Diagnosis: Analysis II
Not Voodoo Home / Comments? / Anatomy of a reaction
A Perfect Reaction (
see all): TLC indicates a single product. A crude 1H NMR
spectrum indicates only your desired compound with few or no
impurities. You can easily identify an optimal method for
A Nightmare Scenario (
see all): You can't find your product.
Common Rookie Mistakes:
Step by Step:
TLC the crude product to make sure it matches the reaction mixture.
If it does not, see
If you do not expect reagent peaks to overwhelm product peaks,
take an NMR of your crude reaction mixture after workup.
Why take a crude NMR or GC?
In most cases, it is important to know about all the products
generated from a reaction, not just the major product.
You can get
the ratio of starting material to product (percent conversion).
You can get the ratio of product to byproduct. This is helpful
You can get ratios of isomers- this is always
Sometimes chemists think they have isolated the
major component of the reaction mixture- a crude NMR can reveal that
the minor component was isolated, and the major component is still at
If you observe two products and can identify
them by NMR or GC, record the ratio of the products.
Decide whether the product(s) are worth
isolating (see Examples): if so, proceed to Purification.
See also: Rookie Mistakes, TLC How to, 1H NMR How to, Reaction Checklist, Troubleshooting the Experiment