# 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

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).