Mass Spectra: 1-pentanol vs 3-pentanol

In mass spectrometry, 1-pentanol can be observed to have no peak at $$m/z = 59$$, whilst 3-pentanol can be observed to have a peak at $$m/z = 59$$.

Clearly the 3-pentanol peak at $$m/z = 59$$ is due to the cleavage creating $$\ce{CH3CH2CHOH | CH2CH3}$$

Why is this the case, as isn't 1-pentanol able to form the fragment $$\ce{CH2CH2CH2OH}$$ by cleaving at the third carbon?

i.e. fragmenting into $$\ce{CH3CH2}$$ and $$\ce{CH2CH2CH2OH}$$ (this should have $$m/z = 59$$?).

• RE: "... Why is this the case, as isn't 1-pentanol able to form the fragment CH2CH2CH2OH by cleaving at the third carbon?..." Of course 1-pentanol can cleave in such a manner. It is just that such cleavage is highly improbable.
– MaxW
Dec 19 '19 at 0:34

1 Answer

In mass spectrometry, organic compounds can be ionized by different methods. The compounds ionized by electron impact ionization display particular fragmentation patterns. When subjected to mass spectroscopy, cleavage of the $$\ce{C-C}$$ bond next to the oxygen or nitrogen can usually occur, when the compound is ionized by electron impact ionization.

Let’s look at what happens to 3-pentanol: $$\ce{CH3CH2CH(OH)CH2CH3 ->[e- beam] CH3CH2CH(O^.+H)CH2CH3 }$$

$$\ce{CH3CH2CH(O^.+H)CH2CH3 -> CH3CH2CH=O^+H + ^.CH2CH3}$$

Now, base peak (most probably), would be $$\ce{[CH3CH2CH=OH]+}$$, which should be at $$59 \ m/z$$. Note that in general, the molecular ion of an alcohol (here it is $$88 \ m/z$$) is either trace or non-existent, probably loosing $$\ce{H2O^.}$$.

Now, we'll look at what happens to 1-pentanol: $$\ce{CH3CH2CH2CH2CH2-OH ->[e- beam] CH3CH2CH2CH2CH2-O^.+H }$$

$$\ce{CH3CH2CH2CH2CH2-O^.+H -> CH2=O^+H + ^.CH2CH2CH2CH3}$$

Now, base peak (again, most probably), would be $$\ce{[CH2=OH]+}$$, which should be at $$31 \ m/z$$ ($$88-31=57$$ mass loss). Note that here again, the molecular ion is either trace or non-existent, probably loosing $$\ce{H2O^.}$$, and possibly $$[M-18]$$ ion should be significant here, since there is equal chance of cleaving $$\ce{C_1-O}$$ bond (giving peak at $$70 \ m/z$$) or $$\ce{C_2-C_3}$$ bond (giving peak at $$31 \ m/z$$):

However, according to the mass spectrum given, $$\ce{C_5}$$-carbon chain has undergone McLafferty type rearrangement (which is common in MS of aldehydes and ketones) to give the base peak at $$42 \ m/z$$ by loosing a water and ethylene simultaneously from parent ion (see the insert in above diagram).