alkenes & alkynes-b
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Alkenes, Alkynes
&
EA
Reaction
Department of
Biochemistry Faculty of Pharmacy
University of Santo Tomas
Alkenes CnH2n
Alkynes
Alkenes
Cycloalkenes
Alkynes
Alkenes
Physical Properties
Alkenes & alkynes are
nonpolar
Physical properties are
similar to those of alkanes
with same C skeletons
Those that are liquid at RT
have densities <1.0g/mL
Physical Property: Boiling point
Alkenes have diffuse e- cloud; alkanes have stronger
interaction; & alkyne’s e- cloud is tighter than alkenes
STRUCTURE OF ALKENES
ISOMERISM
Alkenes: cis & trans isomerism
R
H H
R H
R H
R
All substituents are
on one side
of bond
All substituents are
on different sides
of bond
cis trans
Alkenes: cis & trans isomerism
Alkenes: cis & trans isomerism
Alkenes: cis & trans isomerism
Cis/trans Isomerism in Cycloalkenes
For cycloalkenes with 7 or fewer C
atoms, the cis configuration is preferred
Cis/trans Isomerism in Di-, Tri-, &
Polyenes
Total # of cis-trans isomers possible = 2n
n = # of double bond
2,4-heptadiene
Double bond
C2 – C3 C4 – C5
trans trans
trans cis
cis trans
cis cis
What is the correct systematic name for the following?
Br
1. 1-bromo-2-hexene2. 6-bromo-4-hexene3. cis-1-bromo-2-hexene4. trans-1-bromo-2-hexene5. trans-6-bromo-4-hexene
Alkenes: E/Z System
For more than two substituents the cis/trans,the E/Z system is used.
Alkenes: E/Z System
H3C
I H
F
(E)-3-fluoro-2-iodo-2-propene
H
H3C CH2
CH2
OH
CH3
path goes to O,
not H
path goes to C,
not H
comparison
stops here
C
H
O
H
C
H
C
H
Alkenes: E/Z System
H3C
H CH
CH2
CH2
CH3
CH CH2 C C
C H
C
HH
CH2 CH3 C C
H H
H
HH
Which is the correct structure for E-3-bromo-2-chloro-2-pentene?
C C
CH3
Cl
CH2CH3
Br
C C
CH3
Cl
Br
CH2CH3
C C
CH3
Br
Cl
CH2CH3
C C
CH3
Br
CH2CH3
Cl
1. 2.
3. 4.
Alkenes: Stability
The stability of alkenes depends upon
number of substituents
Stability of Disubstituted Alkenes
H3C
H3C H
H H3C
H H
CH3 H
H3C H
CH3
> >
REACTION OF ALKENES
ELECTROPHILIC ADDITION & OXIDATION
Reactive Sites of Alkenes & Alkynes
CH3
H3C
H3C C CH
Alkenes: Electrophilic Addition
Electrophilic addition to the double bond is a
two-step process
R
R
R
R
+ Y+ + Z-
R
R
Y
Z
RR
electrophile
nucleophile
Alkenes: Electrophilic Addition
Alkenes: Electrophilic Addition
Alkenes: AE - Hydrogenation
CH3
+ H2
Pt/CCH
3H
H
With the help of metal catalysts, H2 adds to
the double bond.
(Catalytic Reduction)
Alkenes: AE - Hydrogenation
Alkenes: AE - Hydrogenation
Alkenes: AE - Halogenation
Alkenes: AE - Halogenation
Alkenes: AE - Hydrohalogenation
Electrophilic reagents we can use are hydrogen halides:
HF, HCl, HBr, HI
Alkenes: AE - Hydrohalogenation
Alkenes: AE - Hydration
H3C
CH2
CH
CH2
+ H2O
HCl, trace
OH
H3C
CH2
CH
CH2
H
1-butene 2-butanol
Alkenes: AE - Hydration
AE – Markovnikov’s Rule
We obtain both products; however, one is the
major product.
HII
H I
H
1-methylcycohexene 1-iodo-1-methyl-cyclohexane
2-iodo-1-methyl-cyclohexane
Major product Minor product
+
We have a regioselective reaction.
AE – Markovnikov’s Rule
In the addition of HX or H2O to an
alkene, H adds to the C of the double
bond having the greater number of
hydrogens.
AE – Markovnikov’s Rule
AE – Markovnikov’s Rule
Carbocation Stability
Carbocation Stability
AE – Markovnikov’s Rule
AE – Markovnikov’s Rule
HBr
Br
H
+H
Br1-bromo-1-methyl-cyclopentane
1-bromomethyl-cyclopentane
A B
AE – Markovnikov’s Rule
Alkenes: AE –Addition of Alcohols
Alcohols react the same way as water
C CH2
H3C
H3C
+ CH3CH2OHH+ H3C
CH3
CH3
O
H2C CH32-methylpropene ethanol
tert-butyl-ethyl ether
Note: the reaction works only in the
presence of acid.
Alkenes: AE –Addition of Alcohols
Alkenes: OxidationCold, alkaline KMnO4
acidic KMnO4
Alkenes: Oxidation
Ozonolysis
Dihydroxylation with OsO4
Alkenes: Oxidation
Hydroboration oxidation
Polymerization of Alkenes
• Polymers are large molecules that
contain a large number of repeating
units of a small molecule (monomer).
• Biopolymers are typically synthesized
by organisms.
• Synthetic Polymers are made in organic
laboratories.
Polymerization of AlkenesAlkenes can be used as a monomeric unit to
form chain-growth polymers
styrene
n
polystyrene
Polymerization of Alkenes
Polymerization of Alkenes
For the polymerization, electrophilic addition
reactions can be used.
BF3F3B
In the initiation step an electrophile (BF3)
adds to the alkene to form a carbocation
intermediate.
Polymerization of Alkenes
F3B
+F
3B
Subsequent propagation steps grow the
polymer until all monomers are consumed.
Polymerization of Alkenes
Termination can also be accomplished by
adding a nucleophile.
F3B
n
Nu
F3B
Nu
n
Polymerization of AlkenesAn alternative way to initiate polymerization is
starting with a radical initiator.
RO ORD
(hn) 2 RO
radicals
RO
RO
R R
R
The radical adds onto the alkene to form a new
radical. Propagation is accomplished via subsequent
radical additions.
PREPARATION OF ALKENES
-ELIMINATION REACTION
Preparation of Alkenes
-Elimination Reactions
Dehydrogenation of alkanes
Dehydration of alcohols
Dehydrohalogenation of alkyl halides
Dehydrogenation of Alkanes
Dehydration of Alcohols
Zaitsev’s Rule
Dehydrohalogenation of RX
CONJUGATED DIENES
ADDITION & DIELS-ALDER REACTIONS
Reaction of Conjugated Dienes
undergo two-step electrophilic addition
reactios just as do simple alkenes
1,2 & 1,4-addition
Reaction of Conjugated Dienes
CH2 CH CH CH3 CH2 CH CH CH3
Less Substituted Double Bond
(Secondary carbocation)
More Substituted Double Bond
(Primary carbocation)
+
Diels-Alder Reaction: Cycloaddition
REACTION OF ALKYNES
ADDITION, OXYDATION, & ACIDITY OF TERMINAL ALKYNES
Reactivity of Alkynes
Hydrohalogenation
Hydrohalogenation
Halogenation
Hydration
Hydration of Alkyne & Tautomerism
Draw the ENOL tautomers of the following ketones:
Catalytic Reduction
Oxidation
Acidity of Terminal Alkynes
HC CH + NH2
HC C + H3N
Acidity of Terminal Alkynes
-FIN-
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