do the same operation with the isomer and you will see that some m/z values (from the ones above) are missing.... try it.

Reply 3

ema farouk

How many molecular ion peaks (parent ion peaks) occur in the mass spectrum of 1,2-dibromoethane, CH2BrCH2Br? Assume the only isotopes present are 1H, 12C, 79Br and 81Br.
a) 1 b) 2 c) 3 d) 4 e) 5

Original post by ema farouk

How many molecular ion peaks (parent ion peaks) occur in the mass spectrum of 1,2-dibromoethane, CH2BrCH2Br? Assume the only isotopes present are 1H, 12C, 79Br and 81Br.
a) 1 b) 2 c) 3 d) 4 e) 5

2 bromine atoms therefore 3 possible combinations ...

Mass spectrum MCQ's

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