CHM 136 HF
Alkene Structure and Reactivity
Chapter 7
McMurry, 9th Ed.
pheromone of the
silkworm moth
male moths can detect
10-13 mol/L in air!
1
25 million
tons/year
OH
OOH
HO
O
O
OH
*
*
n
Cl
Cl
Cl O
O
2
ethylene
polyethylene
1
2
Naming alkenes: drawing structures from IUPAC names
Follows alkane naming rules plus….
a. the parent hydrocarbon is the longest chain
that must contain the double bond(s)
b. C atoms numbered to give C=C the lowest possible numbers;
C=C location designated by number of first alkene carbon
3-ethyl-2,4,5-trimethyl-6-propylnona-2,5-diene
(S)-3-bromo-3-methylcyclohexene
(S)-3-bromo-3-methylcyclohex-1-ene
3-ethyl-2,4,5-trimethyl-6-propyl-2,5-nonadiene
Calculating the degrees of unsaturation of a molecule
A degree of unsaturation refers to a
double bond or a ring in a molecule
Degrees of
Unsaturation 0 1 1
Degrees of unsaturation can be calculated
from the molecular formula
C4H10 C4H8 C6H12
H3C
H2
C
C
H
CH2
H3C
CH2
CH2
CH3
4
3
4
Degrees of unsaturation
H
H
H H
H
H
H H
H
H
H
H
H
H
H
H
H
H
CH4
C2H6
C3H8
CnH2n+2
deg. unsat. = 0 deg. unsat. = 1
5
CnH2n
H
H CH3
H
H
H H
H
C2H4
C3H6
C6H12
C4H6
C2H2H H
C6H10
H
H
H H
H
H
CnH2n-2
deg. unsat. = 2
CH3
deg. unsat. = 4
CnH2n-6
6
Degrees of unsaturation
5
6
Calculating degree(s) of unsaturation:
Degrees Unsaturation = [(# of H in alkane) – (# of H in compound)]/2
= ((2n+2) – #H)/2
CH3
C7H8
7
What to do about X, O, N?
Nitrogen (N): for each N, add an H to the formula
Oxygen (O): ignore oxygen it does not affect calculation
Halogens (X): X is equivalent to H (same valence)
N
O
H
Br
H2N
C9H9N2OBr
8
Overall D.U. = (2n+2 – #Hydrogens – #Halogens + #Nitrogens)/2
n = #Carbons
7
8
Examples: calculating DU
O
O
N
H
O
furan tetrahydrofuran
(THF)
C4H4O C4H8O C8H7N C6H4O2
9
..... because they cannot freely rotate about C=C:
C C
CH3
H
H
H3C
C C
H
CH3
H
H3C

cis-2-butene trans-2-butene
- exist as stereoisomers.....
Alkene structure:
 alkene stereoisomers cannot easily interconvert 10
C C
H H
H3C CH3
9
10
di-substituted alkenes — can use cis/trans or E/Z
tri- and tetra-substituted alkenes - E/Z system must be used
cis-2-butene
or
(Z)-2-butene
Cahn-Ingold-Prelog rules needed to describe alkene stereoisomers
Reminder:
Example:
Cl
11
acid
trans-2-butenecis-2-butene
Steric strain between -CH3 groups
in cis isomer destabilizes molecule
Alkene stabilities can be measured two ways
1. Equilibrium constant
between isomers:
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11
12
- isomeric reactants, same product
- measure and compare H° for each reaction.
2. H° of alkene hydrogenation
1-butene
(E)-2-butene C C
CH3
HH3C
H
+ H2
CH3CH2CH2CH3
13
(Z)-2-butene
CH3CH2CH2CH3
H°Z H°E
(E)-2-butene is _______ stable than (Z)-2-butene
14
Z vs E 2-butene
13
14
C C
CH3
HH3C
H
+ H2
CH3CH2CH2CH3
H°1 H°2
(E)-2-butene is _______ stable than 1-butene
15
1-butene vs 2-butene
More substituted double
bonds are more stable
alkane 16
Ho
15
16
Why are more substituted alkenes more stable?
