Addition Reactions of Alkenes

The most characteristic reaction of alkenes is addition to the double bond. Addition Reactions of Alkenes

Exothermic reaction with heat given off (heat of hydrogenation). Hydrogenation of Alkenes The reaction is slow due to a high activation energy and is normally catalyzed by finely divided transition metals.

Stereochemistry of Alkene Hydrogenation

The alkene is planar so addition of A and B could be either syn (same side) or anti (opposite sides). Possible Stereochemistry of Addition

Mechanism of Hydrogenation Step 1. Hydrogen atoms bond to the catalyst surface. Step 2. Alkenes bond to the catalyst surface.

Mechanism of Hydrogenation Step 4. Transfer of second hydrogen atom Step 3. Hydrogen atom transfer from the catalyst to the alkene.

Mechanism of Hydrogenation The syn addition of H 2 is observed in many specific examples:

Since more substituted alkenes are more stable the heat of hydrogenation of substituted alkenes should be lower. Heats of Hydrogenation and Stability

more stable less stable

Heats of Hydrogenation and Stability

Addition of H-X to an alkene yields an alkyl halide according to the equation: Addition of Hydrogen Halides to Alkenes

The  -bond of the alkene which is electron rich (red in the electrostatic potential map) reacts with the polarized partially positive hydrogen (purple) of the HX. Addition of Hydrogen Halides to Alkenes

More substituted double bonds are more electron rich so they react faster. Addition of Hydrogen Halides to Alkenes HI is the strongest acid so it reacts fastest in the series.

Addition of HX is highly selective: Selectivity of Addition of HX to Alkenes

Markovnikov’s rule: when an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the more hydrogens, and the halogen adds to the carbon that has fewer hydrogens. Markovnikov’s Rule for Addition of HX

Mechanism of Addition of HX to Alkenes

This step is slow and rate determining. Protonation gives the most stable cation. Therefore the proton adds to the alkene carbon that initially has the most protons. Step 1. Protonation.

Mechanism of Addition of HX to Alkenes In the rapid second step the bromide attaches to the carbocation. Step 2. Combination of the anion and cation.

Potential Energy Graph for Addition of HBr

Carbocations rearrange to form more stable cations. The fact that rearrangements occur on addition of HX to alkenes provides further support for the reaction mechanism. Carbocation Rearrangements

The mechanism must explain the formation of both products. Here it involves a hydride shift. Carbocation Rearrangements The final step has chloride attaching to either of the secondary or tertiary carbocations.

Acid-Catalyzed Hydration of Alkenes

Addition of Water (H-OH) to Alkenes Electrophilic addition of water to alkenes is an acid catalyzed reaction. Markovnikov’s rule is followed.

Rate of the Hydration Reaction The indication that more substitued alkenes react fastest suggests carbocation intermediates

Mechanism of Hydration: Step 1 Markovnikov’s rule states that the proton is added onto the alkene carbon that initially has the most hydrogen atoms. Step 1: Protonation.

Mechanism of Hydration: Steps 2 & 3 Step 2: Nucleophilic addition. Step 3: Deprotonation.

The hydration of alkenes is the reverse of the dehydration of alcohols. Hydration and Dehydration These are equilibrium reactions and the real question is: How can you ensure the reaction goes in the desired direction?

Le Chatelier’s Principle A system at equilibrium adjusts to minimize any stress applied to it. For the hydration-dehydration equilibria, the key stress is water. Adding water pushes the equilibrium toward more product (alcohol). Removing water pushes the equilibrium toward more reactant (alkene).

Hydroboration–Oxidation of Alkenes Hydroboration–Oxidation of Alkenes

Anti-Markovnikov Addition of Water An indirect method to add water It involves sequential hydroboration and oxidation. The water is added in an anti-Markovnikov way.

Hydroboration-Oxidation of Alkenes Hydroboration is the addition of boron hydride to the alkene: Oxidation replaces the boron substituent with hydroxyl.

Boron adds to the least substituted C of the alkene: Hydroboration-Oxidation of Alkenes The oxidation then forms the alcohol:

Hydroboration-Oxidation of Alkenes The reaction equation is normally written as:

No carbocation intermediates so no rearrangements. Hydroboration-Oxidation of Alkenes Several different forms of BH 3 can be used.

Stereochemistry of Hydroboration-Oxidation The H and OH are added in a syn orientation.

