1. Alkenes Bonding, Nomenclature, Properties Structure
chapter 5
Alkenes
Bonding, Nomenclature, Properties Z E
Structure
Hydrogen Deficiency
Nomenclature
Physical Properties
Naturally Occurring Alkenes/Terpenes
Lecture 10, February 25, 2003 b

2. R or S?
R or S?

3. Which is 2S,3R-propoxyphen & 2R,3S-propoxyphen?
antitussive NovraD | DarvoN analgesic

4. Unsaturated Hydrocarbons
1. Alkene: contains a carbon-carbon double bond
ethene
2. Alkyne: contains a carbon-carbon triple bond (Ch 7)
ethyne (acetylene)
3. Arene: benzene and its derivatives (Ch 21-22)
phenyl. Ph

5. Structure of Alkenes -bond Double Bond:
1 -bond formed by overlap of 2 sp2 hybrid orbitals
1 -bond formed by overlap of 2 parallel 2p orbitals
-bond
perpendicular
to the plane
-bonded atoms
all in a plane 
trigonal - bond
angles ~ 120° H C

6. Structure of Alkenes No rotation about a C=C bond - why?
Rotation requires breaking  bond
~63 kcal/mol

7. Nomenclature IUPAC: Root name
- longest continuous chain containing the olefin.
Number chain - olefin
lowest number #-
alkene
indicates C=C
in chain
Number of C’s in C=C
chain
Start 8 sept 25, 2001 1 2 3 4 5 6
2-ethyl-4-methyl-1-pentene
2-hexene

8. IUPAC names?
(7S)-7-bromo-4-(2-iodoethyl)-6-methyl-3-octene
7-bromo-4-(2-iodoethyl)-6-methyl-3-octene

9. Number of C’s in chain with an C=C
Nomenclature IUPAC: Root name
- longest continuous chain containing the olefin
#-cycloalkene
# alkene
Number chain - olefin
lowest number #-
alkene
indicates C=C
in chain
Number of C’s in chain with an C=C
Cyclic
Olefin (functional group) - positions 1,2
Number around the ring to best accommodate substituents

10. 4-bromo-4-methylcycloheptene
IUPAC names?
cycloheptene
-bromo- -methyl
4-bromo-4-methylcycloheptene
(3 ,5 )- -chloro- -fluoro- -methylcyclopent-1-ene
(3S,5R)-5-chloro-1-fluoro-3-methylcyclopent-1-ene

11. The Cis,Trans System recall cis/trans isomersCl C Cl H
cis-1,2-dichloroethene
trans-1,2-dichloroethene
trans generally more stable than cis - dipoles and 1,2 interactions

12. The Cis,Trans System trans-3-hexene cis-3,4-dimethyl-2-pentene
Configuration is determined by the orientation of atoms of the main chain
trans-3-hexene
cis-3,4-dimethyl-2-pentene

13. Cycloalkenes- 3 to 7 “cis” olefins
rings not large enough to accommodate trans double bonds
C8 limited stability as trans

14. E,Z Configuration E Z uses priority rules (Chapter 3)
higher priority groups - same side, Z
higher priority - opposite sides, E Z E

15. E/Z - priorities of groups on ends of C=C
(1) Atom assigned a priority, higher atomic number higher priority.
(2) Isotopes - higher atomic mass, higher priority H < 2H < 3H [H < D < T]
(3) If priority the same, go to the next set of atoms: CH2-H < -CH2-O-H < -CH2-Cl
(4) double (triple) bonds replaced by single bonds. C H C H

16. The E,Z System
Example: name each alkene and specify its configuration by the E,Z system

17. ?(trans)-longest chain?
IUPAC names?
Stereochemistry?
?(trans)-longest chain?
(3Z,7S)-7-bromo-4-(2-chloroethyl)-6-methyl-3-octene
(7S)-7-bromo-4-(2-chloroethyl)-6-methyl-3-octene
(S)-6-ethyl-1-fluoro-5,5-dimethylcyclooctene

18. priority goes to the next point of difference
If same atoms,
priority goes to the next point of difference
(E)-
(4E)-
9-bromo-5-(2-methylpropyl)-4-nonene

19. number so enes (enyne, ynes) have the lowest possible number
Nomenclature - more than 1 unsaturated group
Diene (or diyne, en-yne)
longest chain (ring) with both groups:
#,#-alkadiene
#,#-alkadiyne
#-alken-#-yne
(if the same ene > yne).
number so enes (enyne, ynes) have the lowest possible number 5 1 2 3 4 6 7
1,4-cycloheptadiene

20. Dienes, Trienes, and Polyenes
alkenes with n double bonds (that can be cis, trans) 2n stereoisomers are possible
example 2,4-heptadiene

21. Dienes - E or Z per olefin with number
( 2Z, 4S, 6E)-
1,6-dichloro-2,4-dimethyl-2,6-decadiene
( 2Z, 4S, 6E)-
to 6.1

22. Physical Properties Alkenes are nonpolar compounds
attractive forces between molecules are
dispersion forces
The physical properties of alkenes are similar to those of alkanes
to 6.1

23. Common Names used lab & lecture not on tests

24. Index of Hydrogen Deficiency
Index of hydrogen deficiency (IHD):
IHD =  (number of rings + number of  bonds)
Compare Hs of alkane with Hs in a compound
CnH2n CnHx
e.g. C6H10
C6H2(6)+2
IHD = 2 (H
reference
- H
molecule ) =
( ) 2
= 2
to 6.1

25. Index of Hydrogen Deficiency
Other elements present
F, Cl, Br, I add one H - (Group 7)
C6H7ClO
O No correction to unknown formula
N, P subtract one H (Group 5)
C6H8ClNO
e.g.
C6H9Cl
9 + 1
7 + 1
to 6.1

26. Index of Hydrogen Deficiency
Problem: calculate the IHD for niacin, molecular formula C6H6N2O
reference hydrocarbon C6H14
IHD = [14 - (6-2)]/2 = 5
end
to 6.1