Chapter 7
Structure and Synthesis
of Alkenes
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,Prentice Hall
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Chapter 7 2
Functional Group
Pi bond is the functional group.
More reactive than sigma bond.
Bond dissociation energies:
C=C BDE 146 kcal/mol
C-C BDE -83 kcal/mol
Pi bond 63 kcal/mol
=>
Chapter 7 3
Orbital Description
Sigma bonds around C are sp2 hybridized.
Angles are approximately 120 degrees.
No nonbonding electrons.
Molecule is planar around the double bond.
Pi bond is formed by the sideways overlap of
parallel p orbitals perpendicular to the plane
of the molecule,
=>
Chapter 7 4
Bond Lengths and Angles
Hybrid orbitals have more s character.
Pi overlap brings carbon atoms closer.
Bond angle with pi orbitals increases.
Angle C=C-H is 121.7?
Angle H-C-H is 116,6? =>
Chapter 7 5
Pi Bond
Sideways overlap of parallel p orbitals.
No rotation is possible without breaking
the pi bond (63 kcal/mole).
Cis isomer cannot become trans without
a chemical reaction occurring,
=>
Chapter 7 6
Elements of
Unsaturation
A saturated hydrocarbon,CnH2n+2
Each pi bond (and each ring) decreases the
number of H’s by two.
Each of these is an element of unsaturation.
To calculate,find number of H’s if it were
saturated,subtract the actual number of H’s,
then divide by 2,
=>
Chapter 7 7
Propose a Structure:
First calculate the number of elements of
unsaturation.
Remember:
A double bond is one element of unsaturation.
A ring is one element of unsaturation.
A triple bond is two elements of unsaturation,=>
for C5H8
Chapter 7 8
Heteroatoms
Halogens take the place of hydrogens,so
add their number to the number of H’s.
Oxygen doesn’t change the C:H ratio,so
ignore oxygen in the formula.
Nitrogen is trivalent,so it acts like half a
carbon.
C
H
H
C
H
H
N C
H
HH
=>
Chapter 7 9
Structure for C6H7N?
Since nitrogen counts as half a carbon,
the number of H’s if saturated is
2(6.5) + 2 = 15.
Number of missing H’s is 15 – 7 = 8.
Elements of unsaturation is 8 ÷ 2 = 4.
=>
Chapter 7 10
IUPAC Nomenclature
Parent is longest chain containing the
double bond.
-ane changes to -ene,(or -diene,-triene)
Number the chain so that the double
bond has the lowest possible number.
In a ring,the double bond is assumed to
be between carbon 1 and carbon 2.
=>
Chapter 7 11
Name These Alkenes
C H 2 C H C H 2 C H 3
C H 3 C
C H 3
C H C H 3
C H 3
C H C H 2 C H 3
H 3 C
1-butene
2-methyl-2-butene
3-methylcyclopentene
2-sec-butyl-1,3-cyclohexadiene
3-n-propyl-1-heptene
=>
Chapter 7 12
Alkene Substituents
= CH2
methylene
(methylidene)
- CH = CH2
vinyl
(ethenyl)
- CH2 - CH = CH2
allyl
(2-propenyl)
Name,=>
Chapter 7 13
Common Names
Usually used for small molecules.
Examples:
C H 2 C H 2
e t h y l e n e
C H 2 C H C H 3
p r o p y l e n e
C H 2 C C H 3
C H 3
i s o b u t y l e n e
=>
Chapter 7 14
Cis-trans Isomerism
Similar groups on same side of double
bond,alkene is cis.
Similar groups on opposite sides of
double bond,alkene is trans.
Cycloalkenes are assumed to be cis.
Trans cycloalkenes are not stable
unless the ring has at least 8 carbons,
=>
Chapter 7 15
Name these:
C C
C H 3
H
H
C H 3 C H 2
C C
B r
H
B r
H
trans-2-pentene cis-1,2-dibromoethene
=>
Chapter 7 16
E-Z Nomenclature
Use the Cahn-Ingold-Prelog rules to
assign priorities to groups attached to
each carbon in the double bond.
