Deducing Structures from Mass Spectra

Mass Spectra and IR

What is the main purpose of using mass spectra in this topic?

To interpret fragmentation patterns and suggest possible structures of simple organic compounds.

Why are additional fragment m/z values important for compounds containing oxygen?

Oxygen-containing functional groups can produce characteristic fragment ions that help identify the structure.

What possible ion produces a peak at m/z = 15?

CH₃⁺.

What possible ion produces a peak at m/z = 17?

OH⁺.

What possible ion produces a peak at m/z = 28?

CO⁺.

What possible ions can produce a peak at m/z = 29?

CH₃CH₂⁺ or CHO⁺.

What possible ion produces a peak at m/z = 31?

CH₂OH⁺.

What possible ions can produce a peak at m/z = 43?

CH₃CH₂CH₂⁺, CH₃CHCH₃⁺ or CH₃CO⁺.

What possible ions can produce a peak at m/z = 45?

COOH⁺ or CH₃CHOH⁺.

What possible ion produces a peak at m/z = 57?

C₄H₉⁺, which can have four possible structures.

Why can one m/z value correspond to more than one possible fragment ion?

Different ions can have the same relative mass and charge.

Why is it useful to compare the m/z of a fragment ion with the molecular ion?

The difference indicates the mass of the group lost during fragmentation.

A molecular ion is at m/z = 60 and a fragment is at m/z = 45. What has been lost?

A group with a mass of 15, suggesting loss of CH₃.

What should you identify first when deducing a structure from a mass spectrum?

The molecular ion peak, as this gives the relative molecular mass of the compound.

After finding the molecular ion peak, what should you examine?

The major fragment peaks and the possible ions that could produce them.

Why are peaks present in both possible compounds often less useful?

They do not help distinguish between the possible structures.

What are the most useful peaks when comparing mass spectra of structural isomers?

Major fragment peaks that occur in one spectrum but not the other.

Compounds A and B have molecular formula C₃H₆O. What is their relative molecular mass?

58.

What molecular ion peak would C₃H₆O produce?

m/z = 58.

Why does a peak at m/z = 58 not distinguish between two C₃H₆O isomers?

Both isomers have the same relative molecular mass and therefore the same molecular ion m/z.

Both C₃H₆O spectra contain a peak at m/z = 15. Which ion causes this?

CH₃⁺.

Why does the m/z = 15 peak not distinguish between the two C₃H₆O structures?

Both compounds contain fragments capable of producing CH₃⁺.

Compound A, C₃H₆O, has a major peak at m/z = 29. What possible ions could cause it?

CH₃CH₂⁺ or CHO⁺.

Which C₃H₆O structure fits a major m/z = 29 peak particularly well?

Propanal, CH₃CH₂CHO.

Why does propanal fit a major peak at m/z = 29?

It can fragment to form CH₃CH₂⁺ and/or CHO⁺, both with m/z = 29.

Compound B, C₃H₆O, has a major peak at m/z = 43. Which useful oxygen-containing fragment can cause it?

CH₃CO⁺.

Which C₃H₆O structure is suggested by a major CH₃CO⁺ peak at m/z = 43?

Propanone, CH₃COCH₃.

Why is m/z = 43 strong evidence for propanone in this example?

Propanone can readily fragment to form CH₃CO⁺, while the alternative hydrocarbon fragments do not fit the remaining mass appropriately.

What structures are identified for the two C₃H₆O spectra in the worked example?

A is propanal, CH₃CH₂CHO, and B is propanone, CH₃COCH₃.

Compounds C and D have molecular formula C₃H₈O. What is their relative molecular mass?

60.

What molecular ion peak would C₃H₈O produce?

m/z = 60.

Why does m/z = 60 not distinguish between C₃H₈O isomers?

They have the same molecular formula and therefore the same relative molecular mass.

Compound C has a major peak at m/z = 31. Which fragment ion can cause this?

CH₂OH⁺.

What C₃H₈O structure is suggested by a major m/z = 31 peak?

Propan-1-ol, CH₃CH₂CH₂OH.

Why does the m/z = 31 peak support propan-1-ol?

The CH₂OH⁺ fragment is consistent with a terminal CH₂OH group.

Compound D has a major peak at m/z = 45. What possible ions could cause this?

COOH⁺ or CH₃CHOH⁺.

Why can COOH⁺ be ruled out for a compound with molecular formula C₃H₈O?

COOH⁺ contains two oxygen atoms, but C₃H₈O contains only one oxygen atom.

Which fragment therefore explains the m/z = 45 peak in compound D?

CH₃CHOH⁺.

What C₃H₈O structure is suggested by the CH₃CHOH⁺ fragment?

Propan-2-ol, CH₃CH(OH)CH₃.

What structures are identified for the two C₃H₈O spectra in the worked example?

C is propan-1-ol and D is propan-2-ol.

How can a molecular formula help you decide between possible ions with the same m/z?

You can eliminate fragment ions containing atoms that are not present, or more atoms of an element than the original molecule contains.

What is a good overall method for deducing an organic structure from a mass spectrum?

Find the molecular ion to determine Mr, use the molecular formula if given, identify important fragment peaks, assign possible ions to them, and choose the structure consistent with all the evidence.