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Chapter 10
Alkenes
1
Alkenes
Introduction—Structure and Bonding
2
Alkenes
Strength of the p Bonding
Restricted rotation:
Stereoisomerism:
Stability:
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Alkenes
Introduction—Structure and Bonding
• Cycloalkenes having fewer than eight carbon atoms
have a cis geometry.
• trans-Cyclooctene is the smallest isolable trans
cycloalkene
less stable than cis-cyclooctene, making it one of the few
alkenes having a higher energy trans isomer.
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Alkenes
Calculating Degrees of Unsaturation : n-m for CnH2(n-m)
n-m = # of rings + # of p bonds
Quick assessment of molecular structure from molecular formula
• general structural formula CnH2n : acyclic alkenes, Cycloalkanes
• Each p bond or ring removes two hydrogen atoms from a molecule, and this
introduces one degree of unsaturation.
• The number of degrees of unsaturation for a given molecular formula can be
calculated by comparing the actual number of H atoms in a compound to the
maximum number of H atoms possible for the number of carbons present if the
molecule were a straight chain alkane.
• This procedure gives the total number of rings and/or p bonds in a molecule.
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Alkenes
Calculating Degrees of Unsaturation w/ hetero atoms
Halogens (F, Cl, Br, I) - Add the number of halogens to the
number of hydrogens in the formula.
Oxygen – Ignore the number of oxygens in the fomula.
Nitrogen – Subtract the number of nitrogens from the number of
hydrogens in the formula.
Examples. Deduce the number of degrees of unsaturation (d.u.) in the following
molecular formulas and suggest one possible structure for each:
(a) C6H11Cl; (b) C5H8O; (c) C8H9N.
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Alkenes
Nomenclature of Alkenes: alkenes are identified by the suffix –ene.
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Alkenes
Nomenclature of Alkenes
• Compounds with two double bonds : the suffix “–adiene”.
•
three double bonds
trienes, and so forth.
• Always choose the longest chain that contains both atoms of the
double bond.
• In cycloalkenes, the double bond is located between C1 and C2, and
the “1” is usually omitted in the name.
• The ring is numbered clockwise or counterclockwise to give the first
substituent the lower number.
• Compounds that contain both a double bond and a hydroxy group
are named as alkenols and the chain (or ring) is numbered to give
the OH group the lower number.
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Alkenes
Nomenclature of Alkenes : stereoisomers
Entgegen
(opposite)
Zusammen (together)
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Alkenes
Nomenclature of Alkenes
• Some alkene or alkenyl substituents have common names.
• The simplest alkene, CH2=CH2, named in the IUPAC system as
ethene, is often called ethylene.
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Alkenes
Physical Properties
• physical properties are similar to alkanes of comparable
molecular weight.
• Alkenes have low melting points and boiling points.
• Melting and boiling points increase as the number of carbons
increases because of increased surface area.
• Alkenes are soluble in organic solvents and insoluble in water.
• The C—C single bond between an alkyl group and one of the
double bond carbons of an alkene is slightly polar because the
sp3 hybridized alkyl carbon donates electron density to the sp2
hybridized alkenyl carbon.
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Alkenes
Physical Properties
• A consequence of this dipole is that cis and trans isomeric
alkenes often have somewhat different physical properties.
• cis-2-Butene has a higher boiling point (4°C) than trans-2-butene
(1°C).
• In the cis isomer, the two Csp3—Csp2 bond dipoles reinforce each
other, yielding a small net molecular dipole. In the trans isomer,
the two bond dipoles cancel.
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Alkenes
Interesting Alkenes
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Alkenes
Lipids
• Triacyl glycerols are hydrolyzed to glycerol and three
fatty acids of general structure RCOOH.
• As the number of double bonds in the fatty acid
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increases, the melting point decreases.
Alkenes
Lipids
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Alkenes
Lipids
• Fats and oils are triglycerols with different physical
properties.
• Fats have higher melting points—they are solids at room
temperature. Usually from animal sources
• Oils have lower melting points—they are liquids at room
temperature. Usually from vegitable sources
• The identity of the three fatty acids in the triacylglycerol
determines whether it is a fat or an oil.
• An exception to this generalization is coconut oil, which
is largely composed of saturated alkyl side chains.
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Alkenes
Lipids
• Increasing the number of double bonds in the fatty acid
side chains decreases the melting point of the
triacylglycerol.
• Fats are derived from fatty acids having few or no double
bonds.
• Oils are derived from fatty acids having a larger number
of double bonds.
• An exception to this generalization is coconut oil, which
is largely composed of saturated alkyl side chains.
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Alkenes
Preparation of Alkenes
alkenes can be prepared from alkyl halides and alcohols via
elimination reactions.
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Alkenes
Preparation of Alkenes
• elimination
reactions
regioselective.
are
stereoselective
and
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Reactions of Alkenes
Introduction to Addition Reactions
• The characteristic reaction of alkenes is addition—the p bond is
broken and two new  bonds are formed.
