Mass Spectrometry : Examples involving carbocations (carbonium ions)

Summarizing the most important conclusion from the page on carbocations:

Order of stability of carbocations

primary < secondary < tertiary

Applying the logic of this to fragmentation patterns, it means that a split which produces a secondary carbocation is going to be more successful than one producing a primary one. A split producing a tertiary carbocation will be more successful still. Let's look at the mass spectrum of 2-methylbutane. 2-methylbutane is an isomer of pentane - isomers are molecules with the same molecular formula, but a different spatial arrangement of the atoms.

Look first at the very strong peak at m/z = 43. This is caused by a different ion than the corresponding peak in the pentane mass spectrum. This peak in 2-methylbutane is caused by:

The ion formed is a secondary carbocation - it has two alkyl groups attached to the carbon with the positive charge. As such, it is relatively stable. The peak at m/z = 57 is much taller than the corresponding line in pentane. Again a secondary carbocation is formed - this time, by:

You would get the same ion, of course, if the left-hand CH₃ group broke off instead of the bottom one as we've drawn it. In these two spectra, this is probably the most dramatic example of the extra stability of a secondary carbocation.

مواضيع ذات صلة

Spectroscopic Properties of Alkynes

Spectroscopic Properties of Alkenes

Application of Chemical Shifts to Structure Determination

Correlations Between Structure and Chemical Shifts

Electronegativity and Chemical Shifts

Effects of Carbon Bond Type

Mass Spectroscopy

Hydrogen Bonding

Chemical Exchange

Proton-Proton Splittings and Conformational Analysis

Spin-Spin Splitting - What We Observe

Proton-Proton Splittings and Chemical Exchange