Alkene Reactions and Electrophilic Addition

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Questions and Answers

What type of reaction occurs between alkenes and electrophiles?

Hydrogenation

Nucleophilic substitution

Addition (correct)

Elimination

What characterizes syn addition in alkene reactions?

Formation of a cyclic intermediate

Formation of a carbocation

Addition from opposite sides

Addition from the same side (correct)

In hydrohalogenation, which bonds are broken during the reaction?

$ ho$ bonds only

$C=C$ and $H-X$ bonds (correct)

$C-C$ and $H-X$ bonds

$H-H$ and $X-X$ bonds

What results from the Markovnikov addition of hydrogen halides to alkynes?

A more substituted carbocation is formed Signup and view all the answers

According to Markonikov's rule, where does the hydrogen attach in the addition of $H-X$ to an unsymmetrical alkene?

To the less substituted carbon atom Signup and view all the answers

Why is carbocation A more stable than carbocation B?

It is stabilized by resonance Signup and view all the answers

What is the first step in the mechanism of electrophilic addition of $H-X$?

Carbocation formation Signup and view all the answers

What occurs when one mole of X2 is added to an alkyne?

Formation of a trans dihalide Signup and view all the answers

What type of addition occurs when X and Y are added to the opposite sides of a double bond?

Anti addition Signup and view all the answers

Which of the following is true about the formation of resonance structures?

Resonance stabilizes adjacent positive charges Signup and view all the answers

Which of the following best describes the electrophilic addition reaction of alkenes with $HBr$?

Two $ ho$ bonds are broken and two $ ho$ bonds are formed Signup and view all the answers

What describes the structure of carbocation formed when treating alkynes with hydrogen halides?

Vinyl carbocation formed is sp hybridized Signup and view all the answers

During the addition of hydrogen halides to alkenes, which intermediate is typically formed?

Carbocation Signup and view all the answers

If two equivalents of HBr are added to an alkyne, what is the expected outcome?

Formation of a polyhalide Signup and view all the answers

Which statement best explains why trans alkenes are more stable than cis alkenes?

Trans alkenes exhibit fewer steric interactions. Signup and view all the answers

How does a bridged halonium ion function in the addition of X2?

It facilitates a two-step addition process Signup and view all the answers

What is the primary function of the boron atom during hydroboration?

It serves as an electrophile. Signup and view all the answers

What type of hybridization does a substituted carbocation, as described in the content, typically have?

sp2 hybridization Signup and view all the answers

What does the Zaitsev rule state regarding elimination reactions?

The major product is the more substituted double bond. Signup and view all the answers

Which feature characterizes the regioselectivity of hydroboration?

Boron bonds to the less substituted carbon. Signup and view all the answers

In an E2 reaction, why is the reaction considered regioselective?

It yields predominantly the more substituted alkene. Signup and view all the answers

What is the primary mechanism for the acid-catalyzed dehydration of secondary and tertiary alcohols?

E1 mechanism Signup and view all the answers

What is the effect of oxidation on the alkylborane obtained from hydroboration?

It introduces a C-O bond. Signup and view all the answers

How is an addition reaction characterized?

A π bond is broken and two new σ bonds are formed. Signup and view all the answers

What type of addition does hydroboration follow?

Syn addition. Signup and view all the answers

What effect does a better leaving group have on the rate of SN1 and SN2 reactions?

It increases the rate of both reactions Signup and view all the answers

Which statement about the stereochemistry of hydroboration-oxidation is correct?

It produces a racemic mixture of alcohols. Signup and view all the answers

Which of the following is a characteristic of stereoselective reactions?

They form exclusively one stereoisomer. Signup and view all the answers

When reacting with an alkyl halide, what type of product does the E2 reaction favor?

The more substituted alkene. Signup and view all the answers

What role do alkyl groups play during the hydroboration process?

They stabilize the positive charge on the carbon. Signup and view all the answers

Which type of solvent is particularly favorable for SN1 reactions?

Polar protic solvents Signup and view all the answers

What is the primary reason vinyl halides do not undergo SN1 reactions?

The resulting carbocation is highly unstable Signup and view all the answers

For which type of alcohols does the dehydration reaction predominantly use the E2 mechanism?

Primary alcohols Signup and view all the answers

What is produced after the complete hydroboration and oxidation of an alkene?

