Alkenes and Electrophilic Addition Reactions

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Questions and Answers

What is the primary reason why alkenes react with electrophiles?

Alkenes have lone pairs of electrons available.

Alkenes possess a negative charge.

Alkenes contain a high electron density in their double bond. (correct)

Alkenes are stable and do not react easily.

Which reaction is an example of the electrophilic addition of hydrogen halides to alkenes?

Oxidation of ethene to acetaldehyde.

Reduction of ethene to ethane.

Addition of hydrogen bromide to propene. (correct)

Formation of an alcohol from ethene and water.

Which product is typically obtained when an unsymmetrical alkene reacts with hydrogen halide according to Markovnikov’s rule?

A primary carbocation is formed exclusively.

No carbocation is formed during this reaction.

Equal amounts of both possible carbocations.

A secondary carbocation predominates over a primary carbocation. (correct)

In the context of alkenes, what does the term 'oxidation' primarily refer to?

The removal of hydrogen or addition of oxygen to the molecule. Signup and view all the answers

What role do manganese ions play in the oxidation of alkenes?

They cause the rupture of the carbon-to-carbon double bond. Signup and view all the answers

What is necessary for an alkene to exhibit cis-trans isomerism?

Two different groups bonded to each carbon atom of the double bond Signup and view all the answers

Which of the following compounds exhibit cis-trans isomerism?

but-2-ene Signup and view all the answers

Why doesn't but-1-ene exhibit cis-trans isomerism?

It has identical groups bonded to the same carbon atom in the double bond Signup and view all the answers

What type of isomerism is shown by compounds having the same structural formula but differing in spatial arrangement?

Stereoisomerism Signup and view all the answers

In the context of alkenes, what is meant by restricted rotation?

It results from the presence of pi bonds in the double bond Signup and view all the answers

Which is an example of chain isomerism?

but-1-ene and 2-methylpropene Signup and view all the answers

Which statements are true regarding functional group isomerism?

It involves isomers with the same molecular formula but different functional groups Signup and view all the answers

What distinguishes stereoisomers from structural isomers?

Stereoisomers have the same structural formula but different 3D arrangements Signup and view all the answers

What reagents are commonly used for the elimination of water from alcohols to produce alkenes?

Excess concentrated H2SO4 or Al2O3(s) Signup and view all the answers

What type of reaction occurs during the elimination of hydrogen halides from halogenoalkanes?

Elimination reaction Signup and view all the answers

According to Saytzeff's Rule, what determines the major product in an elimination reaction?

The degree of substitution of the alkenes formed Signup and view all the answers

What is the general equation for the elimination of H and X from halogenoalkanes?

R1-R2-X + H → R1=R2 + HX Signup and view all the answers

Which of the following statements about alkenes is false?

Alkenes are typically soluble in polar solvents. Signup and view all the answers

When 2-bromobutane is treated with hot ethanolic KOH, what is likely to happen?

Three different alkenes will be formed in different proportions. Signup and view all the answers

What does the degree of substitution in alkenes refer to?

The number of alkyl groups bonded to the carbon atoms of the C=C bond. Signup and view all the answers

Which solvent is appropriate for the elimination of hydrogen halides from halogenoalkanes?

Ethanol Signup and view all the answers

What is the result of mild oxidation of alkenes using cold KMnO4?

Formation of a diol Signup and view all the answers

Which compounds are formed during oxidative cleavage of alkenes using hot KMnO4?

Ketones and/or carboxylic acids Signup and view all the answers

What observation occurs in an alkaline medium when alkenes undergo mild oxidation with KMnO4?

Brown precipitate of MnO2 Signup and view all the answers

Which statement about potassium dichromate (K2Cr2O7) as an oxidising agent is true?

It is a weaker oxidising agent compared to KMnO4. Signup and view all the answers

What occurs when two hydrogen atoms are bonded to the carbons involved in the C=C bond during oxidative cleavage?

Formation of carbon dioxide and water Signup and view all the answers

What is the condition for performing mild oxidation of alkenes with KMnO4?

