Alkene Reactions and Mechanisms: Electrophilic Addition, Markovnikov, Anti-Markovnikov, and Catalytic Hydrogenation

What type of reactions do alkenes primarily undergo?

Electrophilic additions.

What is the nature of electrons in pi bonds of alkenes?

They are loosely held.

What are carbocations?

Intermediates formed during some reactions of alkenes.

What is the first step in the electrophilic addition mechanism?

Pi electrons attack the electrophile, forming a carbocation.

What happens in the second step of electrophilic addition?

A nucleophile adds to the carbocation.

What is Markovnikov's Rule?

When HX adds to an unsymmetrical alkene, the more stable carbocation is formed.

What is the outcome of Markovnikov's Rule?

The more highly substituted alkyl halide is formed.

What is an anti-Markovnikov addition?

Reactions where the electrophile adds contrary to Markovnikov's rule.

What role do peroxides play in anti-Markovnikov reactions?

They promote free radical mechanisms.

What is the first step in the anti-Markovnikov addition mechanism?

Free radical initiation.

What is the reagent used in the oxymercuration reaction?

Mercury(II) acetate.

What is formed during the oxymercuration reaction?

A three-membered ring called the mercurinium ion.

What is the final product of the oxymercuration-demercuration reaction?

The Markovnikov product with the hydroxy group on the most substituted carbon.

What is the significance of the hydride in the demercuration reaction?

It replaces the mercuric acetate.

What does the addition of water to an alkene produce?

An alcohol.

What is the mechanism for the hydration of alkenes?

Protonation of the double bond forms a carbocation, followed by nucleophilic attack of water.

What is the result of the hydration reaction following Markovnikov's rule?

The Markovnikov alcohol is formed.

What is the difference between synthetic steps and mechanistic steps?

Synthetic steps are individual reactions to form a product; mechanistic steps describe how bonds are broken or formed.

What is the outcome of the alkoxymercuration-demercuration reaction?

An ether is produced when alcohol is used instead of water.

What is the role of borane (BH3) in hydroboration reactions?

It adds to alkenes with anti-Markovnikov orientation.

What is the product of hydroboration after oxidation?

Anti-Markovnikov alcohols.

What is the rate-limiting step in reactions involving carbocations?

The formation of the carbocation.

What happens to carbocations during reactions?

They can rearrange to more stable forms.

What is the significance of the bromide ion in the addition of HBr to alkenes?

It adds to the carbocation to form an alkyl bromide.

What is the effect of using dilute H2SO4 or H3PO4 in hydration reactions?

It drives the equilibrium toward hydration.

What is the mechanism of the oxymercuration reaction?

Mercury(II) acetate acts as an electrophile and forms a mercurinium ion.

What is the result of the addition of HBr in the presence of peroxides?

Formation of the anti-Markovnikov product.

What is the significance of the carbocation stability in Markovnikov's rule?

The most stable carbocation is favored in the reaction.

What is the first step in the hydroboration reaction sequence?

Addition of BH3 to give an alkyl borane (R-BH2).

What is produced after the oxidation of the alkyl borane in hydroboration?

An alcohol with anti-Markovnikov orientation.

What is diborane (B2H6) used for in hydroboration?

It acts as a dimer of borane in tetrahydrofuran (THF) solution.

What is the mechanism of hydroboration characterized by?

Borane adds to the double bond in a single step, with boron adding to the less substituted carbon.

What type of addition occurs during the hydroboration step?

Syn addition, where hydrogen and boron add to the same side of the double bond.

What is the result of the oxidation of the alkyl borane?

The formation of an anti-Markovnikov alcohol.

What is the complete synthesis sequence for converting 1-methylcyclopentanol to 2-methylcyclopentanol?

Dehydration of the alcohol, hydroboration of the alkene, and oxidation of the alkyl borane to alcohol.

What is the stereochemical outcome of hydroboration-oxidation when forming chiral products?

A racemic mixture is obtained due to the planar nature of the intermediate alkene.

What is the mechanism of halogen addition to a double bond?

Formation of a vicinal dihalide through anti addition of halides.

What is the intermediate formed during halogen addition?

A three-membered ring called the halonium ion.

What is the stereochemistry of halogen addition determined by?

The presence of a 3-membered ring and the need for the bromide nucleophile to attack from the opposite side.

What happens when bromine is added to an alkene in CCl4?

The dark red-brown color disappears, indicating the presence of a double bond.

What is formed when a halogen is added in the presence of water?

A halohydrin, which is a Markovnikov addition.

What is the role of water in the mechanism of halohydrin formation?

Water acts as a nucleophile that opens the halonium ion ring.

What is the orientation of addition in halohydrin formation?

Anti orientation due to the mechanism involving a halonium ion.

What occurs when cyclohexene is treated with bromine in saturated aqueous sodium chloride?

A mixture of trans-2-bromocyclohexanol and trans-1-bromo-2-chlorocyclohexane is formed.

What is catalytic hydrogenation of alkenes?

The addition of hydrogen (H2) across the double bond using a catalyst.

What type of stereochemistry is observed in catalytic hydrogenation?

Syn stereochemistry, as both hydrogens add to the same side of the double bond.

What is Wilkinson's catalyst used for?

It is a soluble homogeneous catalyst that catalyzes the hydrogenation of carbon-carbon double bonds.

What is the significance of the syn addition in hydroboration?

It leads to the formation of specific stereoisomers in the final product.

What is the purpose of retro-synthetic analysis in organic synthesis?

To work backward from the target molecule to identify suitable starting materials and reaction pathways.

What is the outcome of the reaction of 1-methylcyclopentene with bromine water?

Formation of a bromonium ion followed by nucleophilic attack by water.

What is the relationship between the starting material and target product in syntheses involving alkenes?

They share the same basic carbon skeleton but require functional group modifications.

What is the result of adding halogens to a double bond?

Formation of vicinal dihalides through anti addition.

What happens to the stereochemistry during halogenation?

The addition is anti due to the mechanism involving a halonium ion.

What is the effect of using a catalyst in catalytic hydrogenation?

It allows for the addition of hydrogen across the double bond.

What is the significance of the exo and endo designations in cyclohexane rings?

They are used to identify substituents in the boat conformation.