An alcohol with syn addition of H and OH. Signup and view all the answers

In a typical elimination reaction, what process is known as dehydrohalogenation?

The loss of elements from the starting material to form a new π bond Signup and view all the answers

Which statement accurately describes the transition state during hydroboration?

The carbon atom bears a partial positive charge. Signup and view all the answers

What type of hybridization do SN1 and SN2 reactions occur at for alkyl halides?

sp3 hybridized carbon atoms Signup and view all the answers

Which of the following statements is true regarding polar aprotic solvents?

They are particularly good for SN2 reactions Signup and view all the answers

Which mechanism is favored when a strong nucleophile is used in a polar protic solvent?

SN1 mechanism Signup and view all the answers

What happens during the β elimination in an elimination reaction?

Protons are removed from the α and β carbons Signup and view all the answers

How does the presence of bulky R groups affect the rate of 𝑆𝑁 2 reactions?

It decreases the rate due to steric hindrance. Signup and view all the answers

What characterizes the stereochemistry of the products formed from an 𝑆𝑁 1 reaction?

The products are a racemic mixture of enantiomers. Signup and view all the answers

Why do unhindered halides react faster in 𝑆𝑁 2 reactions?

They allow for more effective back-side nucleophilic attack. Signup and view all the answers

In an 𝑆𝑁 1 reaction, what happens to the bond between carbon and bromine?

It is broken before the new bond is formed. Signup and view all the answers

What is the shape and hybridization of the carbocation formed during an 𝑆𝑁 1 reaction?

Trigonal planar and sp² hybridized. Signup and view all the answers

What defines racemization in the context of the 𝑆𝑁 1 reaction?

The formation of equal amounts of two enantiomeric products. Signup and view all the answers

Which of the following reactions would most likely undergo an 𝑆𝑁 1 mechanism?

A tertiary alkyl halide with a weak nucleophile. Signup and view all the answers

What is the kinetic expression for the rate of an 𝑆𝑁 1 reaction?

rate = k[substrate] Signup and view all the answers

Study Notes

Alkane Reactions

Alkanes primarily undergo combustion reactions when exposed to oxygen, yielding CO2 and H2O. This reaction releases energy in the form of heat.
Every C-H and C-C bond is converted to a C-O bond in the product.

Oxidation and Reduction of Alkanes

Oxidation increases C-Z bonds and decreases C-H bonds.
Reduction decreases C-Z bonds and increases C-H bonds.

Problems: Oxidation/Reduction

Determine if the organic compound is oxidized or reduced during a transformation.
- Example: Ethanol to acetic acid is an oxidation reaction.
- Example: Ethylene to ethane is a reduction reaction.

Combustion of Alkanes

Complete combustion of alkanes produces carbon dioxide (CO2) and water (H2O).
Incomplete combustion of alkanes can produce carbon monoxide (CO)

Alkyl Halide Reactions

The general substitution reaction of alkyl halides involves a nucleophile reacting with an alkyl halide to replace the halogen.
Common leaving groups are halide anions, water (from ROH), and nitrogen.
Nucleophilic substitution reactions are Lewis acid-base reactions, nucleophile donates its electron pair to the alkyl halide where it cleaves to form the products and leave a leaving group.
- Example reactions in the alkyl halide reaction include: CH3-Cl + :OH⁻ → CH3-OH+Cl−
A neutral nucleophile also forms a substitution product, bearing a positive charge.
If substituents have a proton connected to an O or N atom, then they readily lose a proton to form a neutral product.
- Example: CH3CH2CH₂Br + :N(CH3)3 → CH3CH2CH₂N(CH3)3 + Br⁻

Leaving Group

A good leaving group is a weak base with a strong conjugate acid that has a low pKa value.
Common good leaving groups are halide anions, except F⁻.
Other leaving groups include but are not limited to water and nitrogen functionalities.
In nucleophilic substitution reactions. The better leaving group is the weaker base.
Examples include Cl⁻, Br⁻, I⁻, H₂O, and N₂.

Periodic Trends in Leaving Group

Left-to-right across a row of the periodic table, basicity decreases, therefore the leaving group ability increases.
Down a column of the periodic table, basicity decreases leading to an increase in leaving ability.

