Spin-Spin Splitting in Alkenes

Spin-Spin Splitting in Alkenes
Alkenes are not free to rotate around the C-C bonds; the two
protons attached to the same carbon may or may not be equivalent
Can have two-bond as well as three-bond couplings in alkenes
Three-bond couplings are referred to as vicinal couplings
Typical values are 6-15 Hz for cis
Typical values are 11-18 Hz for trans
Two-bond couplings are referred to as geminal couplings
Typical values are 0-1 Hz
In order to split one another, the protons must be nonequivalent
In cis-difluoroethylene both protons are equivalent and do
not split on another
In trans-difluoroethylene both protons are equivalent and do
not split one another
In monofluoroethylene the three protons are not equivalent
and will split one another
Trans-1,2Cis-1,2difuoroethylene
difuoroethylene
Both protons equivalent Both protons equivalent
Monofluoroethylene
Protons Nonequivalent
Example: Vinyl Acetate (from text)
Each of the three protons (a, b, and c) is split by the others:
Proton a is split by proton c (cis proton) into a doublet and again
by proton b (geminal proton – small splitting) into a doublet of
doublets
Proton b is split by proton by proton c (trans proton) into a doublet
and again by proton a (geminal proton – small splitting) into a
doublet of doublets
Proton c is split by proton by proton b (trans proton) into a doublet
and again by proton a (cis proton) into a doublet of doublets; both
splittings relatively large
The assignments are based on the appearance of the multiplets
Analysis of multiplet a
Small splitting is the interaction with proton b (geminal splitting)
1347.76 – 1373.29 = 1.47 Hz
1368.51 – 1367.04 = 1.47 HZ
Large splitting is the interaction with proton c (cis splitting)
This splitting is the difference between the centers of the two
pairs of peaks
(1347.76 + 1373.29) / 2 = 1374.03
(1468.51 + 1367.04) / 2 = 1367.78
1374.03 – 1367.78 = 6.25 Hz
Analysis of multiplet b
Small splitting is the interaction with proton a (geminal splitting)
1472.57 – 1471.01 = 1.48 Hz
1458.59 – 1456.75 = 1.84 Hz (should be 1.48)
Large splitting is the interaction with proton c (trans splitting)
This splitting is the difference between the centers of the two pairs
of peaks
(1472.57 + 1471.09) / 2 = 1471.83
(1458.59 + 1456.72) / 2 = 1457.67
1471.83 – 1457.67 = 14.16 Hz
Analysis of multiplet c
In this case both splittings are relatively large; only vicinal
couplings are involved
The smaller of the two is due to coupling between proton c and
proton a (a cis splitting)
The larger of the two is due to coupling between proton c and
proton b (a trans splitting)
Again, the splitting is the difference between the centers of
the peaks
(2192.85 + 2186.60) / 2 = 2189.73
(2178.88 + 2172.63) /2 = 2175.76
2189.73 – 2175.76 = 13.97 Hz
Note that if two or more protons are coupled, then the coupling
constant between protons a and b is the same as the coupling
constant between protons b and a.
For protons a and c:
In multiplet a the coupling constant between protons a and c
was measured to be 6.25 Hz
In multiplet c the coupling constant between protons c and a
was measured to be 6.25 Hz
For protons a and b:
In multiplet a the coupling constant between protons a and b
was measured to be 1.47 Hz
In multiplet b the coupling constant between protons b and a
was measured to be 1.48 Hz
For protons b and c:
In multiplet b the coupling constant between protons b and c
was measured to be 14.16 Hz
In multiplet c the coupling constant between protons c and b
was measured to be 13.97 Hz (well within margin of error)
Can say that two protons are coupled if they have the same
coupling constant; can be useful in assigning peaks.
Another Alkene Example: Crotonic Acid
Multiplet a is a pair of doublets and must arise as a result of one
large and one small splitting. This can be assigned to the methyl
group.
Multiplets b and c arise as a result of two splittings; a splitting by a
single proton into a doublet and a second splitting into quartets by
three equivalent protons.
