MPharm PHA111: Functional Group Chemistry 2

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Questions and Answers

What is the primary reason for alkenes being more reactive than alkanes?

Presence of stronger sigma bonds

Presence of π bonds in their structure (correct)

Higher molecular weight

Absence of double bonds

In electrophilic addition reactions, which type of intermediate is formed during heterolytic cleavage?

Cyclic intermediates

Carbocations (correct)

Carbanions

Free radicals

Which of the following statements about the addition of HX to alkenes is TRUE?

Only radical intermediates are formed.

The reaction proceeds via homolytic cleavage only.

It involves a two-step process. (correct)

Asymmetric alkenes result in symmetrical products.

What facilitates the attack of the π electrons on the electrophile in electrophilic addition?

Greater electron density of the π bond Signup and view all the answers

Which type of addition reaction involves the formation of free radicals?

Homolytic addition Signup and view all the answers

What is the first step in the electrophilic addition mechanism to a C=C double bond?

Attack of π electrons on the electrophile Signup and view all the answers

Which reagent is NOT typically involved in the addition reactions of alkenes?

H2O Signup and view all the answers

What type of reaction occurs during the cracking process used for alkene preparation?

Elimination Signup and view all the answers

What type of bonds do alkenes contain?

C-C double bonds and C-H bonds Signup and view all the answers

Which of the following correctly describes the stability of alkenes?

More substituted alkenes are more stable due to hyperconjugation Signup and view all the answers

What is the general formula for alkenes?

CnH2n Signup and view all the answers

How do cis isomers of alkenes compare in stability to trans isomers?

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

What reaction type specifically involves the addition of hydrogen halides to alkenes?

Hydrohalogenation Signup and view all the answers

What principle explains the regioselectivity of electrophilic addition reactions?

Markovnikov’s rule and carbocation stability Signup and view all the answers

Which of the following statements about the polarity of alkenes is true?

Alkenes are slightly polar due to instantaneous dipole-dipole interactions Signup and view all the answers

What property of alkenes contributes to their generally low boiling points?

Weak intermolecular forces Signup and view all the answers

Which method can cause the interconversion of isomers in alkenes?

Irradiation with UV light Signup and view all the answers

What is the pKa of alkenes, indicating their reactivity?

<blockquote> 44 </blockquote> Signup and view all the answers

What determines the stability of a carbocation in electrophilic addition reactions?

The number of alkyl groups bonded to the carbon Signup and view all the answers

In a Markovnikov addition, where does the electrophile typically add?

To the sp2 carbon with the most hydrogen atoms Signup and view all the answers

Which type of carbocation is considered the most stable?

Tertiary carbocation Signup and view all the answers

What is the role of the catalyst in the hydrogenation of an alkene?

To facilitate syn addition Signup and view all the answers

What type of addition does HBr undergo when peroxide is present?

Anti-Markovnikov addition Signup and view all the answers

What is a key characteristic of anti-Markovnikov addition?

Occurs in the presence of peroxides Signup and view all the answers

Which of the following describes the reaction of alkenes with halogens?

Produces vicinal dihalides Signup and view all the answers

What occurs during keto-enol tautomerization of alkynes?

Formation of ketones Signup and view all the answers

Which catalyst can be used to stop the hydrogenation of an alkyne at the alkene stage?

Lindlar catalyst Signup and view all the answers

What effect does the inductive effect have on carbocation stability?

Decreases positive charge concentration Signup and view all the answers

What is required for the addition of H2O to an alkyne?

Acid catalyst Signup and view all the answers

What type of reaction occurs when alkynes react with hydrogen halides (HX)?

A series of sequential additions possibly leading to monoaddition Signup and view all the answers

What is a characteristic of the triple bond in alkynes?

Acts as a stronger nucleophile due to electron density Signup and view all the answers

What is the outcome of syn (cis) addition during hydrogenation of alkynes?

