Chapter 8 Alkenes and Alkynes II: Addition Reactions

ALKENES Addition Reactions Addition of H2 Addition of HX Addition of ICl Addition of H2SO4 Addition of H2O/H+ Addition of X2 Halohydrin Formation Polymerization Oxymercuration-Demercuration Hydroboration Oxydation Oxydation with OsO4 and KMnO4 Ozonolysis

Introduction: Additions to Alkenes Generally the reaction is exothermic because one p and one s bond are converted to two s bonds. E+ Nu- E Nu

Some Specific Addition Reactions of Alkenes

The p electrons of the double bond are loosely held and are a source of electron density, i.e. they are nucleophilic Alkenes react with electrophiles such as H+ from a hydrogen halide to form a carbocation The carbocation produced is an electrophile It can react with a nucleophile such as a halide

1- Addition of Hydrogen Hydrogen adds to alkenes in the presence of metal catalysts Heterogeneous catalysts: finely divided insoluble platinum, palladium or nickel catalysts This process is called a reduction or hydrogenation An unsaturated compound becomes a saturated (with hydrogen) compound

Hydrogenation: The Function of the Catalyst The catalyst provides a new reaction pathway with lower DG‡ values

2- Addition of Hydrogen Halides HX Addition of HBr to propene occurs to give 2-bromopropane as the major product Markovnikov’s Rule (Original): addition of HX to an alkene proceeds so that the hydrogen atom adds to the carbon that already has the most hydrogen atoms

Example:

Theoretical Explanation of Markovnikov’s Rule The product with the more stable carbocation intermediate predominates. The most stable carbocation is formed fastest because it has a lower ∆G‡. The transition state for the rate determining step (first step) resembles a carbocation and is stabilized by factors which stabilize carbocations. Donating group

In addition reactions the alkene changes from a nucleophile in the first step to an electrophile in the second.

Mechanism for Hydrogen Halide Addition to an Alkene Rate determining step

Energy diagram for the addition of HX to alkene The reaction has two steps: In step1: the alkene donates a pair of electrons to the proton of the hydrogen halid and forms carbocation. This step is highly endergonic(endothermic) and has a high free energy of activation. In step2: the highly active carbocation stabilizes itself by combining with a halide ion. This step is highly exergonic(exothermic) and has a very low free energy of activation and takes place very rapidly. Energy diagram for the addition of HX to alkene

Stability of Carbocation Relative carbocation stability: 3° > 2° >> 1° > methyl > >> > 3° 2° 1° methyl Most stable Less stable Result The final product comes from the most stable carbocation intermediate

Addition of HBr to 2-methylpropene gives only tert-butyl bromide

Anti-Markovnikov’s Rule An Exception of Markovnikov’s Rule when alkenes are treated with HX (X= F, Cl, Br, I) in the presence of peroxides, an anti-Markovnikov addition takes place. Markovnikov’s Rule ROOR Anti-Markovnikov’s Rule

3- Addition of ICl to 2-methylpropene 2-chloro-1-iodo- 2-methylpropene

Give the structure and name of the product that would be obtained from the ionic addition of IBr to propene. Give the structure and name of the product that would be obtained from the following addition:

4- Addition of Sulfuric Acid Addition of concentrated sulfuric acid to alkenes leads to alkyl hydrogen sulfates which are soluble in the acid. The addition follows Markovnikov’s Rule. The sulfate can be hydrolyzed by heating with water The net result is Markovnikov addition of water to an alkene

5- Addition of Water to Alkenes: Acid-Catalyzed Hydration The reaction of alkenes with dilute aqueous acid leads to Markovnikov addition of water .

The Mechanism of Acid-Catalysed Hydration The first step in which a carbocation is formed is rate determining. Step 1 Formation of the most stable C+ Step 2 Formation of oxonium ion O+ Step 3 Formation of Markovnikov product

The hydration of alkenes and the dehydration of alcohols are simply reverse reactions of one other. The reaction is governed by the position of all the equilibria. Hydration is favored by addition of a small amount of acid and a large amount of water. Dehydration is favored by concentrated acid with very little water present (removal of water produced also helps favor dehydration). H2O/dil. H+ H-OH H OH Alkenes Alcohols conc. H+

6- Addition of Bromine and Chlorine to Alkenes Addition produces vicinal dihalides (each halide next to other one). This reaction is used as a test for alkenes because the red color of the bromine reagent disappears when an alkene (or alkyne) is present Alkanes do not react with bromine in the dark

Mechanism of Halogen Addition A bromonium ion intermediate results instead of the carbocation seen in other addition reactions

7- Halohydrin Formation If halogenation is carried out in aqueous solvent. The main product is a halohydrin In unsymmetrical alkenes, the bromonium ion will have some of its d+ charge density on the most substituted of the two carbons The most substituted carbon can best accommodate d+ charge The water nucleophile will tend to react at the carbon with the most d+ charge

8- Polymerization of an Alkene

Hydroxy alkyl mercuric compound 9- Oxymercuration-Demercuration Markovnikov Addition The procedure gives high yields of alcohols and avoids rearrangements. The reaction shows Markovnikov selectivity Step 1: Oxymercuration Mercuric acetate Hydroxy alkyl mercuric compound Step 2: Demercuration

Write the structure of the appropriate alkenes and specify the reagents needed for synthesis of the following alcohols by oxymercuration-demercuration

10- Hydroboration-Oxidation Anti-Markovnikov Syn Hydration The reaction leads to anti-Markovnikov addition of water to alkenes.

