Alkenes 2025 PDF

## Alkenes ### Introduction - Alkenes are hydrocarbons with carbon-carbon double bonds. - Alkenes are also called olefins, meaning "oil-forming gas". - The functional group of alkenes is the carbon-carbon double bond, which is reactive. ### Sigma Bonds of Ethylene A diagram depicting the sigma b...

## Alkenes ### Introduction - Alkenes are hydrocarbons with carbon-carbon double bonds. - Alkenes are also called olefins, meaning "oil-forming gas". - The functional group of alkenes is the carbon-carbon double bond, which is reactive. ### Sigma Bonds of Ethylene A diagram depicting the sigma bonding orbitals of ethylene is provided. ### Orbital Description - Sigma bonds around the double-bonded carbon are $sp^2$ hybridized. - Angles are approximately $120^\circ$ and the molecular geometry is trigonal planar. - Unhybridized p orbitals with one electron will overlap forming the double bond (pi bond). ### Bond Lengths and Angles The bond lengths and angles of ethylene and ethane are provided. - $sp^2$ hybrid orbitals have more s character than the $sp^3$ hybrid orbitals. - Pi overlap brings carbon atoms closer, shortening the C-C bond from 1.54 Å in alkanes down to 1.33 Å in alkenes. ### Pi Bonding in Ethylene A diagram depicting the pi bonding in ethylene is provided. - The pi bond in ethylene is formed by overlap of the unhybridized p orbitals of the $sp^2$ hybridized carbon atoms. - Each carbon has one unpaired electron in the p orbital. - This overlap requires the two ends of the molecule to be coplanar. ### Cis-Trans Interconversion A diagram depicting the cis-trans interconversion of an alkene is provided. - Cis and trans isomers cannot be interconverted. - No rotation around the carbon-carbon bond is possible without breaking the pi bond (264 kJ/mole). ### IUPAC and New IUPAC A table summarizing the IUPAC and new IUPAC names of different alkenes is provided. ### Ring Nomenclature A diagram provides two examples of ring nomenclature. - In a ring, the double bond is assumed to be between Carbon 1 and Carbon 2. ### Multiple Double Bonds A diagram provides an example of how to name alkenes with multiple double bonds. - Give the double bonds the lowest numbers possible. - Use di-, tri-, tetra- before the ending "-ene" to specify how many double bonds are present. ### Cis-Trans Isomers A diagram provides examples of different types of cis-trans isomers. - Similar groups on the same side of a double bond, the alkene is cis. - Similar groups on opposite sides of a double bond, the alkene is trans. - Not all alkenes show cis-trans isomerism. ### Cyclic Compounds A diagram depicts examples of different cyclic compounds. - Trans cycloalkenes are not stable unless the ring has at least eight carbons. - Cycloalkenes are assumed to be cis unless otherwise specifically named trans. ### E-Z Nomenclature - Use the Cahn-Ingold-Prelog rules to assign priorities to groups attached to each carbon in the double bond. - If high priority groups are on the same side, the name is Z (for zusammen). - If high priority groups are on opposite sides, the name is E (for entgegen). ### Example A diagram represents the E-Z nomenclature of an alkene. - Assign priority to the substituents according to their atomic number (1 is highest priority). - If the highest priority groups are on opposite sides the isomer is E. - If the highest priority groups are on the same side, the isomer is Z. ### Commercial Uses of Ethylene A diagram depicts various commercial uses of ethylene. ### Commercial Uses of Propylene A diagram depicts different commercial uses of propylene. ### Heat of Hydrogenation A diagram depicts the heat of hydrogenation for different substituted alkenes - Combustion of an alkene and hydrogenation of an alkene can provide valuable data as to the stability of the double bond. - The more substituted the double bond, the lower its heat of hydrogenation. ### Relative Stabilities A diagram depicting the relative stabilities of various substituted alkenes is provided. ### Substituent Effects A diagram shows the substituent effects on the stability of an alkene. - Among constitutional isomers, more substituted double bonds are usually more stable. - Wider separation between the groups means less steric interaction and increased stability. ### Disubstituted Isomers - Stability: cis < geminal < trans isomer - The less stable isomer has a higher exothermic heat of