Chapter 11 Alkenes and IR
I.
H H H
Alkene Nomenclature
A.
Unsaturation
1) Alkanes: CnH2n+2
H C C C H
H H H
H
2)
Alkenes: CnH2n
H
C C
H
3)
C H
H
H
Degree of Unsaturation
a) Tells us how many rings and double bonds in molecule
b) Hsat = 2C + 2 – X + N
(Ignore O, S)
c) Degree of Unsaturation = (Hsat – Hact)/2
d) Example: C5H8NOCl
i. Hsat = 2(5) + 2 – 1 + 1 = 12
ii. (Hsat – Hact)/2 = (12 – 8)/2 = 2 degrees of unsaturation
O
H H Cl H
C N C C C
H
H H
C
H
H
B.
Nomenclature
1. Common Names end with –ylene
H
a. Ethylene
H
H
C
b.
2.
3.
4.
Propylene
H
H
C C
C
H
H
C H
H
H
IUPAC: Replace –ane of an alkane with –ene of an alkene
a. Ethene
b. Propene
Alkenes follow alkane nomenclature, with double bond location
numbered closest to end
H
H
a. 1-butene
CH2 CH3
C CH3
C
C
b. 2-butene
H3C C
H
c. Cylclohexene H
H
Substituents named as prefixes with lowest numbers
a. 3-methyl-1-pentene
b. 3-methylcyclohexene
5.
Disubstituted Alkenes can be cis or trans streoisomers
CH3
CH3
H
CH3
a. cis-2-butene
C C
C C
H
H
b. trans-2-butene
H
CH3
c. Cycloalkenes cis unless large
6. Tri- or Tetra-substituted Alkenes can be E or Z stereoisomers
a. Use priorities from R/S nomenclature
b. Assign 1-2 on each carbon
c. Move from 1-2-1-2 to trace out an E or Z
CH3CH2
CH2CH2CH3
Br
F
C C
C C
ClCH2CH2
CH3
F
H
E-1-chloro-3-ethyl-4-methyl-3-heptene
Z-1-bromo-1,2-difluoroethene
7.
Alcohols have priority over alkene in numbering: Alkenol
H3C
OH
Cl
2-propen-1-ol
CH
CHCH2CH3
Z-5-chloro-3-ethyl-4-hexen-2-ol
C C
CH3
8.
OH
H
H
Alkene substituents are named alkenyl
ethenylcyclohexane
CH2
R
2-propenyl-
R
trans-1-propenyl-
C C
CH3
H
II.
Pi-bonding in Alkenes
A.
The p-bond
1) sp2 hybridization results in 120o angles
2) H1s-Csp2 overlap gives the CH s-bonds
3) Csp2—Csp2 overlap gives the C—C s-bond
4) Cp—Cp overlap gives the C—C p-bond
B.
Bond Strength
1) Bond strength is proportional to orbital overlap
2) The s-bond in ethene is very strong because of good overlap
3) The p-bond in ethene is fairly weak because of poor overlap
4) Overall, the double bond is stronger than a C—C single bond
5) The weak p-bond will be the reactive part of the molecule
6)
Orbital and Energy level diagrams for ethene
7)
Thermal Isomerization tells us the p-bond energy
a. cis/trans interconversion must go through broken p-bond T.S.
b.
c.
d.
8)
Ea = 65 kcal/mol should be about the same as the p-bond strength
The s-bond is slightly stronger than alkane due to better sp2 overlap
C—H bonds are also stronger than in alkanes (110 kcal/mol)
Radical H-atom abstraction doesn’t occur in alkenes because of the strong
C-H bonds. The chemistry is dominated by the weak p-bond.
III. Physical properties of Alkenes
A.
B.
Boiling points are about like alkanes
Melting points depend on the isomer
1) cis-alkenes have a U-shape that disrupts packing in the solid,
giving lower temperatures (Vegetable oils have cis-alkenes)
2) trans-alkenes have melting points close to the alkanes
C.
Polarization
1) Alkenes are more polar than alkanes due to more e-withdrawing sp2
hybrid orbitals (more s-character draws e- closer to nucleus)
2) cis-alkenes are more polar than trans-alkenes due to their shape
H
H
H
R
C C
C C
CH3
D.
H
CH3
R
Acidity of alkenes > alkanes, again because of the greater s-character of sp2
hybrid orbitals.
1) Ethane pKa = 50
2) Ethene pKa = 44
IV. NMR of Alkene
A.
p-electrons deshield hydrogens
1) Alkane H 1.0 ppm
2) Alkene H 5-6 ppm
3) Spectrum of an alkene
B.
Coupling in Alkenes depends on the isomer
C.
13C
NMR of alkenes gives peaks at 100-150 ppm due to deshielding
H3C 122.8 CH3
C C
H3C
18.9 CH3
H
123.7 C
H3C
12.3
H
C132.7
CH2CH3
20.5 14.0