If high priority groups are on the same
side,the name is Z (for zusammen).
If high priority groups are on opposite
sides,the name is E (for entgegen),
=>
Chapter 7 17
Example,E-Z
C C
H 3 C
H
C l
C H 2
C C
H
H
C H C H 3
C l1
2
1
2
2Z
2
1
1
2
5E
(2Z,5E)-3,7-dichloro-2,5-octadiene
=>
Chapter 7 18
Commercial Uses,
Ethylene
=>
Chapter 7 19
Commercial Uses,
Propylene
=>
Chapter 7 20
Other Polymers
=>
Chapter 7 21
Stability of Alkenes
Measured by heat of hydrogenation:
Alkene + H2? Alkane + energy
More heat released,higher energy alkene.
=>
30.3 kcal
27.6 kcal
Chapter 7 22
Substituent Effects
More substituted alkenes are more stable.
H2C=CH2 < R-CH=CH2 < R-CH=CH-R < R-CH=CR2 < R2C=CR2
unsub,< monosub,< disub,< trisub,< tetra sub.
Alkyl group stabilizes the double bond.
Alkene less sterically hindered.
=>
Chapter 7 23
Disubstituted Isomers
Stability,cis < geminal < trans isomer
Less stable isomer is higher in energy,has
a more exothermic heat of hydrogenation.
27.6 kcalTrans-2-butene
28.0 kcal(CH3)2C=CH2Isobutylene
28.6 kcalCis-2-butene
C H 3
C C
C H 3
H H
H
C C
C H 3
C H 3 H =>
Chapter 7 24
Cycloalkene Stability
Cis isomer more stable than trans.
Small rings have additional ring strain.
Must have at least 8 carbons to form a
stable trans double bond,
For cyclodecene (and larger) trans
double bond is almost as stable as the
cis,
=>
Chapter 7 25
Bredt’s Rule
A bridged bicyclic compound cannot
have a double bond at a bridgehead
position unless one of the rings contains
at least eight carbon atoms.
Examples:
Unstable.
Violates Bredt’s rule
Stable,Double bond
in 8-membered ring.
=>
Chapter 7 26
Physical Properties
Low boiling points,increasing with mass.
Branched alkenes have lower boiling points.
Less dense than water.
Slightly polar
Pi bond is polarizable,so instantaneous dipole-
dipole interactions occur.
Alkyl groups are electron-donating toward the pi
bond,so may have a small dipole moment.
=>
Chapter 7 27
Polarity Examples
= 0.33 D? = 0
=>
c i s - 2 - b u t e n e,b p 4°C
C C
H
H 3 C
H
C H 3
t r a n s - 2 - b u t e n e,b p 1° C
C C
H
H
H 3 C
C H 3
Chapter 7 28
Alkene Synthesis
Overview
E2 dehydrohalogenation (-HX)
E1 dehydrohalogenation (-HX)
Dehalogenation of vicinal dibromides (-X2)
Dehydration of alcohols (-H2O)
=>
Chapter 7 29
Removing HX via E2
Strong base abstracts H+ as X- leaves
from the adjacent carbon.
Tertiary and hindered secondary alkyl
halides give good yields.
Use a bulky base if the alkyl halide
usually forms substitution products,
=>
Chapter 7 30
Some Bulky Bases
C
C H 3
H 3 C
C H 3
O
_
te r t- b u t o x i d e
(CH3CH2)3N,
triethylamine
=>
N
H
C H ( C H 3 ) 2
C H ( C H 3 ) 2
d i i s o p r o p y l a m i n e
N C H 3H
3 C
2,6 - d i m e t h y l p y r i d i n e
Chapter 7 31
Hofmann Product
Bulky bases abstract the least hindered H+
Least substituted alkene is major product.
