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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

S3.2 - Functional groups - Classification of organic compounds

3.2.1 Representing Organic Compounds 3.2.2 Functional Group 3.2.3 Homologous Series 3.2.4 Trends and Properties of Homologous Series 3.2.5 Nomenclature 3.2.6 Structural Isomerism 3.2.7 Stereoisomerism and Chirality (AHL) 3.2.8 Mass Spectrometry (MS) of Organic Compounds (AHL) 3.2.9 Infrared (IR) Spectroscopy and Greenhouse Gases (AHL) 3.2.10 Proton NMR Spectroscopy (AHL) 3.2.11 Splitting Patterns in Proton NMR Spectroscopy (AHL) 3.2.12 Interpreting Spectra (AHL)

Mass Spectrometry (MS) of Organic Compounds HL Only

Specification Reference S3.2.8

Quick Notes

  • Mass spectrometry (MS) identifies compounds based on mass-to-charge ratio (m/z) of fragments.
  • The molecular ion peak (M+) corresponds to the Mr (relative molecular mass) of the compound.
  • Fragmentation occurs when bonds break, forming smaller ions.
  • We can use specific fragment masses (given in the data booklet) to deduce structural features.

Full Notes

The Basics of Mass Spectrometry

IB Chemistry diagram of a typical mass spectrum showing molecular ion peak and fragmentation peaks.

Identifying Molecular Mass and Formula

The molecular ion peak (M+) has the highest m/z value on a spectra (peak furthest to the right).

We can use precise atomic masses to confirm the molecular formula based on the M+ peak.

For Example Determining the molecular formula of a compound with an M+ = 58.12

IB Chemistry example spectrum showing M+ peak at 58.12 for butane C4H10.

Fragmentation

Inside the mass spectrometer, molecular ions can break into fragments. Each fragment forms a smaller ion and shows up as a peak at a lower m/z.

These peaks help identify parts of the molecule.

Example Hydrocarbons A and B

Hydrocarbons A and B both have a molecular formula of C4H10 (same molecular ion peak), however they have different fragment patterns in their spectra, showing different structures.

IB Chemistry diagram comparing fragmentation patterns of isomers of C4H10.

Fragment peaks at 15 and 43 show a CH3 fragment and C3H7 fragment. However, no fragment at 29 means no C2H5 group. This means the likely possible structure is CH3CH(CH3)CH3.

IB Chemistry spectrum showing presence of m/z 29 indicating ethyl fragment for C4H10 straight chain isomer.

The extra peak at m/z 29 for Hydrocarbon B means it has a C2H5 group in its structure (as well as a CH3 and C3H7 group). This would indicate CH3CH2CH2CH3 as its structure.

Example Fragmentation Patterns

Certain functional groups and bond types break in characteristic ways.

Examples of useful fragments (data booklet values):

Fragment m/z Common in
CH3+ 15 Alkanes, methyl groups
C2H5+ 29 Alkanes
OH+ / H2O+ 17 / 18 Alcohols
COO+ 45 Carboxylic acids
C6H5+ (phenyl) 77 Aromatic compounds

Summary