Formation of cis-alkene Signup and view all the answers

Which alkene addition reaction is known to be stereospecific?

Hydrogenation reaction Signup and view all the answers

Alkynes can be converted into which of the following through halogenation?

Haloalkenes Signup and view all the answers

What type of carbocation is involved in the first step of Markovnikov addition?

Vinylic carbocation Signup and view all the answers

Which feature makes terminal alkynes unique in terms of acidity compared to internal alkynes?

Acidity results from sp hybridization Signup and view all the answers

Why do alkynes have a lower reactivity than alkenes in certain reactions?

The triple bond is stronger than a double bond Signup and view all the answers

Under what condition does anti-Markovnikov addition specifically occur?

In the presence of a radical initiator Signup and view all the answers

What is the stereospecific aspect of the addition of bromine to an alkene?

Anti addition is observed Signup and view all the answers

What kind of addition is characterized by a trans (anti) mechanism?

Hydrogenation Signup and view all the answers

What type of carbocation is most stable during the addition reactions of alkynes?

Tertiary carbocation Signup and view all the answers

What does adding H2O to an alkyne produce after tautomerization?

Ketones Signup and view all the answers

What is formed from the reduction of alkynes with Na/NH3?

Trans-alkenes Signup and view all the answers

What is the mechanism for halogenation reactions of alkynes?

Electrophilic addition Signup and view all the answers

Which step is necessary to convert an alkyne to a cis-alkene?

Use of Lindlar catalyst 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
Contact: [email protected], 0191 5152760
Date: Dale 1.21
Course: PHA111, Functional Group Chemistry

Learning Objectives

Identify different functional groups in drug molecules, focusing on alkenes/alkynes
Use curly arrows to depict bond making/breaking processes, focusing on alkene chemistry
Demonstrate understanding of alkene/alkyne reactivity, including specific reactions and mechanisms like electrophilic addition, hydrohalogenation, hydration, halogenation and hydrogenation
Explain regioselectivity of electrophilic addition reactions, referencing Markovnikov's rule and carbocation/radical stability, and predict product formation accordingly

Alkenes

Unsaturated Compounds: Contain carbon-carbon double bonds (sp² hybridized carbons), only C-C and C-H bonds, composed of a sigma (σ) bond and a pi (π) bond
Reactivity: Controlled by the electron-rich C=C double bond, which acts as a nucleophile. More substituted alkenes are more stable
Hyperconjugation: Stabilizes more substituted alkenes
Properties: Slightly polar, instantaneous dipole-dipole interactions occur, alkyl groups donate electrons towards the π bond, bond between sp² and sp³ carbons is stronger than a bond between two sp³ carbons
General formula: CnH2n
pKa: > 44 (highly inert); [HCl pKa = -7, H₂O pKa= 15.7, alkanes pKa > 50]. Alkenes generally have low boiling points increasing with molecular weight and length of the carbon chain. Branched alkenes have lower boiling points.
Isomers: Cis isomers (substituents on same side) are less stable than trans isomers (substituents on opposite sides) due to steric strain. Interconversion can occur thermally or using UV/Acid catalysts

Alkenes: Isomers and Vision

Cis-retinal: FITS into and binds with a receptor site of opsin; Visible light causes isomerization of the molecule.
Isomerization: Changes the configuration of retinal from cis to trans. The neurons of the optic nerve fire producing a visual image.

Alkenes: Reactivity

Preparation: Cracking is an industrial process and is used to produce alkenes.
Elimination: Includes dehydrohalogenation and dehydration reactions to form alkenes

Alkenes: Addition Reactions

Involves Electrophilic Addition: Electrons in the π bond are easily accessible to electrophiles (e.g., E+,). Electron movement from double bond (weaker π bond) to electrophile (E+ or δ+), yielding the major product.
Types:
- Homolytic (involves radicals)
- Heterolytic (involves cations).
Markovnikov's Rule: In addition reactions, the electrophile (proton or other atom) bonds to the carbon atom with more hydrogen substituents, forming the more stable carbocation intermediate.

