Download Chemistry 201 C Alkenes

Chemistry 201 C
Alkenes: Structure and
Reactivity
This presentation was created by
Professor Carl H. Snyder
Chemistry Department
University of Miami
Coral Gables, FL 33124
[email protected]
Copyright 2004 by Carl H. Snyder,
University of Miami. All rights
reserved.
Unsaturation
unsaturated
Degree of Unsaturation
(Index of Hydrogen
Deficiency)
saturated
An alkane -- CnH2n+2 -- is fully saturated with hydrogens.
A molecule is unsaturated when it contains fewer
hydrogens (and/or halogens and oxygens) than an
alkane of corresponding carbon content.
The degree of unsaturation or index of hydrogen
deficiency equals the number of H2 molecules it would
take to saturate the molecule.
IUPAC Rules
What is the index of hydrogen deficiency of
each?
IUPAC Rules
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IUPAC Rules
Cycloalkenes
Special Names for Special
Groups
Common Names of Some Alkenes
(note error in title of table)
The term methylene also applies to the -CH2group in alkanes, cycloalkanes, cycloalkenes,
and other organic molecules.
sp2 Hybridization
Two of the three p orbitals combine with the single s
orbital to form three sp2 orbitals, leaving one p oribtal
unhybridized.
The three sp2 orbitals point to the apexes of an
equilateral triangle.
The unhybridized p orbital is perpendicular to the plane
of the triangle.
Combination of Two sp2
Carbons to Form a π-Bond
2 sp2 orbitals, one from each carbon, overlap to
form a σ-bond.
2 p orbitals, one from each carbon, overlap to
form a π (pi) bond.
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Ethene or Ethylene
No Rotation About The π-Bond
Rotation about
the double-bond
requires breaking
then reforming
the π-bond.
Breaking the πbond requires
about 268 kJ/mol
The rotational
barrier in ethane
is only 12
kJ/mole.
Geometric Isomerism in
2-Butene exists as 2-Butene
two geometric
isomers:
cis-2-butene, with
both methyl
groups on the
same side of the
double bond, and
trans-2-butene,
with the methyl
groups on
opposite sides of
the double bond.
Identify These Geometric Isomers
Are these two geometric isomers?
Which is cis, which is trans?
Identical and Nonidentical
Alkenes
The top two structures are mutually identical.
The bottom two structures differ from one another.
If either sp2 carbon bears two identical
substituents, geometric isomerism is not possible.
Chemistry Reaches A Higher
Plane
Are these two geometric isomers?
Which is cis, which is trans?
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The Easy E,Z-System
A general system of geometric designation
Substituents on each sp2 carbon are ranked by atomic
number.
Z-isomer: Substituents of similar rank are on the
same side of the C=C double bond (zusammen).
E-isomer: Substituents of similar rank are on opposite
sides of the C=C double bond (entgegen).
Cahn-Ingold-Prelog Sequence
Rules
Is This An E Or A Z Isomer?
Cahn-Ingold-Prelog Sequence
Rules
Cahn-Ingold-Prelog Sequence
Rules
Chemistry Reaches A Higher
Plane
Are these two geometric isomers?
Which is E, which is Z?
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Stability of Alkenes
Thermodynamic stability - Refers to the
energy content of an alkene.
If alkene A is thermodynamically more stable
than alkene B, then alkene A is lower in
energy than alkene B
Kinetic stability - Refers to the chemical
reactivity of an alkene
If alkene A is kinetically more stable than
alkene B, then alkene A is less likely to react
(or less likely to react rapidly) than alkene B.
Thermodynamic Stabilities of
Alkenes
Thermodynamic stabilities of alkenes can be
determined by:
1) equilibrium constants
2) heats of combustion
3) heats of hydrogenation
Equilibria
Combustion
trans-2-butene is favored over cis-2-butene in acid
catalyzed equilibrium.
The trans isomer is thermodynamically more stable,
by a calculated value of 2.8 kJ/mol.
Catalytic Hydrogenation of An
Alkene
Heat released on combustion of both cisand trans-2-butene show that the trans
isomer is more stable by 3.3 kJ/mol, in
close experimental agreement with the
value of 2.8 kJ/mol found through acidcatalyzed equilibrium studies.
Heats of Hydrogenation
Addition of H2 to an alkene produces an alkane.
Since both isomers of 2-butene produce the
same alkane, butane , . . .
any difference in heats evolved reflects
difference in the energy-contents of the two
geometric isomers.
cis-2-Butene loses 120 kJ/mol, while . . .
trans-2-butene loses only 116 kJ/mol.
trans-2-Butene must contain 4 kJ/mol less
energy than does cis-2-butene.
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Energy Difference: 4 kJ/mole
Origin of The Energy Difference
trans-2-Butene is more stable than cis-2-butene
by 4 kJ/mol.
The energy difference.originates in the CH3 / CH3
crowding present in the cis isomer but not in the trans.