C H 3 C
H
H
C
C H 3
B r
C H 2
H
C H 3 C H 2 O
C H 3 C H 2 O H
_
C C
C H 3
C H 3H
H 3 C
C C
H
HH
3 C
C H 3 C H 2
71% 29%
72%28%
C C
H
HH
3 C
C H 3 C H 2
C C
C H 3
C H 3H
H 3 C_
C H 3 C H 2 O H
C H 3 C
H
H
C
C H 3
B r
C H 2
H
( C H 3 ) 3 CO=>
Chapter 7 32
E2,Diastereomers
Stereospecific reaction,(S,R) produces only
trans product,(R,R) produces only cis.
P h
B r H
H C H 3
P h
H
Ph CH3Br
PhH
H
Ph
CH3
Ph
HBr
CH3Ph
PhH =>
Chapter 7 33
E2,Cyclohexanes
Leaving groups must be trans diaxial,=>
Chapter 7 34
E2,Vicinal Dibromides
Remove Br2 from adjacent carbons.
Bromines must be anti-coplanar (E2).
Use NaI in acetone,or Zn in acetic acid.
I- Br
CH3 HBr
CH3H
C C
C H 3
H
H
H 3 C =>
Chapter 7 35
Removing HX via E1
Secondary or tertiary halides
Formation of carbocation intermediate
Weak nucleophile
Usually have substitution products too
=>
Chapter 7 36
Dehydration of
Alcohols
Reversible reaction
Use concentrated sulfuric or phosphoric
acid,remove low-boiling alkene as it
forms.
Protonation of OH converts it to a good
leaving group,HOH
Carbocation intermediate,like E1
Protic solvent removes adjacent H+
=>
Chapter 7 37
Dehydration
Mechanism
C
H
C
O H
S
O
O
O HOH C
H
C
O H
H
H S O 4
_
C
H
C
O H
H
C
H
C
H 2 O,
C C H 3 O +=>
Chapter 7 38
Industrial Methods
Catalytic cracking of petroleum
Long-chain alkane is heated with a catalyst to
produce an alkene and shorter alkane.
Complex mixtures are produced.
Dehydrogenation of alkanes
Hydrogen (H2) is removed with heat,catalyst,
Reaction is endothermic,but entropy-favored.
Neither method is suitable for lab synthesis
=>
Chapter 7 39
End of Chapter 7
Structure and Synthesis
of Alkenes
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,Prentice Hall
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Chapter 7 2
Functional Group
Pi bond is the functional group.
More reactive than sigma bond.
Bond dissociation energies:
C=C BDE 146 kcal/mol
C-C BDE -83 kcal/mol
Pi bond 63 kcal/mol
=>
Chapter 7 3
Orbital Description
Sigma bonds around C are sp2 hybridized.
Angles are approximately 120 degrees.
No nonbonding electrons.
Molecule is planar around the double bond.
Pi bond is formed by the sideways overlap of
parallel p orbitals perpendicular to the plane
of the molecule,
=>
Chapter 7 4
Bond Lengths and Angles
Hybrid orbitals have more s character.
Pi overlap brings carbon atoms closer.
Bond angle with pi orbitals increases.
Angle C=C-H is 121.7?
Angle H-C-H is 116,6? =>
Chapter 7 5
Pi Bond
Sideways overlap of parallel p orbitals.
No rotation is possible without breaking
the pi bond (63 kcal/mole).
Cis isomer cannot become trans without
a chemical reaction occurring,
=>
Chapter 7 6
Elements of
Unsaturation
A saturated hydrocarbon,CnH2n+2
Each pi bond (and each ring) decreases the
number of H’s by two.
Each of these is an element of unsaturation.
To calculate,find number of H’s if it were
saturated,subtract the actual number of H’s,
then divide by 2,
=>
Chapter 7 7
Propose a Structure:
First calculate the number of elements of
unsaturation.
Remember:
A double bond is one element of unsaturation.
A ring is one element of unsaturation.
A triple bond is two elements of unsaturation,=>
for C5H8
Chapter 7 8
Heteroatoms
Halogens take the place of hydrogens,so
add their number to the number of H’s.
Oxygen doesn’t change the C:H ratio,so
ignore oxygen in the formula.
Nitrogen is trivalent,so it acts like half a
carbon.
C
H
H
C
H
H
N C
H
HH
=>
Chapter 7 9
Structure for C6H7N?
