Alkenes and Electrophilic Addition

• Reactions of alkenes are covered in Chapter 8.

Bonding in Alkenes

• Pi bond electrons are loosely held.
• Double bonds act as nucleophiles attacking electrophilic species.
• Carbocations are intermediates in some reactions.
• Electrophilic additions describe these reactions.

Electrophilic Addition

• Step 1: Pi electrons attack the electrophile.
• Step 2: Nucleophile attacks the carbocation.

Types of Additions

• Additions to alkenes can involve hydration, halogenation, hydrogenation, halohydrin formation, dihydroxylation, HX addition, epoxidation, oxidative cleavage, and cyclopropanation.

Addition of HX to Alkenes

• Step 1 is the protonation of the double bond.
• The protonation step forms the most stable carbocation.
• Step 2 is a nucleophile attacking the carbocation, forming an alkyl halide.
• HBr, HCl, and HI are added accordingly.

Mechanism of Addition of HX

• Step 1: The double bond protonates.
• Step 2: The halide attacks the carbocation nucleophilically.

Hint - Carbocation Stability

• Carbocation stability is ranked: 3° > 2° > 1° > +CH3.
• During electrophilic addition, the electrophile will preferentially add to the double bond to yield the most stable carbocation intermediate.

Regioselectivity

• Markovnikov's rule dictates that when a proton adds to an alkene double bond, it bonds to the carbon atom with the greater number of hydrogens.
• The extended Markovnikov's rule specifies that in electrophilic alkene addition, the electrophile will add to generate the most stable intermediate.

Markovnikov’s Rule Example

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

Examples following Markovnikov Rule

• An appropriate example of a reaction that follows Markovnikov's rule can be viewed in the image provided.

Solved Problem

• Problem 1, in the image provided, shows how to convert 1-methylcyclohexene to 1-bromo-1-methylcyclohexane using the principles of Markovnikov orientation with the addition of HBr to an alkene.

Free-Radical Addition of HBr

• In the presence of peroxides, HBr adds to yield the "anti-Markovnikov" product.
• Peroxides generate free radicals.
• Only HBr has the necessary reactivity for a free-radical chain reaction to take place.
• The peroxide effect is exclusive to HBr; HCl or HI reactions with alkyl radicals are strongly endothermic and do not exhibit this effect..

Reaction Example - Free Radical Addition of HBr

• A reaction between 1-methylcyclohexene and HBr, where R-O-O-R and heat are present in the reaction.

Hydration of Alkenes

• Addition of water to double bonds forms an alcohol.
• The addition follows Markovnikov's rule.
• Hydration is the reverse of alcohol dehydration
• Dilute solutions of H2SO4 or H3PO4 are used to drive equilibrium toward hydration.

Hydration Mechanism

• Step 1: The double bond protonates to form a carbocation.
• Step 2: Water molecules nucleophilically attack.
• Step 3: The alcohol deprotonates to form the alcohol product.

Hydration Orientation

• Protonation follows Markovnikov's Rule.

Hydroboration of Alkenes

• Diborane (B2H6) adds to an alkene with anti-Markovnikov orientation, forming alkylboranes.
• After undergoing oxidation anti-markovnikov alcohols are formed.
• H.C. Brown of Purdue University discovered this.
• Brown was awarded the Nobel Prize in Chemistry in 1979 for his work in the field of borane chemistry.

Diborane

• Diborane (B2H6) exists as a dimer of borane.
• The complex of borane with tetrahydrofuran (BH3•THF) represents the most commonly used borane form.

Hydroboration Mechanism

• Borane adds to the double bond in a single step; boron adds to the less substituted carbon and hydrogen to the more substituted carbon.
• Orientation is such that the partial positive charge resides on the more highly substituted carbon atom during the transition state.

Oxidation to Alcohol

• Alcohol is produced through oxidation of alkyl borane with basic hydrogen peroxide.
• Orientation is anti-Markovnikov.

Stereochemistry of Hydroboration

• In hydroboration, hydrogen and boron add to the same side of the double bond (syn addition).
• When boron is oxidized, the OH group retains the stereochemical orientation.

Solved Problem re Hydroboration

• This is a problem on how to convert 1-methylcyclopentanol to 2-methylcyclopentanol.

Addition of Halogens

• Cl2, Br2 and sometimes I2 add to double bonds to form a vicinal dihalide.
• This consists of an anti addition of halides.

Halogen Addition to Alkenes - Mechanism

• A Halonium Ion intermediate occurs in the reaction.

Stereochemistry of Halogen Addition

Anti Stereochemistry

• Backside attack of nucleophile on bromonium ion results in anti stereochemistry.
• This backside attack is the reason anti stereochemistry of addition is assured.

Examples of Stereospecificity

• A set of examples is provided in the images.

Bromine Test for Unsaturation

• Add Br2 in CCl4 solvent (dark, red-brown) to an alkene.
• The color dissipates as the bromine adds to the double bond.
• The brown stain will remain in the absence of a double bond.
• Result is a a chemical test to chemically test for the presence a double bond..

