4.8 organic synthesis and analysis Flashcards
what does NMR stand for?
nuclear magnetic resonance
low resolution NMRs = from AS level
high resolution = A level (if talk about splitting patterns)
low resolution Hydrogen NMR:
- each hydrogen environment produces one peak
- the peak area is the number of hydrogens producing that peak
- the scale goes from 10 to 0
how many hydrogen environments does benzene ⏣ have?
1
how many hydrogen environments does chlorobenzene have?
3
carbon-13 NMR:
- identical principles to hydrogen NMR but with carbon atoms
- each peak represents a different carbon environment
- scale runs from 0 to 220
- use data sheet to quote numbers
- when the nuclei absorb the radio frequency radiation, the nuclei move from the lower energy state to a higher energy state
- this absorption is detected and converted into an NMR spectrum
what can be deduced from the analysis of a proton NMR spectrum?
- the no. of peaks = no. of different environments of hydrogen atoms in the molecule
- area under each peak = no. of hydrogen atoms of that type
what is the integration factor of a proton NMR?
- the relative intensity of the peaks (area under the peak)
- (usually written above the peak as hard to read directly)
why are NMR values typically expressed in parts per million e.g 2.0ppm?
- the value of (the chemical shift) δ is generally very small
what is every peak in NMR measured relative to?
- the standard
(TMS / tetramethylsilane)
what is the standard used in NMR spectra?
- tetramethylsilane / TMS
why do we use TMS as a standard?
- as TMS has 12 protons which are all equivalent and 4 carbons, which are also all equivalent
- this means that it gives a single, strong signal in the spectrum which turns out to be outside the range of most other signals, especially from organic compounds
- and TMS is unreactive
- i.e the hydrogens in TMS are all effectively shielded (they dont interfere with H peaks from molecule)
- those hydrogens with a large chemical shift (δ) are said to be ______
- and those with lower values of δ are said to be _____
- deshielded
- shielded
is there always an NMR peak at 0?
- yes because all of them are measured relative to TMS
what is the n+1 rule?
- the peaks in proton NMR are often not single peaks - they are split into a number of peaks very close together
- n = no. of hydrogen atoms on adjacent atoms
- a hydrogen atom with no hydrogen atoms on adjacent carbon atoms will have no splitting and will give a single peak
in n+1 what does n stand for?
- the number of adjacent hydrogen atoms - 3 BONDS AWAY
how many peaks will a hydrogen atom with no hydrogen atoms on adjacent carbon atoms give?
1
- it will have no splitting and will give a single peak known as a singlet
what is 1 peak called in H NMR?
a singlet
what are 2 peaks called in H NMR?
a doublet
what are 3 peaks called in H NMR?
a triplet
what are 4 peaks called in H NMR?
a quartet
what are 5 peaks called in H NMR?
a pentent
what are more than 4 peaks called in H NMR?
multiplet