Since nitrogen counts as half a carbon,
the number of H’s if saturated is
2(6.5) + 2 = 15.
Number of missing H’s is 15 – 7 = 8.
Elements of unsaturation is 8 ÷ 2 = 4.
=>
Chapter 7 10
IUPAC Nomenclature
Parent is longest chain containing the
double bond.
-ane changes to -ene,(or -diene,-triene)
Number the chain so that the double
bond has the lowest possible number.
In a ring,the double bond is assumed to
be between carbon 1 and carbon 2.
=>
Chapter 7 11
Name These Alkenes
C H 2 C H C H 2 C H 3
C H 3 C
C H 3
C H C H 3
C H 3
C H C H 2 C H 3
H 3 C
1-butene
2-methyl-2-butene
3-methylcyclopentene
2-sec-butyl-1,3-cyclohexadiene
3-n-propyl-1-heptene
=>
Chapter 7 12
Alkene Substituents
= CH2
methylene
(methylidene)
- CH = CH2
vinyl
(ethenyl)
- CH2 - CH = CH2
allyl
(2-propenyl)
Name,=>
Chapter 7 13
Common Names
Usually used for small molecules.
Examples:
C H 2 C H 2
e t h y l e n e
C H 2 C H C H 3
p r o p y l e n e
C H 2 C C H 3
C H 3
i s o b u t y l e n e
=>
Chapter 7 14
Cis-trans Isomerism
Similar groups on same side of double
bond,alkene is cis.
Similar groups on opposite sides of
double bond,alkene is trans.
Cycloalkenes are assumed to be cis.
Trans cycloalkenes are not stable
unless the ring has at least 8 carbons,
=>
Chapter 7 15
Name these:
C C
C H 3
H
H
C H 3 C H 2
C C
B r
H
B r
H
trans-2-pentene cis-1,2-dibromoethene
=>
Chapter 7 16
E-Z Nomenclature
Use the Cahn-Ingold-Prelog rules to
assign priorities to groups attached to
each carbon in the double bond.
If high priority groups are on the same
side,the name is Z (for zusammen).
If high priority groups are on opposite
sides,the name is E (for entgegen),
=>
Chapter 7 17
Example,E-Z
C C
H 3 C
H
C l
C H 2
C C
H
H
C H C H 3
C l1
2
1
2
2Z
2
1
1
2
5E
(2Z,5E)-3,7-dichloro-2,5-octadiene
=>
Chapter 7 18
Commercial Uses,
Ethylene
=>
Chapter 7 19
Commercial Uses,
Propylene
=>
Chapter 7 20
Other Polymers
=>
Chapter 7 21
Stability of Alkenes
Measured by heat of hydrogenation:
Alkene + H2? Alkane + energy
More heat released,higher energy alkene.
=>
30.3 kcal
27.6 kcal
Chapter 7 22
Substituent Effects
More substituted alkenes are more stable.
H2C=CH2 < R-CH=CH2 < R-CH=CH-R < R-CH=CR2 < R2C=CR2
unsub,< monosub,< disub,< trisub,< tetra sub.
Alkyl group stabilizes the double bond.
Alkene less sterically hindered.
=>
Chapter 7 23
Disubstituted Isomers
Stability,cis < geminal < trans isomer
Less stable isomer is higher in energy,has
a more exothermic heat of hydrogenation.
27.6 kcalTrans-2-butene
28.0 kcal(CH3)2C=CH2Isobutylene
28.6 kcalCis-2-butene
C H 3
C C
C H 3
H H
H
C C
C H 3
C H 3 H =>
Chapter 7 24
Cycloalkene Stability
Cis isomer more stable than trans.
Small rings have additional ring strain.
Must have at least 8 carbons to form a
stable trans double bond,
For cyclodecene (and larger) trans
double bond is almost as stable as the
cis,
=>
Chapter 7 25
Bredt’s Rule
A bridged bicyclic compound cannot
have a double bond at a bridgehead
position unless one of the rings contains
at least eight carbon atoms.
Examples:
Unstable.
