Structural Determination Help!!!!

Hi there, I was given this homework to do having just completed the structural determination section in the AQA textbook.

However, I am struggling with this homework and was wondering whether you could give me some pointers in order to do the questions (e.g. solve one, and then I can do the rest/walk though how to solve the questions

1) Butanal and methylpropanal give slightly different mass spectra. Both give a molecular ion peak at m/z = 72, but butanal gives four other peaks wheras methyl propanal only gives three - state the species responsible for these 4 other peaks and write equations to show their formation from the molecular ion.

2) suggest how pentan-2-one and pentan-3-one could be distinguished in a mass spectrum, write equations to show the formation of any important fragment ions.

3)Write equations to show the formation of at least 2 species which give intense peaks in the formation of: petane, ethyl ethanoate, propanoic acid, and pentanal.
I have also attached the entire sheet - if you could give me step by step pointers to questions that would be most appreciated :-)

Thanks!

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I already had some help with Q1 but was just wondering if:

"You can see that the methyl group should show up as an ion and also the [CHO]+. There will also be the fragments left behind from these leaving, (CH3)2CH+ and CH3CHCHO+

You can do a similar analysis for butanal ... "

So, for question 1:

The species responsible for the peaks would be:

[CH3CH2CH2CHO]+(dot) ---> [CH3]+ AND [CH2CH2CHO](dot) ----------- (and the same but the symbols the alternate way round?)

I think that I've got:
1) [CH3CH2CH2]+
2) [CH3CH2]+
3) [CH3]+
4) [CHO]+

As the species responsible for the four other peaks - but could it not be: [CH2CH2CHO]+, [CH2CHO]+ responsible for the peaks also? Or is it to do with the stability of the group and which one breaks off first - assuming that the CHO would break off first due to the polarity of the C=O bond?

However, when writing the equations is it like this?

[CH3CH2CH2CHO]+(DOT) ----> [CH3CH2CH2]+ AND [CHO](DOT)
[CH3CH2CH2]+(DOT) -----> [CH3CH2]+ AND [CH2](DOT)
etc...???

I am not sure what (dot) means ... The only species that show up in MS are positive ions.

The question is much simpler than you are making it. The longer alkyl chain in butanal will break sequentially giving more fragments than methylpropanal.

The relative stability of fragments plus the bond strength decides which fragments are seen with their intensities.

Butanal|:

CH₃CH₂CH₂CHO fragments to give:

[CHO]⁺
[CH₃]⁺
[C₂H₅]⁺
[C₃H₇]⁺

as well as other minor peaks

[CH₂CH₂CHO]⁺
[CH₂CHO]⁺

The dot refers to the unpaired electron that is formed, from electrons being fired from an electron gun at the ionised sample - thus knocking electrons off, forming a +1 charge (sometimes 2+)

OK, you mean free radicals - these are not detected in MS.

Yep, I mean the free radicals. We have to writer the equations - and so we must include the free radical, because one species will have the +1 charge and the other free radical.

Also, when you take the fragment e.g. [CH3CH2CH2]+ does this then become [CH3CH2CH2]+(dot) for the next equation?