17
H
Compare -
Note: sp2-sp3 bonds
stronger than sp3-sp3
Also π-bonds interact with -CH3
more strongly than with -H
Increasing stability
tetra
tri
di
mono
> 
Substituted alkenes differ in their stabilities
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17
18
Alkene Reactivity
+ –
H Br
carbocation
intermediate
(C+)
C C
H
H
H
H
H
+
Br -–
X is usually Cl, Br, I or OH
The electrophilic addition of HX to an alkene
Recall the reaction of ethene with HBr:
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propene 1-bromopropane 2-bromopropane
+ HBr ether +C C
H
H
H3C
H
The reaction is regioselective - possible constitutional isomer products
but one dominates
Markovnikov’s rule: In the electrophilic addition of HX to an alkene,
H attaches to the C atom which has the most H’s and
X attaches to the C atom with the fewest H’s.
Why??
Reaction of propene with HBr: two isomeric products are possible
21
In the electrophilic addition of HX to an alkene,
H attaches to the C atom which has the most H’s and
X attaches to the C atom with the fewest H’s
A “rule” is what you use to make a prediction when you
DO NOT UNDERSTAND WHAT IS GOING ON!!
Markovnikov’s rule:
Vladimir Markovnikov
December 22, 1838 – February 11, 1904
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22
2° carbocation
1° carbocation
C C
H
H
H3C
H
2-bromopropane
Let's look at the reaction mechanism:
–
–
The 2° carbocation intermediate is formed
preferentially because it is more stable 2 questions:
> > >
3° carbocation, three alkyl groups
donating electron density
inductive effect - alkyl substituents donate electron
density towards C+, stabilizing positive charge
increasing stability
- more highly substituted carbocations are more stable
(lower in energy)
1. Why are carbocations having more alkyl groups more stable?
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23
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Reaction progress
Fr
ee
e
ne
rg
y
H2C=CH(CH3)
+ H–Br
T.S. 1
INTG1‡
2. How does intermediate stability determine the preferred rxn pathway?
- the first elementary step
is the slowest so let's
take a closer look
25
the Hammond postulate:
Fr
ee
e
ne
rg
y
Reaction progress
Reactants
2° INT
T.S. 1
_____ stable 2° intermediate
_____ stable T.S.1
_____ energy barrier
1° INTT.S. 1
less stable 1° intermediate
_____ stable T.S.1
______ energy barrier
in a reaction, the structure of a transition
state resembles that of the nearest stable
species.
Hammond
Hammond
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2° carbocation
1° carbocation
C C
H
H
H3C
H
2-bromopropane
–
–
Markovnikov’s rule:
“the protonation occurs faster to form the more stable carbocation
intermediate than it does to form the less stable one”
faster
slower
much
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Examples of Markovnikov’s Rule
1.
Which carbocation forms faster?
+ HCl ether ~100%
Markovnikov product
28
Cl
27
28
2.
+ HBr ether
Which carbocation forms faster?
~100%
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Reaction stereochemistry 1-butene + HBr:
Will obtain a racemic mixture of products; both reactants are
optically inactive, product is optically inactive also.
–
planar carbocation
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(S)-2-bromobutane(R)-2-bromobutane
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Why are carbocations proposed as intermediates?
molecules sometimes rearrange:
+ HBr ether
31
1,2-methyl shift
H Br
–
–
32
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1,2-Hydride shift can also occur: hydride shifts rather than an alkyl group
33
When a less stable carbocation can rearrange to a more stable
carbocation in a single 1,2 hydride shift or alkyl shift
2o  3o
The most important cases:
2o  benzylic or allylic
1o carbocations will rearrange but 1o carbocations are rarely formed
If a reaction is given with a rearranged product, you should be able to
draw the mechanism that yields that product
C
C
H2
CH3
H
Cl
When are carbocation rearrangements likely to occur?
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Highlights
Electrophilic addition to alkenes; Markovnikov’s rule
Degree of unsaturation of a molecule
- carbocation stability
Identifying/naming E and Z alkenes
Stability of alkenes
- the Hammond postulate and what it means
Carbocation rearrangements
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