Bonding, Nomenclature, Properties

1
chapter 5
Alkenes
Bonding, Nomenclature, Properties
Structure Hydrogen Deficiency Nomenclature Physic
al Properties Naturally Occurring Alkenes/Terpenes
b
2
R or S?
3
Which is 2S,3R-propoxyphen 2R,3S-propoxyphen?
(2R,3S)-
antitussive NovraD DarvoN
analgesic
4
Unsaturated Hydrocarbons

• 1. Alkene contains a carbon-carbon double bond

5
Structure of Alkenes

• Double Bond
• 1 ?-bond formed by overlap of 2 sp2 hybrid
  orbitals
• 1 ?-bond formed by overlap of 2 parallel 2p
  orbitals

6
Structure of Alkenes

• No rotation about a CC bond - why?

Rotation requires breaking ? bond 63
kcal/mol
7
Nomenclature IUPAC Root name - longest
continuous chain containing the olefin.
alkene
Number of Cs in CC chain
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
Nomenclature IUPAC Root name - longest
continuous chain containing the olefin
alkene
indicates CC in chain
Number of Cs in chain with an CC
Cyclic
Olefin (functional group) - positions 1,2 Number
around the ring to best accommodate
substituents
10
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 Systemrecall cis/trans isomers
cis-1,2-dichloroethene
trans-1,2-dichloroethene
trans generally more stable than cis - dipoles
and 1,2 interactions
12
The Cis,Trans System

• 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
14
E,Z Configuration

• uses priority rules (Chapter 3)
• higher priority groups - same side, Z

E
Z
15
E/Z - priorities of groups on ends of CC
(1) Atom assigned a priority, higher atomic
number higher priority. (2) Isotopes -
higher atomic mass, higher priority 1H
lt 2H lt 3H H lt D lt T (3) If priority
the same, go to the next set of atoms
-CH2-H lt -CH2-O-H lt -CH2-Cl (4)
double (triple) bonds replaced by single bonds.
16
The E,Z System

• Example name each alkene and specify its
  configuration by the E,Z system

17
IUPAC names?
?(trans)-longest chain?
(3Z,7S)-7-bromo-4-(2-chloroethyl)-6-methyl-3-octen
e
(7S)-7-bromo-4-(2-chloroethyl)-6-methyl-3-octene
(S)-6-ethyl-1-fluoro-5,5-dimethylcyclooctene
18
If same atoms, priority goes to the next point
of difference
9-bromo-5-(2-methylpropyl)-4-nonene
(E)- (4E)-
19
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 gt yne).
number so enes (enyne, ynes) have the lowest
possible number
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
1,6-dichloro-2,4-dimethyl-2,6-decadiene
( 2Z, 4S, 6E)-
( 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
to 6.1
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
• CnH2n2 CnHx

e.g. C6H10
C6H2(6)2
2
to 6.1
25
Index of Hydrogen Deficiency

• Other elements present
• F, Cl, Br, I add one H - (Group 7)

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