Cold conditions Signup and view all the answers

What species is formed from the reaction of CH2=CH2 with water and KMnO4 during mild oxidation?

Ethylene glycol Signup and view all the answers

Which of the following best describes the type of reaction that occurs when alkenes are oxidized with KMnO4?

Oxidation Signup and view all the answers

What is the result of the nucleophile attacking a trigonal planar carbocation?

The product will be achiral. Signup and view all the answers

What does Markovnikov’s rule state about the addition of hydrogen halides to unsymmetrical alkenes?

The H atom is added to the carbon with more hydrogen atoms. Signup and view all the answers

What type of carbocation is formed when a substituted carbon possesses the most alkyl groups?

Tertiary carbocation Signup and view all the answers

During the electrophilic addition mechanism, what must be shown in the mechanism drawing?

Curly arrows indicating the movement of electron pairs. Signup and view all the answers

Why does a racemic mixture occur in reactions leading to chiral centers?

Equal probabilities of attack from either side of the intermediate. Signup and view all the answers

When a carbocation is formed in a reaction, which characteristic primarily determines the stability of the carbocation?

The number of alkyl groups attached to it. Signup and view all the answers

What happens to the optical activity of products formed from a racemic mixture?

It shows no rotation of plane polarized light. Signup and view all the answers

Which aspect is NOT included in the key points for describing the electrophilic addition mechanism?

Determining the reaction temperature. Signup and view all the answers

What effect do electron-donating groups have on carbocations?

They stabilize the carbocation by dispersing the positive charge. Signup and view all the answers

Which of the following groups is an electron-withdrawing group?

Halogens Signup and view all the answers

How does the presence of multiple alkyl groups influence carbocation stability?

It increases stability by enhancing charge dispersion. Signup and view all the answers

When comparing carbocations, which of the following is the correct order of stability?

Tertiary > Secondary > Primary Signup and view all the answers

What is the main consequence of having an electron-withdrawing group on a carbocation?

Decreased stability as it intensifies the positive charge. Signup and view all the answers

Which reaction would likely form a more stable carbocation?

An alkene reacting with hydrochloric acid with multiple alkyl substituents. Signup and view all the answers

In a distinguishing test for alkenes, what is primarily being identified?

The functional group present in the compound. Signup and view all the answers

What major product is expected from the reaction of 3-methylpent-2-ene with hydrogen chloride?

An alkyl chloride as the primary product. Signup and view all the answers

Study Notes

Guiding Questions

Which class of reagents do alkenes react with and why? What types of reactions do alkenes undergo and why?
How do alkenes react with electrophiles?
What is the major product obtained when an unsymmetrical alkene reacts with hydrogen halide and why?

Learning Objectives

Explain the general reactivity of alkenes towards electrophilic reagents (or electrophiles).
Describe the chemistry of alkenes, relevant to the following reactions of ethene:
- electrophilic addition of water/steam
- electrophilic addition of hydrogen halides
- electrophilic addition of halogens
- reduction via catalytic hydrogenation
- oxidation by cold, dilute manganate(VII) ions to form the diol
- oxidation by hot, acidified manganate(VII) ions leading to the rupture of the carbon-to-carbon double bond to determine the position of alkene linkages in larger molecules
Describe the mechanism of electrophilic addition in alkenes, using bromine with ethene as an example.
Apply Markovnikov's rule to the addition of hydrogen halides to unsymmetrical alkenes, and explain the composition of products in terms of the stability of the carbocation intermediates.
Recognize that the mechanisms of polar reactions involve the flow of electrons from electron-rich to electron-poor sites.

References

Peter Cann and Peter Hughes (2015), Cambridge International AS and A Level Chemistry
Kim Seng Chan and Jeanne Tan (2017), Understanding Advanced Organic and Analytical Chemistry, The Learner's Approach, Revised Edition

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Description

This quiz covers key concepts related to alkenes, including their reactions with electrophiles, Markovnikov's rule, and isomerism. Participants will test their understanding of oxidation processes and the role of manganese ions, as well as different types of isomerism in organic chemistry. Perfect for chemistry students looking to solidify their knowledge on alkenes.