Karteikarten
chem 29
Karten in diesem Stapel (35)
- 1define : chromatographyused to separate individual components from a mixture of substances
stationary + mobile phases - 2define : thin layer chromatographyquick
inexpensive
plastic sheet or glass plate coated with absorbent silica which uses adsorption to hold the different substances to its surface - 3define : Rfretention factor
distance of component
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distance of solvent - 4define : gas chromatographyuseful for separating and identifying volatile organic compounds
- stationary phase is a high boiling liquid absorbed onto an inert solid support
- mobile phase is an inert carrier gas
(more soluble component will move slower and have a higher retention time) - 5gas chromatography spectracomponents are detected as peaks
peak integrations (area under)
used to determine the concentration in the sample - 6define : NMRSNuclear Magnetic Resonance Spectroscopy
- 7combination of strong magnetic field and radio frequency radiationwith correct combination, nuclei of atoms absorb radiation
energy from absorption is measured and recorded on NMR spectrum - 8why is nuclear spin significantnuclear spin is significant when there is an odd number of nucleons (commonly H and 13C isotope) but can also detect isotopes of other elements with odd nucleons
- 9define : resonanceelectron and nucleus both have 2 different spin states, with different energies
with the correct combination of magnetic field + radio frequency radiation --> the nucleus absorbs energy and rapidly flips between two spin states (RESONANCE) - 10what does the NMR spectrometer useuses very strong super-conducting electromagnet cooled to 4K by helium
- 11which has more energy? infrared or radio frequencyRadio Frequency radiation has less energy than Infra Red
- 12when can energy be detected on the NMR spectrometerfrequency required for resonance is proportional to magnetic field strength
only in strong and uniform magnetic fields the small quantity of energy can be detected - 13??????all atoms have e- surrounding the nucleus
shifts the energy and radio frequency needed for nuclear magnetic resonance to take place - 14define : chemical shiftfrequency shift is measured on a scale called chemical shift in units parts per million (ppm)
- 15define : TMSTetraMethyl Sailene is used as a standard reference chemical against which all chemical shifts are measured
amount of chemical shift is determined by chemistry environment and presence of nearby electronegative atoms - 16running the NMR spectrum-sample is dissolved in solvent
-placed in narrow NMR sample tube with small amount of TMS
-tube is placed in spectrometer
-spun to even ?? at any imperfections in magnetic field within sample - 17use of carbon-13 NMR spectroscopyprovides info about
- number of different C environments (from number of peaks)
- types of C environments present (from chemical shift) - 184 main types of C atomsthey absorb over different chemical shift ranges and may also be outside the ranges depending on the solvent , concentration and substituents
- 19define : chemical environmentit's determined by the position of C atom within the molecule
if 2 atoms have the same chemical environment then they absorb radiation at the same chemical shift + contribute to the same peak - 20propanal CH3CH2CHOO
H H //
H - C - C - C - H
H H
it has 3 environments
1,2,3 - 21propanone CH3COCH3H O H
l ll l
H - C - C - C - H
l l
H H
it has 2 environments
1,2,1 - 22interpreting the spectraC atoms matched to different spectra based on number of peaks
chemical shifts allow peaks to be assigned
-draw out structures and identify environments
-compare with number of peaks on spectra
- number of peaks should match the number of environments - 23what does a Proton NMR spectrum tell us about a molecule- number of different proton environments (from number of peaks)
- types of proton environments (from chemical shifts)
- relative numbers of each type of proton (from integration traces or ratio numbers of the relative peak areas)
- number of non-equivalent protons adjacent to a given proton (from the spin-spin splitting pattern) - 24define : equivalent atoms/protonsthey are in the same environment so absorb at the same chemical shift value
- increasing the peak size
(if env1 has double the nmbr of Hs than env2, then the proton NMR will have 2 peaks - one with twice area) - 25use of ratio of the relative areas under each peakratio of the relative areas under each peak gives the ratio of the number of protons responsible for each peak
- 26define : NMR spectrometerNMR spectrometer measures the area under each peak as an integration trace shown as an extra line or printed number of the relative peak areas
- 27what are sub peaks (splitting patterns) caused by?a proton NMR peak can be split into sub-peaks or splitting patterns
these are caused by the proton's spin interacting with the spin states of nearby protons in different environments - 28define : spin-spin splitting (or coupling)it's the splitting a main peak into subpeaks
patterns:
singlet - doublet - triplet - quartet
number of subpeaks = n+1
n= number of protons attached to adjacent carbon atom
(aromatic protons - splitting is difficult to interpret) - 29which peaks may be broad?NH and OH protons may be involved in H bonding and the NMR peaks are often broad (not involved in spin-spin coupling) and of variable chemical shift
- 30explain proton exchange1- proton NMR spectrum is run as normal
2- small volume of denterium oxide (D2O) is added
3- the mixture is shaken and a second spectrum is run
(denterium exchanges and replaces the OH and NH protons in sample with denterium atoms)
4- second spectrum is run on CH3OD.
denterium doesn't absorb in this chemical shift range, OH peak disappears - 31typical sequence for identification of a molecule's structure- elemental analysis
- mass spectra
- infrared spectra
- NMR spectra - 32define: elemental analysisuse of percentage composition by mass to determine the empirical formula of a compound
- 33define : mass spectrause of the molecular ion peak to determine the molecular mass and fragment ions to determine parts of the molecule
the molecular formula can then be determined from the empirical formula and the molecular mass - 34define : infrared spectrause of absorption peaks to identify bonds present and functional groups
- 35define : NMR spectrato determine the number and types of carbon and hydrogen atoms from the chemical shift of peaks and the order of the atoms within molecules from splitting pattern
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- AS level Chemistry (module 6) Karteikarten am chem 29, erstellt von myriam farah am 19/03/2018.
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- myriam farah
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