Mechanism of Hydroboration

Step 1: Addition of BH 3.

Mechanism of Hydroboration Step 2:  -Complex rearrangement.

Mechanism of Oxidation

Step 1: Deprotonation. Step 2: Nucleophilic attack.

Mechanism of Oxidation Step 3: Carbon migration. Step 4: Hydrolytic cleavage.

Addition of Halogens to Alkenes Addition of Halogens to Alkenes

Addition of Halogens to Alkenes Electrophilic addition of halogens to alkenes yields vicinal dihalides (X = Br, Cl)

Trans-dihalides are exclusively formed by anti addition. Stereochemistry of Addition of Halogens

Rate of reaction increases with substitution suggesting that flow of electrons from the alkene to bromine is rate determining. Rates of Addition of Halogens to Alkenes

Mechanism of Bromination

Step 1: Addition.

Bromide must attach to the carbon from the opposite side to the bromonium cation. This is why the reaction is an anti-addition reaction. Mechanism of Bromination Step 2: Bromide attack.

Halohydrin Formation Reaction of bromine with alkenes in the presence of water yields vicinal halohydrins (halogen and a hydroxyl on adjacent carbons).

Halohydrin Formation The reaction is regioselective and the hydroxyl group always attaches to the most substituted end of the alkene.

Mechanism of Halohydrin Formation The regioselectivity arises in the step where water (the nucleophile) attaches to the bromonium cation:

Stereoselectivity of Halohydrin Formation The reaction proceeds by anti addition: Water must attach from the side opposite the chloronium ion:

Epoxidation of Alkenes

Epoxides Epoxides are three membered rings with one oxygen atom. Disparlure, the sex attractant of the female gypsy moth.

Formation of Epoxides Reaction of alkenes with peracids yields epoxides. For example:

Rate of Epooxidation Rate of reaction increases with increasing substitution of the alkene.

Mechanism of Epoxidation Oxygen atom transfer. Transition state:

Ozonolysis of Alkenes

Ozone and Ozonolysis Ozone is the triatomic allotrope of oxygen. Ozone reacts with alkenes to form ozonides which undergo hydrolysis in water to carbonyl compounds.

Ozonolysis of Alkenes Typical reactions are:

Problem Solving with Ozonolysis Determine the structure of the alkene given the products: Since the carbonyl carbons were the two ends of an alkene we can figure out what the alkene was:

Free-Radical Addition of Hydrogen Bromide to Alkenes Free-Radical Addition of Hydrogen Bromide to Alkenes

Addition of HBr to Alkenes Addition of HBr to alkenes can be directed to either of two regioisomers:

Anti-Markovnikov Addition of HBr

Mechanism of Free Radical Addition of HBr Radical reactions have three components: radical initiation, chain propagation and termination. Initiation. Step 1. Bond homolysis. Step 2. Hydrogen atom abstraction. After these two steps the bromine radical is formed and it starts the propagation steps.

Mechanism of Free Radical Addition of HBr The bromine radical adds to the end of the alkene that generates the most stable carbon radical. Steps 2 and 4 repeat many times before termination. Propagation. Step 3. Addition. Step 4. Hydrogen atom abstraction.

Role of Intermediates on Regioselectivity The mechanism explains the regioselectivity of addition. In each reaction the most stable intermediate is formed.

Light Initiation of Addition of HBr The free radical reaction can be initiated by light with, or without, peroxides.

Free-Radical Polymerization of Alkenes

Polymerization of Ethylene Oldest way of preparing polyethylene is the radical polymerization of ethylene:

Mechanism of Polymerization of Ethylene Step 1. Bond homolysis. Step 2. Addition. Step 3. Addition.

Formation of Teflon Teflon is formed by radical polymerization of tetrafluoroethylene: Teflon is used to prepare “nonstick” surfaces.

Common Polymers

Introduction to Organic Chemical Synthesis: Retrosynthetic Analysis

Retrosynthetic Analysis One approach to the synthesis of a given a target is to think backwards to the starting material and the reactions needed. For example: How do you synthesize 1,2-epoxycyclo- hexane from cyclohexanol? The double arrow is used in retrosynthetic analysis.

Retrosynthetic Analysis Determine which compound the epoxidcan be synthesized from and then how that compound can be synthesized from cyclohexanol. Then work forward and fill in the reagents required: