Mass spectrometry please help

What about it are you struggling with?

The question is about predicting spectra from structures (or if you prefer, predicting structural features from spectra), so look at each compound and ask, how could it fragment?

e.g. A, loss of a methyl group is going to be favourable, so you should get a strong 119 as the base peak, Other thqn that you're looking at smaller fragment ions like 91 and 77.

Thanks for your reply. Could you please kindly just go through how compound A produces spectrum I -just to get the idea? Many thanks

for the sake of brevity I am going to omit the positive charges, but be sure that you know that only positive ions are detected.

CH₃C₆H₄COCH₃ m/z = 134

This will fragment at the weakest bonds, i.e. the C-C bonds. The positive charge can reside on either of the two fragments so both are seen

CH₃ m/z = 15
C₆H₄COCH₃ m/z = 134-15= 119
CH₃C₆H₄CO m/z = 119
CH₃C₆H₄ m/z = 91
COCH₃ m/z = 43
CH₃C₆H₄ m/z = 134-43 = 91
C₆H₄ m/z = 76

You get the idea?

Thanks for your reply! This is what I've got so far with spectrum 1:
The m/z value for the major peak is 119 so the compound must have lost a CH₃ group - Compound A, B and C can lose a CH₃ group however D cannot, so spectrum 1 does not represent compound D, and so we eliminate compound D.
The m/z value for a minor peak is 133 so the compound must have lost a H - and compound A, B and C can lose a H, am I right here? This is where I am confused.. Can a CH₃ group in compound A and B lose one of its Hydrogens? Can the COH group in compound C lose its hydrogen? Or can you lose the hydogen from the CH2 group of compound B and C? I am confused about from where you can lose the hydrogen.

Could you please kindly let me know what you think? Many thanks

You can lose a hydrogen atom from an aldehyde, but not easily from a methyl.

You can lose a hydrogen atom from an aldehyde, but not easily from a methyl.

Thanks! So we eliminate A and B because we cannot remove the Hydrogen from a Ch3 or Ch2 group, which means C is the answer. But in the mark scheme, it shows that A is the answer, So Iam confused.

If you get the chance, could you please let me know what you think about this? Many thanks

Thanks! So we eliminate A and B because we cannot remove the Hydrogen from a Ch3 or Ch2 group, which means C is the answer. But in the mark scheme, it shows that A is the answer, So Iam confused.

If you get the chance, could you please let me know what you think about this? Many thanks

I think you've misread the question, there's a major peak at 119, and no major peak at 133 or 105. The aldehydes C & D will easily give an M-1 for the 133, that's typically the major peak for an aldehyde so spectrum I cannot be either of those. The ketone B you could expect to readily lose a ethyl group and leave the M-1 for benzaldehyde, 105. Therefore it can't be that either, leaving A as the answer.

I and IV are not aldehydes as they have no major peak at 133

Therefore II and III are C & D

II has a major peak at M-15 so it must be C, and then III is D.

Now to differentiate A & B and I & IV

A should have a peak at M-43 = 91 due to loss of CH3CO (not shown)
B should have a peak at M-29 = 105 hence IV

Therefore A is I

Thank you so much In one of your previous posts you said You can lose a hydrogen atom from an aldehyde, but not easily from a methyl. Out of interest, what else can you easily lose a hydrogen from (not referring to the question)?

alcohol
carboxylic acid
amine
amide

common functional groups.