Mass Spectrometry of Organic Compounds

Mass Spectra and IR

What is mass spectrometry used for in organic chemistry?

To determine the relative molecular mass and help identify the structure of an organic compound.

Why does the mass spectrum of an organic compound often contain many peaks?

The molecular ion can fragment into smaller ions, producing several different peaks.

What does each peak in the mass spectrum of an element represent?

An isotope of the element.

What does m/z mean?

The mass-to-charge ratio of an ion.

What is the molecular ion peak?

The peak produced by the whole organic molecule after it loses one electron.

How is a molecular ion formed?

A high-energy electron collides with the molecule and removes one electron, forming a positive ion.

Write the general equation for formation of a molecular ion.

M + e⁻ → M⁺• + 2e⁻

What charge does a molecular ion have?

+1.

Why does the m/z value of the molecular ion usually equal the relative molecular mass, Mr?

The molecular ion normally has a charge of +1, so its mass-to-charge ratio equals its relative molecular mass.

How can you usually identify the molecular ion peak?

It is usually the significant peak with the greatest m/z value.

What is the molecular ion peak of butane?

m/z = 58.

What does the molecular ion peak at m/z = 58 tell you about butane?

Butane has a relative molecular mass, Mr, of 58.

What is the M+1 peak?

A small peak one m/z unit higher than the molecular ion peak.

What causes the M+1 peak in many organic mass spectra?

The presence of naturally occurring carbon-13 (¹³C) instead of carbon-12 (¹²C) in some molecules.

Approximately what percentage of carbon atoms are carbon-12?

About 99%.

Approximately what percentage of carbon atoms are carbon-13?

About 1%.

Why can carbon-14 normally be ignored in mass spectrometry?

Its natural abundance is extremely small.

What causes peaks with m/z values lower than the molecular ion peak?

Fragmentation of the molecular ion into smaller ions.

What is fragmentation?

The breaking of a molecular ion into smaller pieces.

How can fragmentation help identify an organic compound?

The fragment ions provide information about groups of atoms present in the molecule and therefore help determine its structure.

What usually happens when a carbon-carbon bond breaks during fragmentation?

A positive fragment ion and a neutral species, usually a free radical, are formed.

Which fragmentation products are detected by a mass spectrometer?

Positive ions.

Why are neutral fragments not shown in a mass spectrum?

They are not detected by the mass spectrometer.

What is a free radical?

A species containing an unpaired electron.

How is a free radical represented?

With a dot to show the unpaired electron.

How is a positive fragment ion represented?

With a + charge.

Why can the same bond breaking sometimes produce different fragment ions?

Either of the two fragments may retain the positive charge.

What fragment ion gives a peak at m/z = 15?

CH₃⁺, the methyl ion.

What fragment ion gives a peak at m/z = 29?

C₂H₅⁺, the ethyl ion.

What fragment ion gives a peak at m/z = 43 in butane?

C₃H₇⁺, the propyl ion.

What ion gives the peak at m/z = 58 in butane?

C₄H₁₀⁺•, the molecular ion.

What are the important peaks in the mass spectrum of butane shown in the textbook?

m/z = 15, 29, 43 and 58.

What is the base peak?

The peak with the greatest abundance in a mass spectrum.

What relative intensity is assigned to the base peak?

100%.

What does the height of a peak in a mass spectrum show?

The relative abundance of that ion.

What does the m/z position of a peak tell you?

Which ion or fragment is present.

What is the difference between m/z and peak height?

m/z identifies the ion, whereas peak height shows its relative abundance.

What is the base peak in the mass spectrum of butane shown?

m/z = 43.

Which fragment produces the base peak of butane?

C₃H₇⁺.

What does a mass spectrum plot on the x-axis?

m/z, with no units.

What does a mass spectrum plot on the y-axis?

Relative intensity or relative abundance, from 0% to 100%.

Why might an exam mass spectrum show only selected peaks?

To remove distracting peaks and leave the peaks useful for determining the structure.

How can you use a mass spectrum to determine the relative molecular mass of an organic compound?

Identify the molecular ion peak and use its m/z value as the relative molecular mass.

How can you use fragment peaks to suggest the structure of an organic compound?

Match their m/z values to possible positive fragment ions and use these fragments to identify parts of the molecule.

Why can mass spectra of organic compounds be complex?

Fragmentation and rearrangement can produce many different ions and therefore many peaks.

What is rearrangement during mass spectrometry?

A process in which atoms rearrange during fragmentation, producing additional fragment ions.

Why are rearrangement reactions often not used when interpreting simple mass spectra?

They can be unpredictable and make the spectrum more complicated.

Is breaking a carbon-hydrogen bond usually described as fragmentation in this topic?

No; fragmentation mainly refers to breaking the molecular ion into larger identifiable pieces such as by breaking carbon-carbon bonds.

What information should you look for first when analysing a simple organic mass spectrum?

The molecular ion peak to determine Mr, followed by important fragment peaks to identify parts of the structure.