7. alkenes: reactions and synthesiswebpages.iust.ac.ir/naimi/lectures/organic chemistry/ch....
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Organic Chemistry
M. R. Naimi-Jamal
Faculty of Chemistry
Iran University of Science & Technology
Chapter 4-1. Alkenes: Reactions and Synthesis
Based on McMurry’s Organic Chemistry, 6th edition
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Diverse Reactions of Alkenes
Alkenes react with many electrophiles to give useful products by addition reactions (often through special reagents)
alcohols (add H-OH)
alkanes (add H-H)
halohydrins (add HO-X)
dihalides (add X-X)
halides (add H-X)
diols (add HO-OH)
cyclopropane (add :CH2)
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Electrophilic Addition of HX to Alkenes
General reaction mechanism:
electrophilic addition
Attack of electrophile (such as HBr) on
bond of alkene produces carbocation and
bromide ion
Carbocation is itself an electrophile,
reacting with nucleophilic bromide ion
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Electrophilic Addition for Syntheses
The reaction is successful with HCl and with HI as well as HBr. Note that HI is generated from KI and phosphoric acid
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Orientation of Electrophilic Addition: Markovnikov’s Rule
In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other
If one orientation predominates, the reaction is regiospecific
Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl
substituents)
This is Markovnikov’s rule
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Addition of HCl to 2-methylpropene is regiospecific – one product forms where two are possible
If both ends have similar substitution, then the reaction is not regiospecific
Example of Markovnikov’s Rule
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Same example:
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Practice Problem:
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Solution:
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Practice Problem:
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Solution:
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Problem: Major products?
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Problem: Which alkene?
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Stability of Carbocations and Markovnikov’s Rule
More stable carbocation forms faster
Tertiary cations and associated transition states are more stable than primary cations
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Carbocation Structure and Stability
Carbocations are planar and the tricoordinate
carbon is surrounded by only 6 electrons in sp2
orbitals
The fourth orbital on carbon is a vacant p-orbital
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Carbocation Structure and Stability
The stability of the carbocation (measured by
energy needed to form it from R-X) is increased
by the presence of alkyl substituents
Therefore stability of carbocations:
3º > 2º > 1º > +CH3
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Addition of Water to Alkenes: according to Markovnikov’s Rule
Hydration of an alkene is the addition of H-OH to to give an alcohol
Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol
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Problem: Which starting Alkenes?
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Addition of Halogens to Alkenes
Bromine and chlorine add to alkenes to give 1,2-dihalides, an industrially important process
F2 is too reactive and I2 too unreactive
Cl2 reacts as “Cl+ Cl-”; Br2 is similar
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Addition of Br2 to Cyclopentene
Addition is exclusively trans (stereospecific)
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Mechanism of Bromine Addition
Br+ adds to an alkene producing a cyclic cation: a bromonium ion, in which bromine shares charge with carbon
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Mechanism of Bromine Addition
Since the Br blocks one face, one must get anti (trans) addition
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Addition of Br2 to Cyclopentene
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Halohydrin Formation
This is formally the addition of HO-X to an alkene (with “+OH” as the electrophile) to give a 1,2-halo alcohol, called a halohydrin
The actual reagent is the dihalogen (Br2 or Cl2 with water in an organic solvent)
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An Alternative to Bromine
Bromine is a difficult reagent to use for this reaction
N-Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source
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Catalytic Hydrogenation
Addition of H-H across C=C
Reduction in general is addition of H2 or its equivalent
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Reduction of Alkenes: Hydrogenation
Requires Pt or Pd as powders on carbon and H2
Hydrogen is first adsorbed on catalyst
Reaction is heterogeneous (process is not in solution)
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Syn Addition
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Mechanism of Catalytic Hydrogenation
Heterogeneous – reaction between phases
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Solid Fats from Liquid Oils
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Chapter 4-1, Questions
25, 26, 33, 36,
37, 40