hydrogenation. ### Stability of Cycloalkene A diagram depicts the stability of different cycloalkenes. - Cis isomer more stable than trans in small cycloalkenes. - Small rings have additional ring strain. - Must have at least eight carbons to form a stable trans double bond. - For cyclodecene (and larger), the trans double bond is almost as stable as the cis. ### Physical Properties of Alkenes - Low boiling points, increasing with mass. - Branched alkenes have lower boiling points. - Less dense than water. - Pi bond is polarizable, so instantaneous dipole-dipole interactions occur. - Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment. ### Polarity and Dipole Moments of Alkenes A diagram depicts the polarity and dipole moments of various alkenes. - Cis alkenes have a greater dipole moment than trans alkenes, so they will be slightly polar. - The boiling point of cis alkenes will be higher than the trans alkenes. ### Alkene Synthesis Overview - E2 dehydrohalogenation (-HX) - E1 dehydrohalogenation (-HX) - Dehalogenation of vicinal dibromides (-X₂) - Dehydration of alcohols (-H₂O). ### Dehydrohalogenation by the E2 Mechanism A diagram depicts the mechanism of dehydrohalogenation by the E2 mechanism. - Strong base abstracts H⁺ as the double bond forms and X leaves from the adjacent carbon. - Tertiary and hindered secondary alkyl halides give good yields. ### Bulky Bases for E2 Reactions A diagram depicts bulky bases utilized in E2 reactions. - If the substrate is prone to substitution, a bulky base can minimize the amount of substitution. - Large alkyl groups on a bulky base hinder its approach to attack a carbon atom (substitution), yet it can easily abstract a proton (elimination). ### Hofmann Products A diagram depicts the formation of Hofmann products in E2 reactions. - Bulky bases, such as potassium tert-butoxide, abstract the least hindered H⁺ giving the less substituted alkene as the major product (Hofmann product). ### Dehalogenation of Vicinal Dibromides A diagram depicts the dehalogenation of vicinal dibromides. - Remove Br₂ from adjacent carbons. - Bromines must be anti-coplanar (E2). - Use NaI in acetone, or Zn in acetic acid. ### E1 Elimination Mechanism - Tertiary and secondary alkyl halides: 3° > 2° - A carbocation is the intermediate. - Rearrangements are possible. - Weak nucleophiles such as water or alcohols. - Usually have substitution products too. ### Dehydration of Alcohols - Use concentrated sulfuric or phosphoric acid, remove low-boiling alkene as it forms to shift the equilibrium, and increase the yield of the reaction. - Carbocation intermediate: 3º alcohols react faster than 2º. Primary alcohols are the least reactive. - Rearrangements are common. - Reaction obeys Zaitsev's rule. ### Dehydration Mechanism: E1 A diagram depicting the mechanism of dehydration via the E1 mechanism is provided. ### Bonding In Alkenes A diagram depicts the bonding in alkenes - Electrons in pi bond are loosely held. - The double bond acts as a nucleophile attacking electrophilic species. - Carbocations are intermediates in the reaction. - These reactions are called electrophilic additions. ### Electrophilic Addition A diagram depicts the electrophilic addition of an electrophile to an alkene. - **Step 1**: Pi electrons attack the electrophile. - **Step 2**: Nucleophile attacks the carbocation. ### Types of Additions A table summarizes the various types of additions that can occur with alkenes. ### Addition of HX to Alkenes - **Step 1**: Protonation of the double bond. - **Step 2**: The nucleophile attacks the carbocation, forming an alkyl halide. - HBr, HCI, and HI can be added through this reaction. ### Mechanism of Addition of HX A diagram depicts the process of addition of HX to an alkene. ### Regioselectivity - **Markovnikov's rule**: The addition of a proton to the double bond of an alkene results in a product with the acidic proton bonded to the carbon atom that already holds the greater number of hydrogens. - **Markovnikov's rule (extended)**: In an electrophilic addition to the alkene, the electrophile adds in such a way that it generates the most stable intermediate. ### Markovnikov's Rule A diagram depicts Markovnikov's rule for the addition of HX to an alkene. - The acid proton will bond to carbon 3 in order to produce the most stable carbocation possible. ### Free-Radical Addition of HBr - In the presence of peroxides, HBr adds to an