3.4 experimental determination of structure Flashcards
what can elemental microanalysis be used to determine the masses of
C H O S 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 can elemental microanalysis can be determined from
combustion product masses and percentage product by mass
what can mass spectrometry be used to determine
the accurate gram formula mass and structural features of an organic compound
describe the process of 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 ratio
what can the mass to charge ratio of the parent ion be used to determine
the gram formula mass of the molecular ion, and so a molecule formula can be determined using the empirical formula
what data can be interpreted to gain structural information
fragmentation data
what can be used to identify certain functional groups in an organic compound
infrared spectroscopy
what happens when infrared radiation is absorbed by organic compounds
bonds within the molecule vibrate (stretch and bend).
what do the wavelengths of IR radiation that are absorbed depend on
the type of atoms that make up the bond and the strength of the bond
what happens in IR spectroscopy
IR 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.
what is the absorbance of IR radiation measured in
wavenumbers, the reciprocal of wavelength
what does proton NMR or 1H NMR stand for
proton nuclear magnetic resonance spectroscopy
what can proton NMR give information about
- the different chemical environments of hydrogen atoms in an organic molecule
- how many hydrogen atoms there are in each of these environments
what do proton nuclei behave like
tiny magnets
where do the proton nuclei align in a strong magnetic field
some align with it, if they have lower energy, and some align against it, if they have higher energy
what does the absorption of radiation in the radio frequency region of the EM spectrum cause
the proton 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
what is the chemical shift
the peak position
what is the chemical shift related to in a proton NMR spectrum
the environment of the proton NMR
what is the area under the peak on a proton NMR spectrum related to
the number of proton atoms in that environment
what is the area under the peak related to the numbre of proton atoms often given by
an integration curve on a spectrum
what is the height of an integration spectrum proportional to
the number of proton atoms in that environment, allowing a ratio of the proton atoms in each environment to be determined
what is the standard reference substance using in proton NMR spectroscopy
tetramethylsilane (TMS)
how can proton NMR spectra be obtained
using low resolution or high resolution NMR
what does high resolution proton NMR use that allows it to produce more detailed spectra than in low resolution proton NMR
higher radio frequencies
what occurs in high resolution proton NMR
an interaction with the proton atoms on neighbouring carbon atoms can result in the splitting of peaks into multiplets. the number of proton 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 proton atoms on the neighbouring carbon atom
which proton NMR spectra can be sketched for any given compound
low resolution
