Functional group 2

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Questions and Answers

Which structural feature of alkenes contributes to their higher reactivity compared to alkanes?

Absence of hydrogen atoms

Increased steric hindrance

Greater electron density in double bonds (correct)

Presence of triple bonds

What type of addition reaction involves the breaking of a π bond to form radicals?

Heterolytic addition

Nucleophilic attack

Electrophilic substitution

Homolytic addition (correct)

What initiates the visual process when light hits the opsin receptor?

Rhodopsin generates additional electrons

Cis-retinal isomerizes (correct)

Neurons in the optic nerve are deactivated

Alkenes react with light

Which of the following reactions is NOT a method for preparing alkenes?

Hydrogenation Signup and view all the answers

In electrophilic addition reactions of alkenes, which species acts as an electrophile?

Cations Signup and view all the answers

What characterizes the double bond in alkenes compared to single bonds in alkanes?

Double bonds have more accessible electrons Signup and view all the answers

Which process is commonly used in the industrial preparation of alkenes?

Cracking Signup and view all the answers

Which type of bond is primarily involved in the reactivity of alkenes during addition reactions?

π bond Signup and view all the answers

What is the general formula for alkynes?

C_nH_{2n-2} Signup and view all the answers

Which of the following statements about the acidity of hydrogen in alkynes is true?

sp hybridized hydrogens have a pKa of approximately 25. Signup and view all the answers

How do the boiling points of internal alkynes compare to those of terminal alkynes?

Internal alkynes have higher boiling points due to stronger Van der Waals forces. Signup and view all the answers

Which reaction involves the addition of H-X to alkynes?

Hydration reaction Signup and view all the answers

How does the reactivity of alkynes compare to that of alkenes?

Alkynes are less reactive than alkenes. Signup and view all the answers

What is a characteristic feature of alkynes regarding their structure?

Contain triple C-C bonds. Signup and view all the answers

Which reaction mechanism do terminal alkynes undergo during alkylation?

Nucleophilic substitution Signup and view all the answers

What is the impact of the presence of an adjacent hydrogen atom on the acidity of terminal alkynes?

Increases acidity significantly. Signup and view all the answers

What is a characteristic property of alkenes?

They contain only C-C and C-H bonds. Signup and view all the answers

Which reaction mechanism is specifically noted for alkenes?

Electrophilic addition. Signup and view all the answers

According to Markovnikov’s rule, what is a determining factor in electrophilic addition reactions?

The stability of the carbocation intermediate. Signup and view all the answers

Which of the following statements about cis and trans isomerism in alkenes is correct?

Cis isomers are generally less stable than trans isomers. Signup and view all the answers

What factor contributes to the stability of more substituted alkenes?

Hyperconjugation. Signup and view all the answers

Which of the following describes the boiling points of alkenes?

They generally increase with molecular weight. Signup and view all the answers

What behavior do the sp2 carbon and sp3 carbon bonds exhibit?

They are stronger than sp3-sp3 bonds. Signup and view all the answers

What type of interactions can occur in alkenes due to the polarization of the p bond?

Instantaneous dipole-dipole interactions. Signup and view all the answers

What role does the double bond in alkenes primarily play in chemical reactions?

It acts as a nucleophile. Signup and view all the answers

What determines the outcome of a reaction involving an asymmetric alkene when adding HX?

The stability of the carbocation formed Signup and view all the answers

What is the primary feature of Markovnikov’s Rule in electrophilic addition reactions?

The most stable carbocation is formed Signup and view all the answers

Which type of carbocation is the most stable and preferred in reactions?

Tertiary carbocation Signup and view all the answers

In the hydration of an alkene, which step is crucial for ensuring Markovnikov addition occurs?

Protonation of water Signup and view all the answers

What type of addition occurs when HBr is added to an alkene in the presence of peroxides?

Anti-Markovnikov addition Signup and view all the answers

Which mechanism describes the addition of halogens like Cl2 or Br2 to alkenes?

Stereospecific trans addition Signup and view all the answers

What intermediate is formed during the addition of halogens to alkenes?

Bromonium ion Signup and view all the answers

What is the significance of hyperconjugation in carbocation stability?

Reduces positive charge concentration Signup and view all the answers

Which addition type produces syn addition in alcohol formation from alkenes?

Hydrogenation Signup and view all the answers

Why is HCl less effective than HBr in radical addition reactions to alkenes?

HCl bond energy is too strong Signup and view all the answers

What characterizes the regioselectivity of the addition of HX to alkenes?

Only one constitutional isomer is produced Signup and view all the answers

What role do alkyl groups play in carbocation stability?

They donate electron density Signup and view all the answers

What is the likely outcome for a reaction involving a primary carbocation?

Low stability and high reactivity Signup and view all the answers

What is the primary requirement for Anti-Markovnikov addition of H-Br to alkynes?

Presence of a radical initiator like H2O2 Signup and view all the answers

What process leads to the conversion of enols to ketones in alkynes?

Tautomerization Signup and view all the answers

Which catalyst is used to stop the reaction at the alkene stage during hydrogenation of alkynes?

Lindlar catalyst Signup and view all the answers

Which of the following describes the typical outcome of hydration of alkynes?

