Lecture outline
Vinylic Hs —
Typically δ 5.0 - 6.5 (but substituents may shift them outside of this range)
Complex splitting patterns are common due to different coupling constants for different
pairs of Hs.
Ha
Hb
C
R
C
Hc
Geminal coupling is observed only when the geminal Hs are different. This can also
show up for a pair of Hs on an sp3 C when these Hs are different (i.e. diastereotopic; we'll get to
that later). Jgem is strongly dependent on the H–C–H angle — it can be anywhere from 0 to 20
Hz; but for vinylic Hs, 1 - 3 Hz is typical. This is a "2-bond" coupling.
R–CH=CH2 usually appears as 1 H downfield (~δ 6) and 2 Hs upfield (~δ 5) within the
vinylic region, e.g.,
!!6
Ha
!!5
Hc
Hb
Tree diagrams take the different couplings into account one at a time to reproduce the observed
pattern. Draw tree diagrams to explain the splitting patterns above.
Note that it is possible to generate the familiar multiplets by turning on the equivalent
couplings one at a time. Draw a tree diagram to show that a 1:2:1 triplet can be produced by
"turning on" the coupling to two neighboring Hs, one at a time. What about a quartet?
Vinylic Hs can also participate in
"normal" vicinal coupling across
a single bond, so even more splitting
results. For example, in propene,
the circled H is split into a d by the
cis H, a d by the trans H, and a
q by the 3 methyl Hs. Thus,
16 peaks in all (!) though some
may end up on top of each other.
d
d
H
q
C
CH 3
H
C
H
Approximately what J-values would you predict for the couplings indicated above?
In addition, allylic (and propargylic) Hs also couple weakly to the distant vinylic (or
acetylenic) Hs. These long-range "4-bond" couplings are usually very small (J ≈ 0 - 3 Hz).
Sometimes this coupling is large enough to produce a visible splitting of peaks, sometimes it's so
small its effect is only to broaden the peaks, and sometimes it's completely absent.
Alkene stereochemistry can often be determined by the J-value. Consider, for example,
β-chloroacrylic acid. What splitting patterns would you expect to see in the vinylic region of its
spectrum? How would you determine the key coupling constant?
Cl–CH=CH–CO2H
Suppose you had a sample of this compound and you recorded its NMR spectrum. The
vinylic Hs displayed a pair of doublets, each with a peak separation of 13 Hz. Does this establish
conclusively that the compound is cis or trans?
Would it be helpful to have a sample of the other stereoisomer? Aha! You're in luck —
you find a sample of the other stereoisomer (but again, this sample vial fails to specify the
stereochemistry... grrr.) You record the NMR spectrum of that one and find a peak separation of
7 Hz for each vinylic doublet. Now do you know the stereochem?
The 60-MHz NMR spectrum below contains a beautiful example of complex splitting.
The spectrum belongs to an ester with the formula C5H8O2. (a) Draw the structure. Don't get
distracted by the splitting — that's not the place we start when determining structures. (b) Use
the splittings to assign the vinylic signals to the Hs of the structure, and (c) draw a tree diagram
that explains the splitting of the signal at δ 7.0. (d) What is the source of the tiny splitting of the
signals around δ 6 and d 2? (e) What is the coupling constant between the vinylic Hs? (Note
that there are two ways to estimate Hz — from the field strength and by comparison with other
splittings whose J values you know (hint: 7Hz)) (f) Explain why the one vinylic signal is so far
downfield.
Here's another case where a tree diagram can help us understand the observed splitting
pattern and establish stereochemistry. Below is part of the 300-MHz 1H NMR spectrum of a
mixture of cis- and trans-4-tert-butylcyclohexanols. Which signal comes from which
stereoisomer? To figure this out, we have to think about (a) the dominant conformation of each,
(b) which H we're looking at, and (c) how that H interacts with its neighbors to produce the
patterns shown.
Also, what's the ratio of the isomers? Recall that it was mentioned in class that axial and
equatorial Hs on a cyclohexane chair appear at surprising different chemical shifts. Which is
upfield? Are the signals above consistent with this?