Violates Bredt’s rule
Stable,Double bond
in 8-membered ring.
=>
Chapter 7 26
Physical Properties
Low boiling points,increasing with mass.
Branched alkenes have lower boiling points.
Less dense than water.
Slightly polar
Pi bond is polarizable,so instantaneous dipole-
dipole interactions occur.
Alkyl groups are electron-donating toward the pi
bond,so may have a small dipole moment.
=>
Chapter 7 27
Polarity Examples
= 0.33 D? = 0
=>
c i s - 2 - b u t e n e,b p 4°C
C C
H
H 3 C
H
C H 3
t r a n s - 2 - b u t e n e,b p 1° C
C C
H
H
H 3 C
C H 3
Chapter 7 28
Alkene Synthesis
Overview
E2 dehydrohalogenation (-HX)
E1 dehydrohalogenation (-HX)
Dehalogenation of vicinal dibromides (-X2)
Dehydration of alcohols (-H2O)
=>
Chapter 7 29
Removing HX via E2
Strong base abstracts H+ as X- leaves
from the adjacent carbon.
Tertiary and hindered secondary alkyl
halides give good yields.
Use a bulky base if the alkyl halide
usually forms substitution products,
=>
Chapter 7 30
Some Bulky Bases
C
C H 3
H 3 C
C H 3
O
_
te r t- b u t o x i d e
(CH3CH2)3N,
triethylamine
=>
N
H
C H ( C H 3 ) 2
C H ( C H 3 ) 2
d i i s o p r o p y l a m i n e
N C H 3H
3 C
2,6 - d i m e t h y l p y r i d i n e
Chapter 7 31
Hofmann Product
Bulky bases abstract the least hindered H+
Least substituted alkene is major product.
C H 3 C
H
H
C
C H 3
B r
C H 2
H
C H 3 C H 2 O
C H 3 C H 2 O H
_
C C
C H 3
C H 3H
H 3 C
C C
H
HH
3 C
C H 3 C H 2
71% 29%
72%28%
C C
H
HH
3 C
C H 3 C H 2
C C
C H 3
C H 3H
H 3 C_
C H 3 C H 2 O H
C H 3 C
H
H
C
C H 3
B r
C H 2
H
( C H 3 ) 3 CO=>
Chapter 7 32
E2,Diastereomers
Stereospecific reaction,(S,R) produces only
trans product,(R,R) produces only cis.
P h
B r H
H C H 3
P h
H
Ph CH3Br
PhH
H
Ph
CH3
Ph
HBr
CH3Ph
PhH =>
Chapter 7 33
E2,Cyclohexanes
Leaving groups must be trans diaxial,=>
Chapter 7 34
E2,Vicinal Dibromides
Remove Br2 from adjacent carbons.
Bromines must be anti-coplanar (E2).
Use NaI in acetone,or Zn in acetic acid.
I- Br
CH3 HBr
CH3H
C C
C H 3
H
H
H 3 C =>
Chapter 7 35
Removing HX via E1
Secondary or tertiary halides
Formation of carbocation intermediate
Weak nucleophile
Usually have substitution products too
=>
Chapter 7 36
Dehydration of
Alcohols
Reversible reaction
Use concentrated sulfuric or phosphoric
acid,remove low-boiling alkene as it
forms.
Protonation of OH converts it to a good
leaving group,HOH
Carbocation intermediate,like E1
Protic solvent removes adjacent H+
=>
Chapter 7 37
Dehydration
Mechanism
C
H
C
O H
S
O
O
O HOH C
H
C
O H
H
H S O 4
_
C
H
C
O H
H
C
H
C
H 2 O,
C C H 3 O +=>
Chapter 7 38
Industrial Methods
Catalytic cracking of petroleum
Long-chain alkane is heated with a catalyst to
produce an alkene and shorter alkane.
Complex mixtures are produced.
Dehydrogenation of alkanes
Hydrogen (H2) is removed with heat,catalyst,
Reaction is endothermic,but entropy-favored.
Neither method is suitable for lab synthesis
=>
Chapter 7 39
End of Chapter 7
