organic - NMR Flashcards
what does NMR stand for
-nuclear magentic resonance
-analyse organic compounds
what does NMR detect/what are the two types
-1H NMR/ proton NMR - gives info abt the hydrogen atoms in organic molecules
-13C NMR - gives info abt the carbon atoms in organic molecules
what type of solvent is used for NMR and why
-compound is usually dissolved in a solvent
-use CCl4 or CDCl3 - D = deuterium which is 2H
-use these bc the solvents shouldn’t contain any 1H atoms as these may hide the signals due to 1H atoms in the compound
what happens in NMR spec
-compound being analysed is put in a very strong magnetic field
-radiowaves are passed through the compound
-the compound absorbs some radioowaves and change the spin direction of the nuclei of the 1H or 13C atoms - producing spectrum
-each nucleus generates a magnetic field - spin at diff energy levels - one spin will one up with the external field and the opposing spin lines up higher - so can then identify
what is the standard that is used for calibration and why
TMS - tetramethylsilane
-only produces one signal - as all H + C are in the same environment
-inert
-non-toxic
-gives signal in an area that does not overlap with those form most organic compounds
what is a chemcial shift and how is it produced
-represents the difference bwt the frequencies at which the compound and TMS absorbs in parts per million (ppm)
-gives signals in a spectrum
what happens to C that are closer to an electronegative or double bond element
-further to the left of the shift
-greater chemical shift
how to tell the difference bwt carbon-13 and 1H spectra
-C13 = 0-200 ppm
-1H = 0-10 ppm
what is the key elements of a 13C spectra
-peaks correspond to carbon in different environments
-closer to electroneg/double bond - further left
-no link bwt area of signal and number of carbon atoms
-no splitting
-label each carbon with its corresponding ppm - so easier to see which is which
how do identify butanone in a spectra
-there would be 4 signals
-closest to the TMS = CH3 - as only bonded to CH2
-the next to the left would be CH2 - has CH£ on one side and C=O on the other
-the next would be CH3 bonded only to C=O
-the furthest away would be C attached to =O
what are the key elements of 1H spectra
-one signal for each H in a different environment
-closer to electroneg/double bond - further left
-relative area of the signals relate to the relative number of 1H atoms
-signal split by 1H atoms on adjacent atoms - into double,triplets etc
-the number of h correspond to the rate
eg Ch3 = 3 h
CH2 = 2
h = 1
-find number of signals then find the ratio of hydrogens
-add similar ones tg
eg if 2 methyls joined to a carbon - then 6 H
how many peaks would ethanol have on an 1H spectra - low resolution
Ethanol has the structural formula CH3CH2OH
There are 3 chemical environments: -CH3, -CH2 and -OH
The hydrogen atoms in these environments will appear at 3 different chemical shifts - 3 peaks
-CH3, -CH2, -OH are in the ratio of 3:2:1 respectively
what does coupling mean
-when the 1H atoms are not equivalent to each other on neighbouring C atoms
-does not happen bwt equivalent atoms
what is the n +1
-the number of lines of each peak = 1 + the number of inequivalent H atoms on adjacent C atoms
-neighbouring
what is a singlet
- if the C has a molecule attached to a C that has no hydrogens on it then it is a singlet
what is a doublet
- if the C atom has a C with i H on it then the number of lines is 2
-neighbouring
-relative size of lines is 1:1
what is a triplet
-if the C atom has a C with 2 hydrogens on it then the number of lines is 3
-neighbouring
-relative size of lines is 1:2:1
what is a quartet
-if the C atom has a C with 3 hydrogens on it then the number of lines is 4
-neighbouring
-relative size of the lines is 1:3:3:1
what is a multiplet
-more than 3 neighbouring inequivalent H atoms
which H do not couple each other
-equiv H atoms eg 2 CH2
-H atoms to H on an OH group
-H atoms to H atoms of an NH2
how to find the ratio of NMR - hydrogen
-divide by smallest number
-put in whole number ratio
what are some common reacting ratios for 1H NMR
- 1.2:1.2:1.8 = 2:2:3
- 2.1:2.8 = 3:4
- 1.5:0.5:2.0 = 3:1:4
- 0.3:0.15:0.3:0.6 = 2:1:2:4
how can the structure of a compound be deduced by 1H NMR
-look at each signal to workout what group it represents in a molecule
-look for the ends of the molecule
-then work out where te non-end groups fit in
what does a quartet and triplet indicate in NMR
-CH3-CH2
-2:3
-dont have to be next to each other
-atom joined to the CH2 cannot have a H on it