Formation of ketones through tautomerization Signup and view all the answers

What type of addition does the H-Br reaction represent for alkynes under normal conditions?

Markovnikov addition Signup and view all the answers

Which of the following products is formed when a terminal alkyne is treated with NaNH2?

Acetylide Signup and view all the answers

In the presence of a radical initiator, which addition to alkynes occurs?

Anti-Markovnikov addition Signup and view all the answers

What intermediate is formed during the initial stages of alkynes undergoing hydration?

Vinylic carbocation Signup and view all the answers

What type of addition occurs when hydrogen is added to alkynes in a trans manner?

Radical mechanism addition Signup and view all the answers

What does the term 'geminal dihaloalkanes' refer to in alkyne chemistry?

Alkynes with two halogens on the same carbon Signup and view all the answers

Which statement about vinylic carbocations is true?

They are involved in the first step of certain addition reactions. Signup and view all the answers

What product is typically not formed from hydrogenation of alkynes?

Aromatic compounds Signup and view all the answers

Which of the following processes does NOT involve the same mechanism as with alkenes?

Alkyne polymerization Signup and view all the answers

What indicates that the most stable carbocation is being formed during the addition to alkynes?

Following the Markovnikov rule Signup and view all the answers

Study Notes

MPharm Programme - PHA111 Functional Group Chemistry 2

Course Instructor: Dr. Stephanie Myers, Senior Lecturer in Medicinal Chemistry
Date: Dale 1.21
Contact: [email protected], Telephone: 0191 5152760

Learning Objectives

Identify functional groups in drug molecules: Focus on alkenes/alkynes.
Use curly arrows: To denote bond making/breaking processes, focusing on alkene chemistry.
Reactivities of alkenes/alkynes: Understanding of specific example reactions and their mechanisms (electrophilic addition, hydrohalogenation, hydration, halogenation, hydrogenation).
Regioselectivity of electrophilic addition reactions: Explaining Markovnikov's rule and carbocation/radical stability to predict product formation.

Unsaturated Compounds - Alkenes

Structure: Contain C-C double bonds (sp² hybridised carbons), only C-C and C-H bonds, a σ bond and a weaker π bond.
Reactivity: Controlled by electron rich C-C double bond, which acts as a nucleophile (electron rich).
Stability: More substituted alkenes are more stable due to hyperconjugation.
Bond lengths and angles: A double bond consists of one σ bond and one π bond. Typical bond lengths and angles are shown in the slides (e.g., C-C bond length, C-H-C bond angle).

Properties of Alkenes

Polarity: Slightly polar due to polarisable π bond, leading to dipole-dipole interactions.
Alkyl groups: Electron-donating, contributing to a small dipole moment.
Bond strength: Bond between sp² and sp³ carbon is stronger than between two sp³ carbons.
Boiling points: Generally increase with molecular weight and with the length of the main carbon chain. Branched alkenes have lower boiling points.
Solubility: Virtually insoluble in water.

Alkenes: Isomers

Cis-trans isomers: Cis isomers are typically less stable than trans isomers due to strain between substituents.
Barrier to rotation: Interconversion between cis and trans isomers does not occur spontaneously but can be thermally or by UV light, with strong acids.

Alkenes: Isomers and Vision

Cis-retinal: Only cis-retinal fits into and binds with a receptor site.
Rhodopsin isomerisation: Visible light is absorbed causing rhodopsin isomerization, triggering nerve signals.

Alkenes: Reactivity

Preparation: Cracking is an industrial process to produce alkenes.
Elimination Reactions: Dehydrohalogenation and dehydrations are common elimination reactions.
Addition Reactions: Electrophilic addition reactions, involving the attack of an electrophile on the π bond.
Markovnikov's Rule: The proton of an acid (H-X) adds to the least substituted sp2 carbon in the double bond.
Carbocation stability: Outcome of product formation determined by carbocation stability (primary, secondary, tertiary).

Alkynes: Structure

Structure: Contain C-C triple bonds; internal/terminal triple bonds.
Reactivity: Controlled by electron rich C-C triple bond, which acts as a nucleophile.
Properties: Very reactive, similar to alkenes.
General formula: CnH2n-2
Acidity: Terminal alkynes are more acidic than alkenes or alkanes.
Solubility: Virtually insoluble in water.

Alkynes: Physical Properties

Boiling points: Internal alkynes have higher boiling points than terminal alkynes of comparable molecular weights due to greater Van der Waals forces.

Alkynes: Reactivity

Addition reactions: Alkynes are less reactive than alkenes but undergo similar addition reactions, sequential addition of H-X, as well as addition of halogenation and hydration.
Anti-Markovnikov addition: Requires peroxide as a radical initiator
Alkylation of terminal alkynes: Nucleophilic substitution of terminal alkynes takes place through deprotonation with a strong base.
Hydrogenation: Syn (cis) addition reaction leads to alkene formation.

Studying That Suits You

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Description

Test your knowledge on the structural features, reactivity, and addition reactions of alkenes and alkynes in this engaging quiz. Explore fundamental concepts such as electrophilic addition reactions and the general formulas of hydrocarbons. Ideal for students studying organic chemistry.