56
3 Combination Tables
3.2 Alkenes, Cycloalkenes
Assignment
Range
Comments
C=C
C–(C=C)
100–150 ppm
10–60 ppm
Considerable differences between Z and E:
X
H
C
H
X
C
H–(C=C)
4.5–6.5 ppm
Coupling constants: |J gem | 0–3 Hz
J cis 5–12 Hz
J trans 12–18 Hz
CH 3 –(C=C)
≈1.7 ppm
Coupling constants: 3 JCH
3 –CH=C ≈ 7 Hz
CH 2 –(C=C)
≈2.0 ppm
3 JCH
2 –CH=C ≈ 7 Hz
In rings, |J| smaller:
H
H
H
C n
n = 2, 3 J ≈ 0.5 Hz
n = 3, 3 J ≈ 1.5 Hz
n = 4, 3 J ≈ 4.0 Hz
Long-range coupling, 4 J HC–C=CH 0–2 Hz
H–C(=C) st
3100–3000 cm -1
C=C st
1690–1635 cm -1 Of variable intensity
H–C(=C) δ oop 1000–675 cm -1
CH 2 –(C=C) δ
1440 cm -1
Molecular ion m/z 14n
m/z 14n - 2
Alkenes: moderate intensity
Monocycloalkenes: medium intensity
Fragments
m/z 14n - 1
m/z 14n - 3
Local maxima for alkenes
Local maxima for monocyclic alkenes
Usually, double bonds cannot be localized
Rearrangements
n-Alkenes: unspecific except for:
Cyclohexenes: retro-Diels–Alder reaction:
+
+.
+.
C=C π π*
< 210 nm
(log ε 3–4)
Isolated double bonds; for highly substituted
double bonds often absorption tail
(C=C) 2 π π* 215–280 nm
(log ε 3.5–4.5)
13 C
1 H
IR
MS
UV
R
R
R H
X
- X C H =CH 2
R
R
R H
+ .
+ .