• Alkenes are electron rich.
• Because alkenes are electron rich, simple alkenes do not react
with nucleophiles or bases, reagents that are themselves
electron rich. Alkenes react with electrophiles.
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Introduction to Addition Reactions
• Stereochemical outcome of addition to alkenes
When the addition is selective, only one set of enantiomers forms.
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Addition Reactions to Alkenes
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Alkenes
Hydrohalogenation—Electrophilic Addition of HX
Addition reactions are exothermic
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Alkenes
Hydrohalogenation—Electrophilic Addition of HX
What about the stereochemistry & Regiochemistry
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of the reaction ?
Alkenes
Mechanism of Hydrohalogenation
• The mechanism of electrophilic addition consists of two
successive Lewis acid-base reactions.
Step 1 Electrophilic attack of H+ on the p bond
H
CH3
Br
1st new bond
CH3
C
CH3
C
H
H
H
+
C
H
slow
C
CH3
+
_
Br
H
Carbocation
Step 2 Nucleophilic attack of Br- on carbocation
CH3
H
+
C
H
_
Br
C
H
CH3 H
CH3
H
fast
C
C
Br
H
2nd new bond
CH3
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Alkenes
Hydrohalogenation—Electrophilic Addition of HX
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Alkenes
Markovnikov’s Rule : Regioselectivity of addition
Markovnikov’s rule (1869)
in the addition of HX to an unsymmetrical alkene, the H
atom adds to the less substituted carbon atom—that is,
the carbon that has the greater number of H atoms to
begin with.
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Alkenes
Markovnikov’s Rule
•The basis of Markovnikov’s rule :
the formation of a carbocation in the rate-determining step of the
mechanism.
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Alkenes
Hydrohalogenation—Markovnikov’s Rule
According to the Hammond postulate, Path [2] is faster
because formation of the carbocation is an endothermic
process.
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• The Hammond postulate relates reaction rate to stability.
• It provides a quantitative estimate of the energy of a transition
state.
• The Hammond postulate : the transition state of a reaction
resembles the structure of the species (reactant or product)
to which it is closer in energy.
in an endothermic step, TS resembles the products,
in an exothermic step, TS resembles the reactants.
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Alkenes
Hydrohalogenation—Markovnikov’s Rule
According to the Hammond postulate, Path [2] is faster
because formation of the carbocation is an endothermic
process.
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CH3
H
C
H
H
C
C
Cl
H
HCl
CH3
C
H
H
diethyl
ether
H
CH3
2-Chloropropane ONLY
Propene
CH3
H
C
C
H
Cl
H
NOT 1-Chloropropane
CH3
CH3
HBr
Br
diethyl
ether
1-methylcyclohexene
H
NOT
H
Br
1-bromo-1-methylcyclohexane
ONLY
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Alkenes
Hydrohalogenation—Reaction Stereochemistry
• Recall that trigonal planar atoms react with reagents from two
directions with equal probability.
• Achiral starting materials yield achiral products or racemic
mixture.
A racemic mixture 
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Hydrohalogenation—Reaction Stereochemistry
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Alkenes
Hydrohalogenation—Summary
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Alkenes
Hydration—Electrophilic Addition of Water
• Hydration forms an alcohol.
- Reaction mechanism is very siminar to hydrohalogenation 37
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Alkenes
Hydration—Electrophilic Addition of Alcohols
• Alcohols add to alkenes, forming ethers by the same
mechanism in presence of acid.
•
Note that there are three consequences to the formation of
carbocation intermediates:
1. Markovnikov’s rule holds.
2. Addition of H and OH occurs in both syn and anti fashion.
3. Carbocation rearrangements can occur.
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Alkenes
Halogenation—Addition of Halogen
• Halogenation is the addition of X2 (X = Cl or Br) to an
alkene to form a vicinal dihalide.
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Alkenes
Halogenation—Addition of Halogen
• Halogens add to p bonds because halogens are polarizable.
• Two facts demonstrate that halogenation follows a different
mechanism from that of hydrohalogenation or hydration.
No rearrangements occur
Only anti addition of X2 is observed
These facts suggest that carbocations are not
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intermediates.
Addition reaction of Br2 to alkenes : the fact!
CH3
C
C
H
CH3
Br2
CH3
H
H
cis-2-Butene
CH3
H
C
+
C
Br
Br
C
Br
CH3
H
Racemic mixture of enantiomers
H
C
CH3
H
C
CH3
Br
Br2
C
CH3
H
Br
CH3
H
C
CH3
CH3
C
+
C
Br
H
Br
Br
C
CH3
H
trans-2-Butene
Achiral meso compound
How can we explain this outcome ?
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Alkenes
Mechanism of Halogenation—Addition of Halogen
Step 1
..
: ..
X
..
X
.. :
.. +
X
..
C C
Slow
C
C
+
.. _
:X:
..
