3d- Experimental determination of structure Flashcards
What is elemental microanalysis 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
What does an empirical formula show?
The simplest ratio of the elements in a molecule
What two things can elemental microanalysis be determined from?
- combustion product masses
- percentage product by mass
What can mass spectrometry be used to determine?
The accurate GFM and structural features of an organic compound
What happens during 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
What can the mass-to-charge ratio of the parent ion 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 identify?
Certain functional groups in an organic compound
What happens when infrared radiation is absorbed by organic compounds?
Bonds within the molecule vibrate (stretch and bend). The wavelengths of infrared that are absorbed depend on the type of atoms that make up the bond and the strength of the bond
What happens during 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 cm-1)
What can proton NMR give information on?
The different chemical environments of hydrogen atoms in an organic molecule, and about how many hydrogen atoms there are in each of these environments
How do 1H 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 a proton NMR spectrum what is the chemical shift related to, and what is it measured in?
It is related to the environment of the proton atom and is meaured in parts per million (ppm)
What is the standard reference substance used in proton NMR?
Tetramethylsilane (TMS), which is assigned a chemical shift value equal to zero
How is high-resolution proton NMR different to low-resolution proton NMR?
High-resolution proton NMR uses higher radio frequencies and provides more detailed spectra
