Not Voodoo Home / Comments? / Anatomy of a reaction
A Perfect Reaction (
see all): You obtain a proton NMR spectrum
of a pure compound without
solvent peaks. Each peak can be assigned to a proton of your target compound, and
the integrations are consistent with expectations. Other spectra can be obtained
A Nightmare Scenario (
see all): All of the peaks in your proton NMR
spectrum overlap, the compound is
not the one you expected, and you don't know what it is.
Common Rookie Mistakes:
Spectroscopy Step by Step:
Remove all the solvent from your compound by rotoevaporating thoroughly.
If the compound is not volatile, put it on a high vacuum line for 10-20
Some compounds cling to ethyl acetate. To get rid of residual peaks, try
dissolving the compound in a little dichloromethane, and then rotoevaporating.
Repeat three times.
Acquire a proton NMR spectrum. Analyze the spectrum to confirm your
Label any solvent peaks or known impurities.
Label the peaks that correspond to starting material, or known byproducts, if
present. This will be a factor only when purification does not effectively
Label peaks corresponding to isomers, if applicable. Write the ratio of
isomers on the spectrum.
Label the protons of your product (Ha, Hb, etc.) Assign each peak in the
spectrum to a proton of your product.
Consider measuring the spectrum of a single fraction from a column, if
purification is inefficient.
If you have an OH or NH proton you'd like to assign, add a drop of D 2O to the
sample, shake thoroughly, and take another proton spectrum.
Often, enough information is gained from proton NMR spectroscopy to identify the
product. Sometimes it is appropriate to gather more characterization data, and
Continue with characterization when
Your advisor recommends it.
The compound is new and publication is planned. The spectrum should contain
only product peaks.
You don't know what the compound is.
Do NOT continue with characterization when
Your advisor does not recommend additional characterization.
Your compound has been reported previously in the literature and your data
matches the data reported.
It is a routine procedure, you know exactly what you have, and the compound is
an intermediate in a longer synthetic sequence.
Beyond 1H NMR:
Carbon NMR spectroscopy*
Other specialized NMR experiments; DEPT, HMQC, HMBC
* Necessary for publication often useful for structure determination
See also: Rookie Mistakes, 1H NMR How to, Reaction Checklist, Troubleshooting the Experiment