Alkenes and Their Properties Quiz

Alkenes Introduction

• Alkenes are hydrocarbons with carbon-carbon double bonds.
• They 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

• Sigma bonds around the double-bonded carbon are sp² hybridized.
• Angles are approximately 120°, and the molecular geometry is trigonal planar.

Orbital Description

• Unhybridized p orbitals with one electron will overlap forming the double bond (pi bond).

Bond Lengths and Angles

• sp² hybrid orbitals have more s character than sp³ 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

• The pi bond in ethylene is formed by overlap of the unhybridized p orbitals of the sp² hybrid 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

• 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

• IUPAC naming conventions are used to name alkenes.
• For example, 1-butene and but-1-ene are both correct names for the same structure.

Ring Nomenclature

• In a ring, the double bond is assumed to be between Carbon 1 and Carbon 2.

Multiple Double Bonds

• Give the double bonds the lowest possible numbers.
• Use di-, tri-, tetra- before the ending "-ene" to specify how many double bonds are present.

Cis-Trans Isomers

• Similar groups on the same side of the double bond, the alkene is cis.
• Similar groups on opposite sides of the double bond, the alkene is trans.
• Not all alkenes show cis-trans isomerism.

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

• 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/Propylene

• Alkenes have several commercial applications, for example, producing polymers

Heat of Hydrogenation

• 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

• Ethylene, unsubstituted is less stable.
• Trisubstituted is more stable.

Substituent Effects

• Among constitutional isomers, more substituted double bonds are usually more stable.
• Wider separation between 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

• Cis isomer more stable than trans in small cycloalkenes.
• Small rings have additional ring strain.
• At least eight carbons are needed to form a stable trans double bond.
• In 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.
• Slightly polar (pi bonds are polarizable and instantaneous dipole-dipole interactions occur. Alkyl groups are electron-donating toward the pi bond).

Polarity and Dipole Moments of 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

• Strong base abstracts H⁺ as double bond forms and X leaves from the adjacent carbon.
• Tertiary and hindered secondary alkyl halides give better yields.

Bulky Bases for 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

• Bulky bases, such as potassium tert-butoxide, abstract the least hindered H+ giving the less substituted alkene as the major product.

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 to shift the equilibrium, and increase the yield of reaction.
• Carbocation intermediate: 3º alcohols react faster than 2°; primary alcohols are the least reactive. Note rearrangements are common and follow Zaitsev's rule.

Dehydration Mechanism: E1

• Detailed step-by-step mechanism of dehydration, involving protonation, carbocation formation, and removal of a leaving group.

Bonding in Alkenes

• Electrons in pi bonds are loosely held.
• The double bond acts as a nucleophile and attacks electrophilic species.
• Carbocations are intermediates in these electrophilic addition reactions.

Electrophilic Addition

• Pi electrons attack the electrophile.
• Nucleophile attacks the carbocation.

Types of Additions to Alkenes

• Table of different reactions possible with alkenes (hydration, hydrogenation, hydroxylation, oxidative cleavage, epoxidation, cyclopropanation.

Addition of HX to Alkenes

• Step 1 is the protonation of the double bond.
• The protonation step forms the most stable carbocation possible.
• In step 2, the nucleophile attacks the carbocation, forming an alkyl halide.
• HBr, HCl, and HI can be added through this reaction.

Mechanism of Addition of HX

• Detailed step-by-step mechanism showing protonation and nucleophilic attack.

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

• The acid proton will bond to carbon 3 to produce the most stable carbocation.

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 for the free-radical reaction to work. HCl and HI are too strong so they do not work.

Free-Radical Initiation

• The peroxide bond breaks homolytically to form the first radical.
• Hydrogen is abstracted from HBr.

Propagation Steps

• Bromine adds to the double bond forming the most stable radical possible.
• Hydrogen is abstracted from HBr.

Anti-Markovnikov Stereochemistry

• The intermediate tertiary radical forms faster because it is more stable.

Hydration of Alkenes

• 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

• Detailed step-by-step mechanism showing protonation, nucleophilic attack by water, and deprotonation.

Orientation of Hydration

• Protonation follows Markovnikov's rule.
• Hydrogen adds to the less substituted carbon in order to form the most stable carbocation.

Rearrangements

• Rearrangements can occur with carbocation intermediates.
• A methyl shift after protonation will produce the more stable tertiary carbocation.

Solved Problem 1

• Converting 1-methylcyclohexene to 1-bromo-1-methylcyclohexane requires addition of HBr with Markovnikov orientation.

Solved Problem 2

• Converting 1-methylcyclohexanol to 1-bromo-2-methylcyclohexane involves dehydration to generate 1-methylcyclohexene followed by the anti-Markovnikov addition of HBr.

Solved Problem 2 (Continued)

• Detailed two-step synthesis summarizing the conversion of 1-methylcyclohexanol to 1-bromo-2-methylcyclohexane.

Hydroboration of Alkenes

• The reaction adds water across the double bond with anti-Markovnikov orientation.
• Diborane (B₂H₄) is a dimer of borane in equilibrium with a small amount of BH₃.
• BH₃•THF is the most commonly used form of borane.

Mechanism of Hydroboration

• 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.

Oxidation to Alcohol

• Oxidation of the alkyl borane with basic hydrogen peroxide products the alcohol.
• The orientation is anti-Markovnikov.

Stereochemistry of Hydroboration

• The hydroboration steps add hydrogen and 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

• Converting 1-methylcyclopentanol to 2-methylcyclopentanol requires hydroboration-oxidation to form 2-methyl cyclopentanol from 1-methylcyclopentene

Oxidation of a Trialkylborane

• Detail mechanism to oxidize a trialkylborane with hydrogen peroxide and form the alcohol.

Addition of Halogens

• 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

• The intermediate is a three-membered ring called the halonium ion.

Examples of Stereospecificity

• Examples of stereospecificity in addition reactions of halogens to cis and trans-2-butene.

Test for Unsaturation

• 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, the brown color remains (right side).
• "Decolorizing bromine" is the chemical test.

Formation of 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

• Detailed step-by-step mechanism of halohydrin formation.

Solved Problem 7

• When cyclohexene is treated with bromine in saturated aqueous sodium chloride, a mixture of trans-2-bromocyclohexanol and trans-1-bromo-2-chlorocyclohexane results. A detailed mechanism is outlined demonstrating the formation of both products through bromonium ion attack by water or chloride respectively

Hydrogenation of Alkenes

• 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

• 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 or Cold, dilute solution of KMnO₄ in base.

Mechanism 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 hydrolyzed to produce a cis-glycol and regenerate the osmium tetroxide.

Ozonolysis

• 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

• Ozonolysis-reduction of an unknown alkene gives an equimolar mixture of cyclohexanecarbaldehyde and 2-butanone. Determine the original alkene by removing the oxygen atoms and connecting the carbon atoms with a double bond.

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

• Comparison between permanganate cleavage and ozonolysis, showing the potential formation of aldehydes.

Test your knowledge on the properties and characteristics of alkenes, including molecular geometry, bond length, and isomerism. This quiz will cover key concepts related to elimination mechanisms and the behavior of cis and trans isomers. Dive into the world of alkenes and enhance your understanding of organic chemistry!