Strained ring
Bridged Halonium ion
Step 2
.. +
X
..
C C
..
: X:
Fast
.. _
:X:
..
C
C
: X:
..
Carbocations are unstable because they have
only six electrons around carbon.
Halonium ions are unstable because of ring
strain.
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CH3
CH3
: Br
C
F
C
CH3
H
+
Br
SbF5
SbF6CH3
CH3
SO2(l)
CH3
H
SbF5
Non-classical carbocation
By George Olah
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Halohydrin Formation : addition of X-OH
Step 1
..
: ..
X
..
X
.. :
Slow
C
C
.. +
X
..
C C
Step 2
(Step 3)
..
H2O:
.. +
X
..
C C
+
..
: X:
Fast
C
+
H2O:
.. _
:X:
..
proton
transfer
X
C
C
C
HO
Halohydrin
Even though X¯ is formed in step [1] of the mechanism, its concentration is small compared
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to H2O (often the solvent), so H2O and not X¯ is the nucleophile.
Alkenes
Halohydrin Formation
• Bromohydrins
are
also
formed
with
N-bromosuccinimide
(NBS)
in
aqueous
DMSO
[(CH3)2S=O].
• In H2O, NBS decomposes to form Br2, which then goes
on to form a bromohydrin by the same reaction
mechanism.
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Alkenes
Halohydrin Formation : Selectivity
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Halohydrin Formation
application
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Alkenes
Halohydrin Formation
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Alkenes
Hydroboration—Oxidation
Hydroboration—oxidation is a two-step reaction sequence
that converts an alkene into an alcohol.
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Alkenes
Hydroboration—Oxidation
Hydroboration—oxidation results in the addition of H2O to
an alkene.
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Alkenes
Hydroboration—Oxidation
BH3 is a reactive gas that exists mostly as a dimer, diborane (B2H6).
Borane is a strong Lewis acid that reacts readily with Lewis bases.
The first step in hydroboration
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Alkenes
Hydroboration—Oxidation
• The proposed mechanism involves concerted addition of
H and BH2 from the same side of the planar double bond:
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Alkenes
Hydroboration—Oxidation
BH3 can react with three equivalents of alkene to form a
trialkylborane.
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Alkenes
Hydroboration—Oxidation
9-borabicyclo[3.3.1]nonane (9-BBN)
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Hydroboration—Selectivity
With unsymmetrical alkenes, the boron atom bonds to the
less substituted carbon atom.
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Alkenes
Hydroboration—Oxidation
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Alkenes
Hydroboration—Oxidation
• Oxidation replaces the C—B bond with a C—O bond,
forming a new OH group with retention of configuration.
• The overall result of this two-step sequence is syn
addition of the elements of H and OH to a double bond
in an “anti-Markovnikov” fashion.
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Alkenes
Hydroboration—Oxidation
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Summary of Addition Reactions of Alkenes
Reagent
Product
Mechanism
Regioselectivity
Stereochemistry
HX
Alkyl
halide
2 step via rate-determining
formation of R+.
Rearrangements possible
Markovnikov
Syn and anti
addition
H2O
Alcohol
As above
As above
As above
X2
Vicinal (1,2) 2 step via rate-determining
dihalide
formation of bridged
halonium ion
No rearrangements
-
Anti addition
Stereospecific
X2/H2O
Halohydrin 3 steps, but similar to
(2halogenation
haloalcohol)
Markovnikov:
X+ bonds to less
substituted C.
Anti addition
AntiMarkovnikov
Syn addition in
hydroboration
step; retention of
configuration in
oxidation step
BH3 (or
Alcohol
equivale
nt), then
H2O2/OH
-
2 steps: one-step
hydroboration, then
oxidation.
No rearrangements
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keeping track of all the reactions?
Alkyl halides – Substitution and elimination (they have good leaving groups).
Alcohols – As alkyl halides, but only if OH group has been converted to a good
leaving group.
Alkenes – Addition (p bond is easily broken)
Firstly, check the basic reaction types for a functional group. This provides an
overall organization of reactions.
Then, learn the specific reagents for each reaction. This helps to classify the
reagent according to its major properties.
Is it basic or acidic?
Is it electrophilic or nucleophilic?
Is it an oxidizing agent or a reducing agent?
Think mechanism ! – reasonable ones….
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Alkenes
Alkenes in Organic Synthesis: combination of reactions
Suppose we wish to synthesize 1,2-dibromocyclohexane from
cyclohexanol.
To solve this problem we must:
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Alkenes
Alkenes in Organic Synthesis
Working backwards from the product to determine the starting
material from which it is made is called retrosynthetic analysis.
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Homework
10.40, 10.46, 10.51, 10.52,
10.53, 10.57, 10.61, 10.63,
10.64
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Preview of Chapter 11
Alkynes
Extension of chapter 10
Preparation of alkynes
dehydrohalogenation
Reactions of alkynes
Addition reaction
Reaction of acetylide anions
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