For example in case of NMR spectrum n-Propanol in CDCl3 why don't we get the signal of ( -OH) functional group in the absolute right position of the spectra instead the peak appears after the (-CH2) Group as a triplet. What is the possible reason for that?
Well, the right position for hydroxylic protons is between 1-5 ppm, for aliphatic alcohols. And sometimes one can see the coupling of those exchangeable protons with the closest ones. As in this case that you get a triplet due to the coupling with the closest methylene protons.
With aliphatic alcohols you only get exchange of the hydroxyl protons if there is something to exchange with - normally if water is present.
In CDCl3 the solvent breaks up hydrogen-bonding interactions (that would be present in pure propanol) so the chemical shift of the -OH protons is reduced and will appear between the CH3 and CH2 chemical shifts.
The same thing happens in ethanol and is a classical example of these effects and as such is explained in "Physical Chemistry" by W.J.Moore.
Hallo Tanvir
I agree with Leonardo and Dr. Stephenson.
regarding chem. shift: The peak position depends on the electron density around the proton. More electon density shifts the peak upfield to the lower ppm scale, less shifts it downfield to the higher ppm scale. the electron density around the excahngable proton again depends on parameters like H-bond strength.... Therefore the ppm range is wide and also depends strongly on the solvent.
That the porton forms a triplett opposed to a broad singlet tells that the proton isnot really exchanging within the timescale of the experiment. Because exchange would decouple the J-coupling.
Many greetings
Kristof
Dear Kristof Grohe Sir , Thank you for your answer. Extremely grateful to you , my pleasure.
Exchangeable protons have chemical shifts that are affected by many different things. Temperature, concentration, pH, water content, solvent, etc. This is due to (at least) chemical exchange, hydrogen bonding, conformational change and sometimes even micelle formation. Hydroxyls can be particularly impacted. You may get more consistent shifts in non-polar solvents such as DMSO but exchangeable chemical shifts in CDCl3 can be very difficult to reproduce.
Summing up: there is no such thing as the "absolute right position" for signals due to exchangeable protons; everything depends strongly on experimental conditions. Please do not take respective data too serious...
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