Heats of Hydrogenation
A Generalization
In general, alkyl group subsitution on the
carbons of a C=C double-bond, stabilizes the
alkene (lowers its energy).
Electrophilic Additions of HX To
C=C
A Reaction Mechanism
The C=C bond of alkenes and cycloalkenes
readily undergoes electrophilic addition.
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Electrophile and Nucleophile
Nucleophilic π-Electrons of
C=C
The reagent that seeks electrons is an
electrophile.
An electron-rich center that is subject to attack
by an electrophile (and that seeks a center of
positive charge) is a nucleophile.
Pi-electrons of the typical C=C double bond are
more subject to attack by an electrophile -- are
themselves more nucleophilic than . . .
Sigma-electrons of a typical C-C single bond.
The 2-Step Mechanism
The two-step
mechanism of
electrophilic
addition of HBr
to isobutylene
A carbocation
forms as an
intermediate.
The two-step
mechanism of
electrophilic
addition of HBr
to isobutylene
A carbocation
forms as an
intermediate.
sp2 Carbon and Carbocation
•
The 2-Step Mechanism
a carbocation
Step 1
p electron
sp2 Hybridized carbon
Carbocation, also sp2
hybridized
Ionization of HBr to H+ and BrConversion of C-C π electrons to C-H σ
electrons
Formation of a 3o carbocation
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Step 2
A Review of The Two Steps
Reaction of the Br- anion (a nucleophile) with
the carbocation (an electrophile) to form tertbutyl bromide.
A Reaction Coordinate
vs. reaction
Diagram Energy
coordinate
Think of the
reaction coordinate
(x-axis) as a strip
of movie film, with
each frame
showing the atomic
and molecular
structures as they
exist at each given
moment.
Carbocation Formation
In the first step,
isobutylene and
HBr react to form
the t-butyl
carbocation.
The transition state
for the first step
resembles a
structure
intermediate
between
isobutylene and the
carbocation.
The 2-step
mechanism of
the electrophilic
additon of HX
to a C=C
double bond . .
.
as applied to
HBr and
isobutylene.
Transition State
transtion states
The transition state
represents the
highest-energy
structure involved in
each step of a
reaction. It is unstable
and cannot be
isolated.
Transition State For
Carbocation Formation
δ+ represents a partial
positive charge. The
single partial charge of
the H+ is now
distributed between
the H and the C.
Dashed or dotted lines
represent covalent
bonds being formed
and being broken.
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Product Formation
In the second step,
the carbocation
reacts with a
bromide ion to form
t-butyl bromide.
The transition state
for the second step
is intermeidate
between the
carbocation and
the product.
Transition State For Product
Formation
The Br’s negative
charge and the
carbocation’s positive
charge are in the
process of being
neutralized.
The dashed or dotted
line represents a bond
being formed.
Orbital View of The Reaction
Activation Energy
activation
energy
sp3/sp2
The C receiving the H+ is sp2 in the alkene,
intermediate between sp2 and sp3 in the
transition state, and
sp3 in the carbocation intermediate.
Electrophilic Addition to Alkenes
Can Be Regiospecific
The activation
energy of each
step represents the
energy required to
convert the
reactant(s) to the
transition state.
There is an inverse
relationship
between the size of
the activation
energy and the rate
of the reaction.
Markovnikov’s Rule
Regiospecific: Addition of XY to a C=C double
bond occurs with only one of two possible
orientations.
Regioselective: Addition of XY to a C=C
double bond occurs predominantly but not
exclusively with one orientation.
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Markovnikov’s Rule Does Not
Operate When . . .
The Basis of Markovnikov’s Rule
The Basis of Markovnikov’s Rule:
Stability of The Intermediate
Carbocation
Q: Why Should The Energy of
The Intermediate Affect The
Rate of The Reaction?
A: Hammond’s Postulate
The basis of Markovnikov’s Rule lies in the stability of
the intermediate carbocation.
Stability of carbocations: 3o > 2o > 1o > CH3
The more stable the carbocation, the lower the activation
energy for its formation and the faster it is formed.
Hammond’s Postulate
In an endergonic reaction -- one in which
energy flows into the system -- the transition
state looks like the product(s).
In an exergonic reaction -- one in which
energy flows out of the system -- the transition
state looks like the reactant(s)
Hammond’s Postulate in
Action
Formation of the
intermediate
carbocation is
endergonic
The T.S.
resembles the
carbocation
3o carbcation is
more stable than
1o carbocation.
T.S. for formation
of 3o is more
stable and forms
faster.
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An Unexpected Product
Formation of 2-chloro-3-methylbutane is the
expected product of electrophilic addition of HCl
to 3-methyl-1- butene.
Formation of 2-chloro-2-methylbutane is
unexpected.
Carbocation Rearrangement:
Hydride Shift
A shift of a H- converts a less stable 2o to a
more stable 3o carbocation.
Carbocation Rearrangement:
Methyl Shift
End
Alkenes: Structure and Reactivity
A shift of a CH3 converts a less stable 2o to a
more stable 3o carbocation.
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