[s30.93a]

The second rearrangement (if any) gives the same ion mass as one of α-cleavage (the more probable one)

Thank you, but I want to know why the second rearrangement would give base peak ion and not the first one?

[AWK]

Base peak shows odd number and cannot come from rearrangement.

[s30.93a]

I'm sorry but I'm not able to understand you.
There are two possibilities or two fragment according to Mc Lafferty then why one one is base peak ?

[s30.93a]

These not give ions 105 and 77 which are most abundant in the spectrum of butyrophenone. Check α-cleavages.

α cleavages ? what should I look for when I check it, I don't know how to look for it, sorry

[AWK]

Just broke bond left or right to CO and leave charge on oxygen (in the case of bond to phenyl group charge may be also left at phenyl group.

Elimination of molecule (ethane or propane) from butyrophenone should give even number for ion. MS spectrum of this compound show only abundant odd mass numbers. It means that McLafferty rearrangement is unimpotrant for this compound.

[s30.93a]

Simply Put:
If I ask you to tell me Base peak for 1-phenyl-1-butanone what would you say ? and why ? please help it's a question asked in my exam

[AWK]

The most intensive one - 105
Peaks 106 (partially) and 120 may come from McLafferty rearangement.

[s30.93a]

and why it's 105 ? not any other cation ?

[AWK]

This ion is the most stable C₆H₅CO⁺. this does not surprise me.

[s30.93a]

Please correct me if I'm wrong:
It's not imperative that ion formed after Mc Lafferty rearrangement will always show base peak.
Hence stability is preferred over Mc Lafferty.

[AWK]

Search google for mass spectra of some ketones (aldehydes) starting from 5 carbon atoms (2-pentanone, 2- and 3-hexanone ...). Compare base peaks (the strongest ones) to that of M-28, M-42... and so on (all with even m/e)... and find the answer on your own.