alkene to form the "anti-Markovnikov" product. - Peroxides produce free radicals. - Only HBr has the right bond energy. - The HCl bond is too strong, so it will add according to Markovnikov's rule, even in the presence of peroxide. - The HI bond tends to break heterolytically to form ions, it too will add according to Markovnikov's rule. ### Free-Radical Initiation A diagram depicts the free-radical initiation of the addition of HBr to an alkene. - The peroxide bond breaks homolytically to form the first radical. - Hydrogen is abstracted from HBr. ### Propagation Steps A diagram depicting the propagation steps in the addition of HBr to an alkene. - Bromine adds to the double bond forming the most stable radical possible. - Hydrogen is abstracted from HBr. ### Anti-Markovnikov Stereochemistry A diagram depicting the anti-Markovnikov stereochemistry for the addition of HBr to an alkene. - The intermediate tertiary radical forms faster because it is more stable. ### Hydration of Alkenes A diagram depicts the hydration of an alkene via the addition of water. - The Markovnikov addition of water to the double bond forms an alcohol. - This is the reverse of the dehydration of alcohol. - Uses dilute solutions of H₂SO₄ or H₃PO₄ to drive equilibrium toward hydration. ### Mechanism for Hydration A diagram depicts the mechanism for the addition of water to an alkene. - **Step 1**: Protonation of the double bond forms a carbocation. - **Step 2**: Nucleophilic attack by water. - **Step 3**: Deprotonation to the alcohol. ### Orientation of Hydration A diagram depicts the orientation of hydration of an alkene. - The protonation follows Markovnikov's rule: The hydrogen is added to the less substituted carbon in order to form the most stable carbocation. ### Rearrangements A diagram depicts the rearrangement of a carbocation intermediate in a hydration reaction. - Rearrangements can occur when there are carbocation intermediates. - A methyl shift after protonation will produce the more stable tertiary carbocation. ### Solved Problem 1 A diagram shows the conversion of 1-methylcyclohexene to 1-bromo-1-methylcyclohexane. - This synthesis requires the addition of HBr to an alkene with Markovnikov orientation. Ionic addition of HBr gives the correct product. ### Solved Problem 2 A diagram shows the conversion of 1-methylcyclohexanol to 1-bromo-2-methylcyclohexane. - This synthesis requires the conversion of an alcohol to an alkyl bromide with the bromine atom at the neighboring carbon atom. This is the anti-Markovnikov product, which could be formed by the radical-catalyzed addition of HBr to 1-methylcyclohexene. - 1-Methylcyclohexene is easily synthesized by the dehydration of 1-methylcyclohexanol. The most substituted alkene is the desired product. ### Solved Problem 2 (Continued) A diagram summarizes the two-step synthesis from Solved Problem 2. ### Hydroboration of Alkenes A diagram depicts the hydroboration-oxidation of an alkene. - The reaction adds water across the double bond with anti-Markovnikov orientation. - BH₃ (borane) is a strong Lewis acid. - Diborane (B₂H₆) is a dimer of borane and it is in equilibrium with a small amount of BH₃. - BH₃•THF is the most commonly used form of borane. ### Mechanism of Hydroboration A diagram depicts the mechanism of hydroboration of an alkene - Borane adds to the double bond in a single step, with boron adding to the less substituted carbon and hydrogen adding to the more highly substituted carbon. - This orientation places the partial positive charge in the transition state on the more highly substituted carbon atom. ### Oxidation to Alcohol A diagram depicts the oxidation of an alkyl borane to form an alcohol. - Oxidation of the alkyl borane with basic hydrogen peroxide produces the alcohol. - Orientation is anti-Markovnikov. ### Stereochemistry of Hydroboration A reaction diagram shows the stereochemistry of hydroboration. - The hydroboration steps adds the hydrogen and the boron to the same side of the double bond (syn addition). - When the boron is oxidized, the OH will keep the same stereochemical orientation. ### Solved Problem 4 A diagram shows the conversion of 1-methylcyclopentanol to 2-methylcyclopentanol. - Working backward, use hydroboration-oxidation to form 2-methylcyclopentanol from 1-methylcyclopentene. The use of (1) and (2) above and below the reaction arrow indicates individual steps in a two-step sequence. - The 