11- Oxidation Reaction with OsO4 or KMnO4 Syn 1,2-Dihydroxylation (cold oxidation) Syn 1,2-Dihydroxylation is an important oxidative addition reaction of alkenes. Strong oxidizing agents can be used such OsO4 (ozmium tetroxide) or KMnO4 (potassium permenganate). This type of oxidation used to synthesize 1,2-diols (1,2-dihydroxylation), sometimes called (glycols). cold

Mechanism for Syn Hydroxylation of Alkenes Cyclic intermediates result from reaction of the oxidized metals. The initial syn addition of the oxygens is preserved when the oxygen-metal bonds are cleaved and the products are syn diols Cyclic intermediate

Oxidative Cleavage of Alkenes (hot oxidation) Reaction of an alkene with hot KMnO4 results in cleavage of the double bond and formation of highly oxidized carbons. Monosubstituted carbons become carboxylates . Disubstituted carbons become ketones . Unsubstituted carbons become CO2 . This be used as a chemical test for alkenes in which the purple color of the KMnO4 disappears and forms brown MnO2 residue if alkene (or alkyne) is present. heat heat

Solved Problem 1 An unknown alkene with formula C8H16 was found on oxidation with hot basic KMnO4, to yield only the following products. What was the structure of this alkene ?

Solved Problem 2 An unknown alkene with formula C7H12 yields only the following product on oxidation with hot KMnO4 Answer: Since no carbons are missing in the product, the alkene must be part of a ring in the original. molecule

12- Ozonolysis of Alkenes Cleavage of alkenes with ozone and workup with zinc in acetic acid leads to less highly oxidized carbons than products from cleavage with hot KMnO4 monosubstituted carbons are oxidized to aldehydes disubstituted carbons are oxidized to ketones unsubstituted carbons are oxidized to formaldehyde

Aldehyde and/or ketone Mechanism of Ozonolysis of an Alkene Initial ozonide ozonide (1) O3 Aldehyde and/or ketone (2) Me2S

Predict the products of the following ozonolysis reactions:

Give the structure of an unknown alkene with the formula C7H12 that undergoes ozonolysis to yield the following compounds:

Write the structures of the alkenes that would yield the following carbonyl compounds when treated with ozone and then with dimethyl sulfide:

Summary of the Addition Reactions of Alkenes polymer H2/ catalyst H2O/H+ KMnO4 O3 polymerization HX X2 H2O/X2

1- Addition of Bromine and Chlorine to Alkynes Addition of halogen to alkynes can occur once or twice depending on how many equivalents (mol) of the halogen are added. Addition of one equivalent usually proceeds to give the trans-dihalide.

ALKYNES Addition Reactions Addition of X2 Addition of HX Ozonolysis Catalytic Hydrogenation

1- Addition of Bromine and Chlorine to Alkynes Addition of halogen to alkynes can occur once or twice depending on how many equivalents (mol) of the halogen are added. Addition of one equivalent usually proceeds to give the trans-dihalide.

2- Addition of Hydrogen Halides to Alkynes Addition of hydrogen halides occurs once or twice depending on how many molar equivalent of hydrogen halide are added. Both additions are Markovnikov and give geminal-dihalides. HBr can be generated by reaction of acetyl bromide and alumina instead of aqueous HBr. gem-dihalides

Anti-Markovnikov addition Anti-Markovnikov addition of HBr occurs in the presence of peroxide Anti-Markovnikov addition

3-Ozonolysis of Alkynes Reaction of alkynes with ozone or basic potassium permanganate (KMnO4/OH-) both lead to formation of carboxylic acids. carboxylic acids carboxylic acids

Mechamism of Ozonolysis of Alkyne

4- Catalytic Hydrogenation of Alkyne Catalytic Hydrogenation of an alkyne using a typical transition-metal catalyst, such at Pt, Pd, of Ni, yields an alkane. alkyne alkane Syn Addition of Hydrogen: Synthesis of cis-Alkenes Hydrogenation of the alkene can not be prevented by using one molar equivalent of H2. To prevent hydrogenation of the alkene, a less active catalyst must be employed.  Lindlar catalyst, which is a mixture of Pd/CaCO3, and lead salts, is the catalyst of choice in catalytic hydrogenation of alkynes to give only cis- alkenes. alkyne cis-alkene

Anti Addition of Hydrogen: Synthesis of trans-Alkenes A dissolving metal reaction which uses lithium or sodium metal in low temperature ammonia or amine solvent produces trans-alkenes Net anti addition occurs by formal addition of hydrogen to the opposite faces of the double bond alkyne trans-alkene

Summary of the Addition Reactions of Alkynes

Summary of the Addition Reactions of Alkynes