Experimental Determination Of Structure Flashcards
What can be used to determine the masses of C, H, O,
S and N in a sample of an organic compound in order to determine its empirical formula.
Elemental microanalysis
What does an empirical formula show?
The simplest ratio of the elements in a
molecule
How can elemental microanalysis be determined?
- combustion product masses
- percentage product by mass
What can mass spectrometry be used to do?
Determine the accurate gram formula mass (GFM) and structural features of an organic compound
Describe mass spectrometry.
In mass spectrometry, a small sample of an organic compound is bombarded by high-energy electrons. This removes electrons from the organic molecule generating positively charged molecular ions known as parent ions. These molecular ions then break into smaller positively charged ion fragments. A mass spectrum is obtained
showing a plot of the relative abundance of the ions detected against the mass-to-charge (m/z) ratio.
The mass-to-charge ratio of the parent ion can be used to determine the GFM of the molecular ion, and so a molecular formula can be determined using the empirical formula.
What is infrared spectroscopy used to do?
Identify certain functional groups in
an organic compound
How does infrared spectroscopy work?
When infrared radiation is absorbed by organic compounds, bonds within the molecule vibrate (stretch and bend). The wavelengths of infrared radiation that are absorbed depend on the type of atoms that make up the bond and the strength of the bond.
Describe infrared spectroscopy.
In infrared spectroscopy, infrared radiation is passed through a sample of the organic compound and then into a detector that measures the intensity of the transmitted radiation at different wavelengths. The absorbance of infrared radiation is measured in wavenumbers, the reciprocal of wavelength, in units of cm-1.
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What can proton nuclear magnetic resonance spectroscopy (proton NMR or 1H NMR) give information about?
The different chemical environments of hydrogen atoms (protons or 1H) in an organic molecule, and about
how many hydrogen atoms there are in each of these environments.
Describe proton NMR.
1
H nuclei behave like tiny magnets and in a strong magnetic field some align with the field (lower energy), whilst the rest align against it (higher energy). Absorption of radiation in the radio frequency region of the electromagnetic spectrum causes the 1H nuclei to ‘flip’ from the lower to the higher energy alignment. As they fall back from the higher to the lower energy alignment the emitted radiation is detected and plotted on a spectrum.
In proton NMR what is the chemical shift related to?
In a 1H NMR spectrum the chemical shift, δ , (peak position) is related to the environment of the 1H atom and is measured in parts per million (ppm).
What is the area under the peak related to in proton NMR?
The number of 1H atoms in that environment and is often given by an integration curve on a spectrum. The height of an integration curve is proportional to the number of 1
H atoms in that environment, and so a ratio of 1
H atoms in each environment can be determined.
What is the standard reference substance used in proton NMR?
Tetramethylsilane (TMS), which is assigned a chemical shift value equal to zero
What is the difference between high resolution and low resolution proton NMR?
High-resolution 1H NMR uses higher radio frequencies than those used in low-resolution 1H NMR and provides more detailed spectra
Describe high resolution proton NMR.
In a high-resolution 1H NMR an interaction with 1
H atoms on neighbouring carbon atoms can result in the splitting of peaks into multiplets. The number of 1
H atoms on neighbouring carbon atoms will determine the number of peaks within a multiplet and can be determined using the n+1 rule, where n is the number of 1
H atoms on the neighbouring carbon atom.
