Alkenes: Structure and Reactivity
2302261
Dr Anawat Ajavakom
1
Alkene - Hydrocarbon With Carbon-Carbon
D bl B
Double
Bond
d
„
Also sometimes called “olefin”.
„
Important industrial products
‰ Feedstock for industrial processes
„
Includes many naturally occurring materials
‰ Essential oils,, flavors,, fragrances,
g
, vitamins
Isoprene
Limonene
Myrcene
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
Zingiberine (Ginger)
2
2
Naming of Alkenes
„
Find longest continuous carbon chain for root
„
Number carbons in chain so that double bond carbons have
lowest possible numbers
„
Rings have “cyclo” as prefix.
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
3
3
Many Alkenes Are Known by Common
Names
„
„
„
„
Ethylene
y
= ethene
Propylene = propene
Isobutylene = 2methylpropene
Isoprene = 2-methyl-1,3butadiene
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
4
4
Rotation of π Bond Is Prohibitive
„
This prevents rotation on a carbon-carbon double bond
((unlike a carbon-carbon single
g bond).
)
„
Therefore, alternative structures are possible.
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
5
5
Cis-Trans Isomerism in Alkenes
„
The presence of a carbon-carbon
double can create two possible
structures
‰
‰
„
cis isomer - two similar groups on
same side of the double bond
trans isomer similar groups on
opposite sides
Each carbon must have two
different groups for these isomers
to occur
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
6
6
Cis, Trans Isomers Require That End Groups
M
Must
Differ
Diff iin P
Pairs
i
„
„
180o rotation superposes
Bottom pair cannot be
superposed without
breaking C=C
X
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
7
7
Sequence Rules: The E, Z -Designation
„
Neither compound is
cclearly
ea y “cis”
cs o
or “trans”
a s
‰
‰
„
Substituents on C1 are
different than those on
C2
We need to define
“similarity” in a precise
way to distinguish the two
stereoisomers
Cis, trans nomenclature
only works for
disubstituted double
bonds
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
8
8
Develop a System for Comparison of Priority
off S
Substituents
b tit
t
„
Assume a valuation
system
‰
‰
„
„
If Br has a higher “value”
than Cl
If CH3 is higher than H
Then, in A, the higher
value groups are on
opposite sides
In B, they are on the
same side
‰
Requires a universally
accepted “valuation”
valuation
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
9
9
E, Z Stereochemical Nomenclature
„
„
„
„
Priority rules of Cahn,
Ingold and Prelog
Ingold,
Compare where
higher priority group is
with
i h respect to b
bond
d
and designate as
prefix
E -entgegen, opposite
sides
Z - zusammen,
zusammen
together on the same
side
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
10
10
Ranking Priorities: Cahn-Ingold-Prelog
R l
Rules
„
„
Must rank atoms that are connected at comparison
p
p
point
Higher atomic number gets higher priority
‰
Br > Cl > O > N > C > H
In this case,The higher priority
groups are opposite:
(E ))-2-bromo-2-chloro-propene
bo o
o o p op
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
11
11
Extended Comparison
„
„
„
If atomic numbers are the same, compare at next
connection point at same distance
Compare until something has higher atomic number
Do not combine – always compare
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
12
12
Dealing With Multiple Bonds
„
Substituent is drawn with connections shown and no
double
doub
eo
or triple
p e bo
bonds
ds
„
Added atoms are valued with 0 ligands themselves
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
13
13
Alkene Stability
„
Cis-alkenes are less stable than trans alkenes
„
Compare heat given off on hydrogenation: ΔHo
„
L
Less
stable
bl iisomer iis hi
higher
h iin energy
‰
‰
‰
And gives off more heat
tetrasubstituted > trisubstituted > disubstituted >
monosusbtituted
hyperconjugation stabilizes alkyl
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
14
14
Comparing Stabilities of Alkenes
„
Evaluate heat given off when C=C is converted to C-C
„
More stable alkene gi
gives
es off less heat
‰ Trans butene generates 5 kJ less heat than cis-butene
McMurry Organic Chemistry 6th edition
Chapter 6 (c) 2003
15
15