Formation of Halohydrins

• If a halogen is added in the presence of water as solvent, then a halohydrin is formed.
• Water serves as the nucleophile here.
• Markovnikov addition occurs; the bromide (electrophile) adds to the less substituted carbon.

Mechanism of Halohydrin Formation

• Halohydrin formation goes through a halonium ion intermediate and proceeds through anti stereochemistry and Markovnikov orientation as shown in the example.

Stereochemistry of Halohydrins

• The mechanism includes a halonium ion, similar to halogenation, leading to anti addition.

Orientation of Halohydrin Formation

• In chloronium ions, more substituted carbons bear a greater positive charge than less substituted carbons
• Water attacks the more substituted carbon site to yield a Markovnikov product.

Hint About Halonium Ions

• Opening of a halonium ion is driven by its electrophilic nature. The carbon atom bearing the greater positive charge experiences weak nucleophile attack.

Solved problem using Halo-hydrin reaction

• In the image is an example of a problem where 1-methylcyclopentene reacts with bromine to give bromonium ion.

Reaction for forming Trans-2-bromocyclohexanol

• Also can form Trans-1-bromo-2 Chlorocyclohexane.

Catalytic Hydrogenation of Alkenes

• Hydrogen (H2) is added across the double bond using catalytic hydrogenation.
• The reactions involved only occur if the catalysts are in use.

Mechanism of Catalytic Hydrogenation

• Metal surface adsorbs both hydrogen and alkene.
• Hydrogens are inserted across the same face of the double bond.
• Reaction has syn-stereochemistry

Chiral Hydrogenation Catalysts

• Homogeneous catalysts for hydrogenation are rhodium and ruthenium phosphines.
• Chiral ligands create a new asymmetric carbon by asymmetric induction.

Requirement for Chiral Catalysts

• Only (-)-enantiomer of dopa can cross the blood-brain barrier and transform into dopamine.

Epoxidation

• Alkenes react with peroxyacids to form epoxides (or oxiranes).
• Meta-chloroperoxybenzoic acid (MCPBA) is a commonly used peroxyacid.

Reaction Mechanism - Epoxidation

• The peroxyacid and alkene react with each other in a one-step reaction to produce epoxide and a molecule of acid.

Stereochemistry of Epoxidation

• A set of examples are provided.

Opening the Epoxide Ring

• Catalysation done with acid.
• Water attacks the ring at the point of the protonated epoxide.
• Anti-diol generated.

Syn Hydroxylation of Alkenes

• Alkene is converted to a syn-1,2-diol.
• Two reagents can be used:
  • Osmium tetroxide, OsO4 then followed by hydrogen peroxide
  • Cold, dilute solution of KMnO4 in base.

Permanganate Dihydroxylation

• A cold, dilute solution of KMnO4 can hydroxylates alkenes via syn stereochemistry.
• In the basic solution, the manganate ester is hydrolyzed, freeing the glycol and causing a brown precipitate of manganese dioxide (MnO2).

Oxidative Cleavage with KMnO4

• If the solution is warm/acidic or too concentrated, oxidative cleavage may occur.
• Disubstituted and monosubstituted carbons will generate ketones and carboxylic acids correspondingly.

Oxidative Cleavage Examples

• A set of examples is provided for oxidative cleavage.

Ozonolysis

• Oxidative cleavage of double bonds to generate aldehydes and ketones.
• Is milder than using KMnO4 and does not oxidize aldehydes further.
• The intermediate is reduced by zinc or dimethyl sulfide.

Permanganate Cleavage and Ozonolysis

• A comparison is presented in the image.

hint

• Osmimum tetroxide, cold & diluted KMnO4, epoxidation reactions will only oxidize the the pi bonds of alkenes. Also it will leave the sigma bond intact. Ozone (O3) and heat will cause the breakage off of the alkene which produces carbonyl components.

Solved problem

• The solution process to the problem stated in the image is provided.

Polymerization

• An alkene (monomer) can form a polymer chain by adding to another similar molecule.
• The three categories of methods:
  • Cationic, using a carbocation intermediate
  • Free radical
  • Anionic, using a carbanion intermediate, which is rare.

Cationic Polymerization

• Protonation: H2SO4 reacts with isobutylene.
• Attack by a second molecule of isobutylene, leading to dimer formation.
• Attack by a third molecule to give a trimer, ultimately forming a polymer.

Termination Step of Cationic Polymerization

• Chain growth ends when the acid's weak base abstracts a proton, thus initiating the reaction.
• When hydrogen gets lost, an alkene is formed which is how termination starts.
• An example is given.

Using BF3 as a Catalyst for Cationic Polymerization

• An example reaction chain is provided.

Radical Polymerization

• Formula: ROOR with the use of HEAT yields 2RO!
• Requires an initiator such as peroxide.
• Radical Polymerization happens.

Anionic Polymerization

• Strong molecules that extract molecules are require for an Alkene to pull molecules.
• Carbonyl & Cyna & Nitro has to be connected to Carbons in the molecule.

Alkenes feature loosely held electrons in their pi bond, acting as nucleophiles. Electrophilic additions involve the double bond attacking electrophiles. Markovnikov's rule governs the regioselectivity in these reactions, where the proton bonds to the carbon with more hydrogens.