The split into a doublet will be the same for both protons, but
for one the coupling constant with the methyl group is much
stronger than the other.
The proton cis to the methyl group will experience a much
larger coupling constant with the methyl group; multiplet c is
assigned to this proton on this basis
The proton trans to the methyl group will experience a much
smaller coupling constant with the methyl group; this is a
long-range interaction.
Analysis of Multiplet a (methyl group)
The methyl protons are split into a doublet of doublets by the two
single protons
The large splitting is due to interaction with the proton cis to the
methyl group (a normal three-bond coupling)
This coupling constant is calculated to be 6.07 Hz
The small splitting is due to interaction with the proton trans to the
methyl group (a long-range coupling, will be small)
Two values calculated from spectra – 1.46 Hz and 1.80 Hz
Ideally these should be the same
Analysis of Multiplet b
Multiplet b arises from the hydrogen atom trans to the methyl
group
The signal from proton b is split into a doublet by proton c
(trans proton – large coupling constant)
Each doublet is split into quartet by the methyl group (proton a)
(long range coupling – small coupling constant)
The small coupling constant is 1.64 Hz
The large coupling constant is 15.51 Hz
Analysis of multiplet c
Multiplet c arises from the proton cis to the methyl group
The signal from proton c is split into a doublet by proton b
(trans proton –large coupling constant)
Each doublet is split into a quartet by the methyl proton
This coupling is much larger than in the preceding case
(6.89 Hz versus 1.64 Hz)
As a result there is partial overlap of these multiplets
The trans splitting is calculated by taking the difference between
the centers of the quartets
Works out to be 14.91 Hz
Close to 15.51 Hz – well within a margin of error
Spin-Spin Splitting in Aromatics
Benzene – all protons equivalent, give rise to a singlet
Monosubstituted benzene – protons divided into three sets
those ortho to the substituent group
those para to the substituent group
those meta to the substituent group
Typical values for coupling constants are:
7-10 Hz for ortho protons
2-3 Hz for meta protons (may not be observed)
0-1 Hz for para protons (may not be observed)
Often do not get the predicted number of peaks for symmetrically
substituted benzene
Toluene (methyl benzene) gives a single line for all the
aromatic protons on a low-field instrument
Can also get more than the predicted number of peaks due to a
breakdown of first-order splitting
Only asymmetrically substituted compounds follow expected
splitting patterns
Aromatic Example 1: 3-nitrobenzaldehyde
300-MHz spectrum of 3-nitrobenzaldehyde
In this case the ortho couplings appear to be the only ones
observed
Ha experiences only meta and para couplings; appears as a singlet
Hb and Hd experience one ortho coupling and appear as doublets
Not possible to assign based on the above spectrum alone
Some of the meta couplings show up if spectrum is expanded
Hc experiences two ortho couplings and appears as a triplet
The aldehyde proton appears downfield at 10.147 ppm
Aromatic Example 2: 1,10-phenanthroline
360-MHz Spectrum of 1,10-Phenanthroline
Appears that only ortho couplings are observed
Ha and Hc should appear as doublets; the signal at 9.18 ppm is
assigned to Ha because it is closer to the electronegative nitrogen
atoms and should be shifted furthest downfield
The signal at 7.62 ppm is assigned to Hb since this is spit by both
Ha and Hc; should appear as triplet or doublet of doublets
The signal at 7.76 ppm is assigned to the Hd protons since this
signal appears as a singlet; no other protons in ortho positions
Aromatic Example 3: 2-nitrophenol (from text)
Both ortho and meta couplings are present in this case
The b and d protons experience two couplings, one large and one
small, and appear as a doublet of doublets
The large coupling is an ortho coupling
The small coupling is a meta coupling
The a and c protons experience two large couplings (causing a split
into doublet of doublets, which appear as a triplet) and one small
coupling (causing each component of the triplet to be split)
The large coupling is an ortho coupling
The small coupling is a meta coupling