Carbocation Stability

Outcome: Product stability depends on carbocation intermediates. Trivalent carbon (sp²) is hybridized with a vacant p orbital to stabilize the intermediate
Types:
- Tertiary (3°) has three carbon substitutions
- Secondary (2°) has two carbon substitutions
- Primary (1°) has one carbon substitution
- Methyl (0°) has no carbon substitutions
Hyperconjugation: Alkyl groups donate electrons to stabilize the positive charge on the carbocation (electron donating). Inductive effect also contributes to carbocation stabilization.

Alkenes: Hydration

Follows Markovnikov's rule.
Reverse of dehydration of alcohol
Acid catalysed; use very dilute solutions of H2SO4 to drive the equilibrium towards hydration

Alkenes: Anti-Markovnikov Addition

Anti-Markovnikov addition occurs in the presence of peroxides, where HBr adds to the less substituted carbon.
Follows a radical mechanism, initiated by peroxide breaking into radicals.

Alkenes: Addition of Halogens

Cl2, Br2, and I₂, form vicinal dihalides. Reaction is stereospecific; Anti addition of electrophile to alkene.
π electrons attack bromine molecule; loss of bromide ion, forming a bromonium ion (intermediate). Halide attacks opposite side, forming a vicinal dihalide.

Alkenes: Hydrogenation

Syn (cis) addition: Reaction proceeds with a catalyst (Pt, Pd, or Ni) to produce alkanes. Stereochemistry is cis-specific.
Catalyst is required because the reaction is not spontaneous; the reactions can be stopped at the alkene stage with a "poisoned" catalyst called a Lindlar catalyst.

Alkynes: Structure

Unsaturated Compounds: Contain triple C-C bonds, internal/terminal triple bonds and two weaker pi (π) bonds. The triple bond acts as a nucleophile, making alkynes very reactive.
General formula: CnH2n-2
Reactivity: Very reactive; similar reactivity to alkenes. Alkynes are virtually insoluble in water.

Alkynes: Physical Properties

Alkynes have stronger Van der Waals forces than alkenes, and Internal alkyne BP is higher than terminal alkyne BP

Alkynes: Relative Acidity

Alkynes are more acidic than alkenes and alkanes. The s-orbital character of sp hybridized carbon increases the acidity

Alkynes: Reactivity

Alkynes completely burn in O2.
They undergo similar reactions to alkenes like, H-X addition.
X-X addition, which is bromination reactions
H-H additions which are hydrogenation reactions.
H-OH additions which are hydration reactions

Alkylation of Terminal Alkynes

Involves nucleophilic substitution. Alkynes undergo deprotonation in the presence of strong bases producing alkynide ions, which can be alkylated via SN2 mechanisms

Alkynes: Addition of HX

Sequential Addition of H-X
Follows Markovnikov's Rule
Vinylic Carbocations involved in first step
Can be primary or secondary

Alkynes: Addition Reactions

Anti-Markovnikov H-Br addition possible but requires peroxide initiator
Halogenation follows a similar mechanism as with alkenes

Alkynes: Anti-Markovnikov Addition

Anti-Markovnikov addition occurs only under very specific conditions. HBr adds as a nucleophile assisted by a radical initiator such as H₂O₂

Alkynes: Hydration

Similar mechanism to alkenes, but leads to ketone formation via tautomerization (enol intermediate)

Alkynes: Hydrogenation

Syn addition. Reaction can be stopped at the alkene stage with a "poisoned" catalyst (Lindlar catalyst)
Trans addition with hydrogen is possible using a radical mechanism

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Description

Test your knowledge of functional groups in drug molecules, particularly focusing on alkenes and alkynes. Explore reactivity, mechanisms, and regioselectivity in electrophilic addition reactions. This quiz assesses your understanding of key concepts like Markovnikov's rule related to alkene chemistry.