Question

a. What would be the m/z value for the molecular ion peak of acetaldehyde (ethanal)? Remember m/z values are whole numbers. Please show work and explain why.

Answer 1

In the mass spectrum, the heaviest ion (the one with the greatest m/z value) is likely to be the molecular ion.

A mass spectrum is an intensity vs. m/z (mass-to-charge ratio) plot.

m/z stands for the mass by charge ratio of ion. the m/z value of molecular ion peak for ethanol is 44.

In case of Aldehydes cleavage of bonds next to the carboxyl group results in the loss of hydrogen to give peak with good intensity at m/z =43 (molecular ion having mass less by 1 than parent ion) or the loss of CHO (molecular ion less by 29). it also results into a peak at m/z =29.

For ethanal, hydrogen next to the carboxyl group is usually stripped away yielding CH₃CO+ (m/z = 43). The whole carboxyl group can break off, forming a CHO+ ion (m/z=29). This is why the peaks at m/z=43 and 29 are amongst the highest in the mass spectrum of ethanal.

Following is given some experimental data for mass specta of ethanal which i have done myself in lab. The data shows m/z values of some peaks and their intensities , m/z at 29 and 44 being most intense.

Exhaust gas species after ionization(m/z) 27 29 31 42 43 44 45

Peak intensity 3.59 99.99 0.3 12.79 47.49 82.59 2.79

CH3-CH=O → CH₃CHO⁺ → CH₃CO⁺ + H^.

Parent ion   m/z=44 m/z=43

CH₃CHO⁺ → CH₃⁺ + CHO⁺.

m/z=15 m/z=29