Structure 3.2.7—Stereoisomers have the same constitution (atom identities, connectivities and
bond multiplicities) but different spatial arrangements of atoms. (AHL)
Structure 3.2.8—Mass spectrometry (MS) of organic compounds can cause fragmentation of
Molecules. (AHL)
Structure 3.2.9—Infrared (IR) spectra can be used to identify the type of bond present in a molecule. (AHL)
Structure 3.2.10—Proton nuclear magnetic resonance spectroscopy (1H NMR) gives information on the different chemical environments of hydrogen atoms in a molecule. (AHL)
Structure 3.2.11—Individual signals can be split into clusters of peaks. (AHL)
Structure 3.2.12—Data from different techniques are often combined in structural analysis. (AHL)
Stereoisomerism
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Same structural formula but different spatial arrangement
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Stereoisomers
Structural isomer | Stereoisomer |
molecules with the same molecular formula but different | |
positional arrangement of atoms, substituents or functional groups (structural formula) | spatial arrangement of atoms or substituents in the compound |
Cycloalkanes
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Alkanes with the general formula of alkenes via end C atoms joining to form a ring (.˙. 2 less H atoms needed)
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Can have cis/trans isomers even without a double bond as the ring structure prevents free rotation about the sigma bond.
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Optical isomers (Enantiomers)
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Optical = Chiral center = asymmetric = stereocenter
1.
4 different atoms or substituent groups on central C atom (Chiral center often marked with ‘*’)
2.
Form non-superimposable mirror images
a.
Analogously, our left and right hands are non-superimposable mirror images
3.
Enantiomers rotate plane of polarized light in opposite directions (optical activity)
4.
Racemic mixture (50:50 of each optical isomer) has no optical activity
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Physical and Chemical Characteristics of Optical Isomers
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Optical isomers have identical chemical and physical characteristics
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Can only be distinguished by the direction of rotation of the plane of polarized light
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There may be more than 1 chiral center in which case the the non-superimposable stereoisomer may not be mirror image, i.e. diastereomer
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Mass Spectrometry
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Used to identify the structure unknown or new compounds
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When a molecule is ionised it forms a molecular ion which can also undergo fragmentation to produce particles of smaller mass, where only particles with a positive charge will be deflected and detected
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The Molecular Ion
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When the whole molecule is ionised it forms a molecular ion
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The molecular ion produces the peak of the largest m/z value, which gives the molar mass
Fragments
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Each peak on the spectra is shown due to a particular fragment with a certain m/z value
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It’s position provides information about the molecular mass of a substance
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The tallest peak comes from the most stable species
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It is possible to identify the type of compound from its spectrum by looking at the position of peaks and differences between major peaks
Mass lost from Mr | Fragment Lost |
15 | CH3 |
17 | OH |
18 | H2O |
28 | CH2=CH2 , C=O |
29 | CH3CH2 ,CHO |
31 | CH3O |
45 | COOH |
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provided in data booklet
E.g. Mass spectrometry of propan-1-ol, CH3 CH2 CH2 OH
Isotopes
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Halogen compounds produce multiple molecular ion peaks due to presence of isotopes
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Bromine consists of 50% 79Br and 50% 81Br
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the presence of one Br atom produces two molecular ion peaks of equal abundance at two units apart
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Chlorine consists of 75% 35Cl and 25% 37Cl
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The presence of one Cl atom produces two molecular ion peaks in the ratio 3:1 at two units apart
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hence in dihalogenoalkanes the combination of halogens can be 3 ways e.g. C2H4(35Cl)2+, C2H4(37Cl)2+, C2H4(35Cl37Cl)+
Infrared Spectroscopy
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Used to detect bonds present in a molecule
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When molecules absorb energy in the infrared region of the spectrum they vibrate at a particular frequency
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Molecules that are polar can only be detected using vibration
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Only vibrational motions which result in a change of dipole moment of a molecule will absorb infrared radiation
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Types of vibrations:
Identifying Bonds
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A graph of absorption against wavenumber is produced
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The region from 500-1500cm-1 is known as the fingerprint region, which is specific for each molecule
1H NMR spectroscopy
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Detects unique hydrogen environments in a organic compound
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Reference Standard : The position of absorption (the chemical shift) is measured relative to the absorption of TMS mixed with the sample
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TMS is used as a reference because:
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All protons are in the same chemical env. → produces 1 strong peak/singlet
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It is chemically inert, therefore, it doesn’t react with the compound being analysed
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It’s chemical shift/signal is outside the range of the common chemical shift/signal
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Analysing 1H NMR
1.
Number of peaks : number of different hydrogen environments
2.
Chemical shift: the nature of the hydrogen environment
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More deshielded H (next to EN groups) will be downfield
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More shielded H (next to non-EN groups) will be upfield
3.
Area under peak/Integration trace: number of H atoms in each environment
4.
Splitting: number of H atoms on adjacent C atoms
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Represented in splits of one big peak due to spin-spin coupling
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No.of lines = no. of non-equivalent H atoms on neighbouring carbons + 1 (n+1)
H atom on neighbouring carbon | Splits |
0 | singlet |
1 | doublet |
2 | triplet |
3 | quartet |
4 | quintet |
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the H atom on O-H is not affected by hydrogens on adjacent atoms, hence is always a singlet
Example: ethanal
Applications of H NMR
1.
Structural determination: distinguishing between cis/trans isomers
2.
Magnetic resonance imaging (MRI): obtaining images of internal organs