2-methylcyclopentanol that results from this synthesis is the pure trans isomer. This stereochemical result is discussed in Section 8-7C. 1-Methylcyclopetene is the most substituted alkene that results from the dehydration of 1-methylcyclopentanol. Dehydration of the alcohol would give the correct alkene. ### Oxidation of a Trialkylborane A diagram depicts the stepwise process of oxidation of a trialkylborane. ### Addition of Halogens A diagram depicts the addition of halogens to alkenes. - Cl₂, Br₂, and sometimes I₂ add to a double bond to form a vicinal dibromide. - This is an anti addition of halides. ### Mechanism of Halogen Addition to Alkenes A diagram depicts the mechanism of halogen addition to an alkene. - The intermediate is a three-membered ring called the halonium ion. ### Examples of Stereospecificity A diagram shows the stereospecific halogen addition of Br₂ to the cis and trans isomers of 2-butene. ### Test for Unsaturation A schematic diagram shows the test for unsaturation using Br₂ in CCl₄. - Add Br₂ in CCl₄ (dark, red-brown color) to an alkene. - The color quickly disappears as the bromine adds to the double bond (left-side test tube). - If there is no double bond present the brown color will remain (right side). - “Decolorizing bromine" is the chemical test for the presence of a double bond. ### Formation of Halohydrin A diagram shows the formation of a halohydrin. - If a halogen is added in the presence of water, a halohydrin is formed. - Water is the nucleophile. - This is a Markovnikov addition: The bromide (electrophile) will add to the less substituted carbon. ### Mechanism of Halohydrin Formation A diagram depicts the mechanism for the formation of a halohydrin. ### Solved Problem 7 A diagram shows the formation of trans-1-bromo-2-chlorocyclohexane and trans-2-bromocyclohexanol. - Cyclohexene reacts with bromine to give a bromonium ion, which will react with any available nucleophile. The most abundant nucleophiles in saturated aqueous sodium chloride solution are water and chloride ions. Attack by water gives the bromohydrin, and attack by chloride gives the dihalide. Either of these attacks gives anti stereochemistry. ### Hydrogenation of Alkenes A diagram depicts the hydrogenation of an alkene. - Hydrogen (H₂) can be added across the double bond in a process known as catalytic hydrogenation. - The reaction only takes place if a catalyst is used. The most commonly used catalysts are palladium (Pd), platinum (Pt), and nickel (Ni), but there are other metals that work just as well. - Syn addition of hydrogen. ### Epoxidation A diagram depicts the epoxidation of an alkene with a peroxyacid. - Alkene reacts with a peroxyacid to form an epoxide (also called oxirane). - The usual reagent is peroxybenzoic acid. ### Syn Hydroxylation of Alkenes - Alkene is converted to a syn-1,2-diol. - Two reagents: - Osmium tetroxide, OsO₄, followed by hydrogen peroxide. - Cold, dilute solution of KMnO₄ in base. ### Mechanism with OsO₄ A diagram shows the mechanism of syn hydroxylation of an alkene with OsO₄. - The osmium tetroxide adds to the double bond of an alkene in a concerted mechanism forming an osmate ester. - The osmate ester can be hydrolized to produce a cis-glycol and regenerate the osmium tetroxide. ### Ozonolysis A diagram depicts the ozonolysis of an alkene. - Ozone will oxidatively cleave (break) the double bond to produce aldehydes and ketones. - Ozonolysis is milder than KMnO₄, and will not oxidize aldehydes further. - A second step of the ozonolysis is the reduction of the intermediate by zinc or dimethyl sulfide. ### Solved Problem 8 A diapgram shows the ozonolysis-reduction of an unknown alkene to give an equimolar mixture of cyclohexanecarbaldehyde and 2-butanone. - We can reconstruct the alkene by removing the two oxygen atoms of the carbonyl groups (C=O) and connecting the remaining carbon atoms with a double bond. One uncertainty remains, however: The original alkene might be either of two possible geometric isomers. ### Cleavage with KMnO₄ - Permanganate is a strong oxidizing agent. - Glycol initially formed is further oxidized. - Disubstituted carbons become ketones. - Monosubstituted carbons become carboxylic acids. - Terminal =CH₂ becomes CO₂. ### Comparison of Permanganate Cleavage and Ozonolysis A diagram contrasts the cleavage of alkenes using KMnO₄ and ozonolysis. - Aldehydes can be isolated using ozonolysis.