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Alkene, Alkyne, and Conjugated Pi System Chemistry: Addition Reactions, Mechanisms, and Properties

Study Guide - Smart Notes

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Ch. 12: Chemistry of Alkenes

12-1 Electrophilic Addition to Alkenes

Alkenes undergo electrophilic addition reactions, where small molecules are added to the π bond. The π bond is electron-rich and susceptible to attack by electrophiles.

  • General Reaction: Addition of H-H, H-OH, H-X, Br-Br, etc. to the π bond.

  • Regioselectivity: Determines which carbon of the double bond the new group attaches to.

  • Stereoselectivity: Determines the spatial arrangement of the added groups (syn vs. anti addition).

Example: Addition of HBr to an alkene can yield two isomers depending on which carbon receives the H and which receives the Br.

12-3 Hydrohalogenation

Hydrohalogenation involves the addition of H-X (where X = Cl, Br, I) to the π bond of an alkene.

  • Mechanism: The alkene acts as a nucleophile, attacking the electrophilic H of H-X, forming a carbocation intermediate, which is then attacked by X-.

  • Markovnikov's Rule: The H atom adds to the carbon with more hydrogens (less substituted), and X adds to the more substituted carbon, stabilizing the carbocation intermediate.

Example: Addition of HBr to propene yields 2-bromopropane as the major product.

12-4 Hydration of Alkenes

Hydration adds H-OH to the π bond under acidic conditions, forming alcohols.

  • Mechanism: Follows Markovnikov's rule; the OH group attaches to the more substituted carbon.

  • Conditions: Dilute H2SO4 (low temperature) or concentrated H2SO4.

Example: Hydration of ethene yields ethanol.

12-5 Halogenation of Alkenes

Halogenation involves the addition of X2 (Br2, Cl2) to the π bond.

  • Mechanism: Formation of a halonium ion intermediate, followed by anti-addition of halide.

  • Stereochemistry: Anti-addition leads to enantiomers.

Example: Addition of Br2 to cyclohexene yields trans-1,2-dibromocyclohexane.

12-6 Halohydrin Formation

Halohydrin formation adds X and OH across the double bond.

  • Mechanism: Halonium ion intermediate, followed by nucleophilic attack by water.

  • Product: Halohydrin (X and OH on adjacent carbons).

Example: Addition of Br2 and H2O to an alkene yields a bromohydrin.

12-7, 12-8, 12-9: Other Alkene Addition Reactions

  • Oxymercuration-Demercuration: Markovnikov hydration without rearrangement.

  • Hydroboration-Oxidation: Anti-Markovnikov hydration; syn addition, no rearrangement.

  • Cyclopropane Synthesis: Addition of CH2 (carbene) to alkenes forms cyclopropanes; stereospecific syn addition.

Example: Simmons-Smith reaction uses Zn(Cu) and CH2I2 to generate cyclopropanes.

12-10 Epoxidation and Dihydroxylation

  • Epoxidation: Formation of epoxides (oxiranes) using peracids (e.g., mCPBA).

  • Anti-Dihydroxylation: Epoxide opening with water yields trans-diols (stereospecific).

  • Syn-Dihydroxylation: OsO4 or KMnO4 (cold, dilute) yields cis-diols.

12-12 Ozonolysis

Ozonolysis cleaves double bonds to form carbonyl compounds.

  • Mechanism: Ozone reacts with alkene to form ozonide, which is reduced to aldehydes/ketones.

  • Reagents: O3, DCM, Me2S or Zn/AcOH.

12-13 Radical Addition: Anti-Markovnikov Hydrohalogenation

Radical conditions (HBr, peroxides) lead to anti-Markovnikov addition.

  • Mechanism: Initiation, propagation, and termination steps involving alkyl and bromine radicals.

  • Specificity: Only works for HBr, not HCl or HI.

Ch. 13: Chemistry of Alkynes

13-1 Naming Alkynes

Alkynes are hydrocarbons with a triple bond. