7. Alkenes: Reactions and Synthesiswebpages.iust.ac.ir/naimi/Lectures/Organic Chemistry/Ch. 4-1... · 7 Orientation of Electrophilic Addition: Markovnikov’s Rule In an unsymmetrical

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