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13.4: Reactivity of Alkenes

Last updated

Aug 9, 2023
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- 13.3.8: Zaitsev's Rule
- 13.4.1: Addition of Sulfuric acid to Alkenes

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Alkenes are a family of hydrocarbons (compounds containing carbon and hydrogen only) containing a carbon-carbon double bond. Alkenes are relatively stable compounds, but are more reactive than alkanes because of the reactivity of the carbon–carbon π-bond. Most reactions of alkenes involve additions to this π bond, forming new single bonds.

13.4.1: Addition of Sulfuric acid to Alkenes
The carbon-carbon double bond in alkenes such as ethene react with concentrated sulfuric acid. It includes the conversion of the product into an alcohol.
13.4.2: Addition Reactions of Alkenes
13.4.3: Addition Reactions of Dienes
Addition reactions of isolated dienes proceed more or less as expected from the behavior of simple alkenes. Thus, if one molar equivalent of 1,5-hexadiene is treated with one equivalent of bromine a mixture of 5,6-dibromo-1-hexene, 1,2,5,6-tetrabromohexane and unreacted diene is obtained, with the dibromo compound being the major product (about 50%).
13.4.4: Catalytic Hydrogenation of Alkenes
The double bond of an alkene consists of a sigma (σ) bond and a pi (π) bond. Because the carbon-carbon π bond is relatively weak, it is quite reactive and can be easily broken and reagents can be added to carbon. Reagents are added through the formation of single bonds to carbon in an addition reaction.
13.4.5: Catalytic Hydrogenation of Alkenes II
Alkene hydrogenation is the syn-addition of hydrogen to an alkene, saturating the bond. The alkene reacts with hydrogen gas in the presence of a metal catalyst which allows the reaction to occur quickly. The energy released in this process, called the heat of hydrogenation, indicates the relative stabily of the double bond in the molecule (see Catalytic Hydrogenation).
13.4.6: Catalytic Hydrogenation of Alkenes II
The carbon-carbon double bond in alkenes react with hydrogen in the presence of a metal catalyst. This is called hydrogenation. It includes the manufacture of margarine from animal or vegetable fats and oils.
13.4.7: Diazomethane, Carbenes, and Cyclopropane Synthesis
13.4.8: Diels-Alder Cycloaddition
The unique character of conjugated dienes manifests itself dramatically in the Diels-Alder Cycloaddition Reaction. A cycloaddition reaction is the concerted bonding together of two independent pi-electron systems to form a new ring of atoms. When this occurs, two pi-bonds are converted to two sigma-bonds, the simplest example being the hypothetical combination of two ethene molecules to give cyclobutane. This does not occur under normal conditions, though.
13.4.9: Electrophilic Addition of Halogens to Alkenes
13.4.10: Electrophilic Addition of Hydrogen Halides
Due to the nature of the π bond, π bonds can act like a nucleophile and undergo addition of electrophiles. The π electrons have a high electron density in the π electron cloud which can be easily polarized (give up the electrons) and can act like a nucleophile (nucleus-loving due to too many electrons; wants to give them away to electrophile). This behavior is very similar to the behavior that the lone pair electrons have on a Lewis base.
13.4.11: Electrophilic Addition of Hydrogen Halides II
Symmetrical alkenes (like ethene or but-2-ene) are dealt with first. These are alkenes where identical groups are attached to each end of the carbon-carbon double bond.
13.4.12: Free Radical Reactions of Alkenes
13.4.13: Hydration of Alkenes
- 13.4.13.1: Formation of alcohols from alkenes
13.4.14: Hydroboration-Oxidation of Alkenes
Hydroboration-Oxidation is a two step pathway used to produce alcohols. The reaction proceeds in an Anti-Markovnikov manner, where the hydrogen (from $BH_3$ or $BHR_2$ attaches to the more substituted carbon and the boron attaches to the least substituted carbon in the alkene bouble bond. Furthermore, the borane acts as a lewis acid by accepting two electrons in its empty p orbital from an alkene that is electron rich. This process allows boron to have an electron octet.
13.4.15: Oxacyclopropane Synthesis
13.4.16: Oxidation of Alkenes with Potassium Manganate
The carbon-carbon double bonds in alkenes such as ethene react with potassium manganate(VII) solution (potassium permanganate solution).
13.4.17: Ozonolysis
Ozonolysis is a type of weak oxidative cleavage where we cleave alkenes (double bonds) into either ketones, aldehydes or carboxylic acid using ozone.
13.4.18: Ozonolysis of Alkenes and Alkynes
Ozonolysis is a method of oxidatively cleaving alkenes or alkynes using ozone ( $O_3$ ), a reactive allotrope of oxygen. The process allows for carbon-carbon double or triple bonds to be replaced by double bonds with oxygen. This reaction is often used to identify the structure of unknown alkenes by breaking them down into smaller, more easily identifiable pieces.
13.4.19: Polymerization of Alkenes
13.4.20: Reactions of Alkenes with Halogens
This page looks at the reaction of the carbon-carbon double bond in alkenes such as ethene with halogens such as chlorine, bromine and iodine. This is called halogenation. Reactions where the chlorine or bromine are in solution (for example, "bromine water") are slightly more complicated and are treated separately at the end.
13.4.21: Stereoselectivity in Addition Reactions to Double Bonds
13.4.22: Vicinal Syn Dihydroxylation
Osmium tetroxide oxidizes alkenes to give glycols through syn addition. A glycol, also known as a vicinal diol, is a compound with two -OH groups on adjacent carbons.

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