Nucleophilic Substitution Reactions

Nucleophilic substitutions are Lewis acid-base reactions.
Nucleophile donates its electron pair, the alkyl halide accepts it, and the C - X bond is heterolytically cleaved.
Curved arrow notations can be used to show the movement of electron pairs.

Nucleophilicity Versus Basicity

The strength of nucleophilicity parallels basicity, especially when considering the same nucleophilic atom.
A negatively charged nucleophile is always stronger than its conjugate acid
Right-to-left across a row of the periodic table, nucleophilicity increases as basicity increases.
Steric hindrance decreases nucleophilicity but does not affect basicity. Nonnucleophilic bases are sterically hindered.

Stearic Effects and Nucleophilicity

Nucleophilicity does not correlate to basicity when steric hindrance is significant.
Bulky groups decrease reactivity, for instance, the strength of the nucleophile's attack when there are bulky groups at the site of the reaction.

Polar Protic and Polar Aprotic Solvents

Polar protic solvents are capable of intermolecular hydrogen bonding, solvating cations and anions well.
Cations are solvated by ion-dipole interactions, and anions are solvated by hydrogen bonding.
Polar aprotic solvents exhibit dipole-dipole interactions but cannot hydrogen-bond, solvating only cations.
Anions are not well solvated in polar aprotic solvents because the solvent cannot hydrogen bond to them.

Possible Mechanisms for Nucleophilic Substitution

One step-mechanism-Bond breakage and bond making occur at the same time.
Two-step mechanism- Bond breaking occurs before bond making..
Ten electrons around carbon atom violates octet rule.
Bond making occurs before bond breaking.

SN1 and SN2 Reactions

SN1 Mechanism (substitution nucleophilic unimolecular): two-step mechanism where the rate-determining step involves the alkyl halide only.
SN2 Mechanism (substitution nucleophilic bimolecular): one-step mechanism where the C – X bond breaks as the C–Nu bond forms.
Stereochemistry of SN2 reactions results in the inversion of configuration at a stereogenic center.
SN2 reactions usually proceed smoothly and readily with the alkyl halide having a methyl, primary, or secondary carbon structure.
SN1 reactions have more significant implications for stability as the halogen bonded to an alkyl halide with a tertiary carbon structure will favor this reaction.

Carbocation Stability

Carbocation stability increases as the number of R groups attached to the positively charged carbon increases.
- 3° >2° 1°.

E1 and E2 Reactions

E1 Reaction (elimination unimolecular): two-step mechanism where the leaving group leaves, forming a carbocation, prior to the removal of a proton.
E2 Reaction (elimination bimolecular): one-step mechanism where the removal of the proton and the leaving group happens simultaneously.

The Zaitsev Rule

The major product in an elimination is the more substituted alkene..

Addition Reactions

Alkenes are electron-rich and undergo addition reactions where the pi bond is broken to form a double bond in the product.
Syn and anti addition occur, meaning elements are added to the same or opposite sides of the alkene.

Hydration of Alkenes

The addition of water to an alkene followed by the formation of an alcohol by addition in either a syn or anti fashion via either a concerted (E2) or a stepwise (E1) mechanism, depending on the alkene.
The addition occurs via a carbocation intermediate, or when the nucleophile attacks, followed by the loss of a proton from this carbocation intermediate.

Halogenation of Alkenes

Halogens add to the double bond of the alkene in an anti addition pathway from either side, forming a vicinal dihalide, as a two step mechanism..
The addition has two reaction steps: electrophilic, and nucleophilic.

Hydroboration-Oxidation

It's a two-step sequence converting an alkene into an alcohol.
Adding BH3 followed by oxidation with H2O2 and OH⁻ forms an alcohol.
Hydroboration occurs by syn addition, where the H₂ and BH₃ add with retention of configuration
The overall result of hydroboration-oxidation is the syn addition of H and OH across a double bond, the OH group attaches to the less substituted carbon.

Preparation of Alkynes

Preparing an alkyne involves the elimination reactions of vicinal or geminal dihalide to yield an alkyne via a strong base by two successive E2 eliminations.

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Description

Explore the fascinating world of alkene reactions and their mechanisms in this quiz. Focused on electrophilic addition, it covers concepts like Markovnikov's rule, carbocation stability, and types of addition. Test your understanding of how alkenes interact with electrophiles and what characterizes different addition reactions.