KA 4: experimental determination of structure Flashcards
what is elemental microanalysis used to determine?
masses of C,H,O,S and N in a sample of an organic compound to determine it’s empirical formula
what does empirical formula show?
simplest ratio of elements in a molecule
two ways elemental microanalysis can be determined
-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
describe the process of mass spectrometry
-small sample of organic comp. bombarded by high-energy elecs
-electrons therefore removed from organic molecule generating + charged molecular ions (parent ions)
-these break into smaller + charged ion fragments
how is a mass spectrum 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?
gfm of molecular ion
.:. molecular formula can be determined using emp. formula
the fragmentation data can be interpreted to gain…
structural information
what is infrared spectroscopy used to identify?
certain functional groups in an organic compound
describe what happens when infrared radiation is absorbed by organic compounds
-bonds within molecule vibrate (stretch and bend)
-wavelengths of i.r that are absorbed depend on type of atoms that make up bond and strength of the bond
describe what happens in infrared spectroscopy
-infrared radiation passed through a sample of the organic compound
-then into detector that measures the intensity of the transmitted radiation at different wavelenghts
how is the absorbance of infrared radiation measured
wavenumbers (reciprocal of wavelength) cm-1
where are charateristic absorptions by particular vibrations given?
data booklet
what does proton NMR or 1H NMR stand for?
proton nuclear magnetic resonance spectroscopy
what info does proton NMR provide?
-different chemical environments of H atoms (protons or 1H in an organic molecule)
-how many hydrogen atoms there are in each of these environments
describe the behaviour of 1H nuclei
-behave like tiny magnets
-in a strong magnetic field, align w/ the field (lower E) while the rest align against it (higher E)
describe what happens when 1H nuclei absorb radiation in the radio frequency
-they “flip” from lower energy to higher energy alignment
-as they fall back from higher to lower E alignment the emitted radiation is detected and plotted on the spectrum
describe a 1H NMR 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?
the number of 1H atoms in that environment and is often given by an intergration curve in a spectrum
what is the height of an intergration curve proportional to?
the NO. of 1H atoms in that environment, and so a ratio of 1H atoms in each environment can be determined
what is the standard reference substance used in 1H NMR spectroscopy?
tetramethylsilane (TMS), which is assigned a chemical shift value equal to zero
what can 1H NMR spectra can be obtained using?
low-resolution or high-resolution NMR
difference between high/low-resolution 1H NMR?
high: uses higher radio frequencies than low res
pro of high-res over low-res
provides more detailed spectra
