[FandangoDingo]

Hi everyone! I'm in dire need of help, and was wondering if any of you could lend a helping hand!

In our lab, we carried out an Aldol condensation of an unknown substituted benzaldehyde and an unknown substituted acetophenone.

I have to determine the identity of the starting materials / products from:

    Mass Spectra of the two unknowns
    Proton NMR of the Products

I've made an imgur album of all the above (with gorey details) here: http://imgur.com/a/LCsKL

I think that the weight of the benzaldehyde derivative is 142g/mol. Since benzaldehyde is 106g/mol, that would mean that the substitute is 36g/mol, which sounds like Chlorine to me. Because of the few distinct aryl protons on the NMR (see the NMR zoomed) I think there's high symmetry, which would make it 4-chlorobenzaldehyde.

I think that the weight of the acetophenone derivative is 135g/mol. Since acetophenone is 120g/mol, that would mean that the substitute is 15g/mol, which sounds like CH3 to me. Because of the few distinct aryl protons on the NMR (see the NMR zoomed) I think there's high symmetry, which would make it 4-methylacetophenone.

How far off are my guesses? Any better analysis?

REMEMBER THE SPECTRA IS ALL HERE
http://imgur.com/a/LCsKL

[FandangoDingo]

I'd be grateful for any kind of help

[MOTOBALL]

For interpretation of MS data,

1) you MUST nail the molecular ion correctly,

2) you MUST apply the Nitrogen rule.

Check your lecture notes or a textbook to learn the implications of the N rule.

Also check to determine the typical fragmentations of aldehydes (what atoms or group of atoms are typically lost from the molecular ion). Are there any ions in the mass spectrum of the substd. benzaldehyde to support your claim of a chlorine substituent ??

You are correct about the substd. benzaldehyde, but molecular weight is not 142; replacement of H by Cl (35Cl/37Cl) gives molecular ions at m/z 140/142 in ~3:1 ratio.

Are there any ions in the mass spectrum of the substd. acetophenone to support your claim of a methyl substituent ??

[FandangoDingo]

First off, thanks so much for taking the time to reply!

1) you MUST nail the molecular ion correctly,
2) you MUST apply the Nitrogen rule

I hear you. If I understand what you mean by Nitrogen rule is that

If MW is even, most fragments will have odd mass
If MW is odd, most fragments will have even mass
Only compounds with an odd number of N will have an odd MW

I feel like the majority of the peaks in the benzaldehyde unknown are odd, which would mean that the molecular ion is at 142. I'm also discounting 143 because it looks like C13 based off the ratios.

Also check to determine the typical fragmentations of aldehydes (what atoms or group of atoms are typically lost from the molecular ion).

I've been using this site for that, though I'm not sure how it helps: http://www.chem.arizona.edu/massspec/example_html/examples.html . I'm given that it's a benzaldehyde, so presence of an aldehyde is not something I'm out to prove.

Are there any ions in the mass spectrum of the substd. benzaldehyde to support your claim of a chlorine substituent ??
You are correct about the substd. benzaldehyde, but molecular weight is not 142; replacement of H by Cl (35Cl/37Cl) gives molecular ions at m/z 140/142 in ~3:1 ratio.

Ah! Perhaps the molecular weight is 140, and that 142 is M+2? Would that still make Cl a likely substitute?

Are there any ions in the mass spectrum of the substd. acetophenone to support your claim of a methyl substituent ??

Not that I can really see. I really made this deduction based on the 15g/mol discrepancy. I'm not aware of any particular pattern that you usually see, and I don't believe it's a nitrogen group, because I'd need a NH₂ group, which wouldn't fit the molecular weight.

[MOTOBALL]

Your instructor could have just given you the MW's of the two starting products, however a mass spectrum was supplied for each.  My guess is that an interpretation of each spectrum is expected, so that you deduce from the spectra the location of each substituent.

For instance, why not CH3CH2COC6H5 rather than CH3COC6H4CH3  from the MS ??

If you are interested in a career in chemistry then learning MS interpretation (rather than just knocking off another homework question as quickly as possible) is something you should pursue.

A hardball instructor will declare your answers correct, but may only give half-marks because you have really only performed simple arithmetic on the MS data.

I prefer the following statement of the N rule,

"If the MW is even, then the molecule contains an even number of N atoms (0, 2, 4,... etc), AND the converse is true.

If the MW is odd, then the molecule contains an odd number of N atoms (1, 3, 5,.  etc), AND the converse is true."

Good Luck !!!