Transcript CH 9-6 Power Point

CH 9-6: (4) Alcohol (R-OH) Protons in 1H NMR
•How many proton signals do you expect for ethanol?
(a)
(b)
(c)
•The –OH proton signal is usually found around 3-4 ppm.
•Since the –OH proton is acidic we rarely observe splitting!
The –OH signal is always a singlet and does not split
adjacent protons.
•When a few drops of D2O are added to an alcohol sample,
the acidic –OH proton will undergo acid/base exchange with
D2O forming –OD, and the –OH NMR signal disappears!
•The deuterium atom is invisible in 1H NMR. We can take
advantage of this chemistry to ID –OH NMR signals.
Identifying the R-OH acidic proton in 1H NMR
•All acidic protons can be exchanged with deuterium (“D”)
from D2O.
•Can be acid or base catalyzed, or uncatalyzed.
•Since “D” is invisible in 1H NMR, the signal for the –OH
proton will disappear.
NMR Background Review
(1) Chemical Equivalence of Hydrogen atoms.
(2) The “Chemical Shift” scale is defined as ppm, and
ranges from 0 - 13 ppm.
(3) Signal Splitting: The magnetic field of a specific H
nucleus will be influenced by all H nuclei on adjacent
carbon atoms. This causes the signal to be “split”. The
N=1 Rule is used to predict splitting.
(4) Alcohol Protons. The NMR signal from acidic R-OH
protons is not “split” by adjacent protons. Acidic protons
will also disappear when a few drops of D2O are added to
the sample.
Identification of an Unknown: IR & NMR Data
•Determine the Hydrogen Deficiency. What does this tell you?
•If the chemical formula is given, what does that tell you?
•Based on the IR, what functional groups are obviously present?
•In the 1H NMR spectrum, identify all signals and label as (a), (b), (c), etc.
How many protons are associated with each set of signals? How many
total protons are in the molecule? Summarize in a table.
•Are there any diagnostic chemical shifts in the NMR spectrum (>4.5
ppm)? What functional groups are represented?
•Are any of the protons exchangeable with D2O (acidic protons of
alcohols, carboxylic acids, amines).
•Each NMR signal (a,b,c…) represents a unique H or group of
equivalent H’s. Based on the splitting pattern for each signal, draw an
organic structural fragment (“puzzle piece”) that will produce the
splitting for that specific H or equivalent group of H’s.
•Once you have all the “puzzle pieces”, piece them together to
complete the unknown structure.
CH 9-6: (4) Alcohol (R-OH) Protons in 1H NMR
•After studying this video, try working through the
following problem to solve the structure of an unknown.
We will work on this problem in class (no quiz).
(a) The unknown formula is C4H10O. Calculate the Hydrogen Deficiency. What
information does the (i) HD and (ii) formula tell you about the unknown?
(b) What functional group information do you get from the IR spectrum? List
the frequency and the name of the functional group.
(c) Note that I am giving you the NMR data in table format. Note that there are
four signals (a-d) suggesting four non-equivalent groups of protons. Start by
adding up the total number of protons. Does that match the H’s in the formula?
(d) Now look at the chemical shifts. If you have chemical shifts (ppm) > 4.5, you
may be able to identify additional functional groups. Do any signals disappear
when D2O is added? What does this suggest? If this consistent with the IR?
(e) Now look at the splitting for each signal. Based on the # of protons and the
splitting you should be able to draw a partial structural “puzzle piece”. For
example, if you have a signal for a 3H (t), this may suggest a puzzle piece that
looks like: -CH2CH3
1
H NMR Data Table
(e) Finally, take all of your
puzzle pieces and try
putting them together
into a complete structure.
Check that your structure
matches all of the given
data (formula, HD, IR,
NMR), and that all
carbons have four bonds!
*This signal disappears when D2O was added to the sample.