Six Clues to Help Solve a Structure
to think of solving spectroscopy problems as analogous to
a detective in the dark with a flashlight, searching for whodunit.
Methodical clue gathering is the best way to determine the
culprit (those mischievous compounds in our case) rather than
blindly waving the flashlight (or pencil) about, hoping for
some sort of chance epiphany (which invariably comes not during
an exam but immediately following).
talk about how to systematically find those clues! It's a
good idea to write down all of the clues on a piece of scratch
paper as you come across them, so they are in front of you
when you go to solve the structure.
1: Determine the degrees of unsaturation from the molecular
formula or mass spec. From this you can determine if double
bonds, triple bonds, or rings may be present on the unknown
compound, and how many. Click
here for a tutorial on how to determine degrees of unsaturation.
2: Look at the IR (if applicable) to determine the major
functional groups present in the compound, confirming (if
possible) their presence in the NMR spectrum (easy ones to
confirm include aldehydes, carboxylic acids, and benzene rings).
3: Find out how many hydrogens each set of peaks on an
NMR represents by comparing the ratio of the integration to
the molecular formula.
4: Break the NMR peaks into fragments using the integration
found in clue 3 and write each fragment down on a piece of
scratch paper. For a helpful list of common fragments found
with certain hydrogen integrations, click
5: Combine all of the fragments in a way that makes sense
with the NMR peak splitting. This is the hardest part, and
sometimes the best way is simply guessing and checking. Put
together a possible structure and check to see if it makes
sense in the NMR. If not, try another one until it does.
6: Recheck your structure with the NMR and the IR to make
sure it is an exact match.
Tutorial :: An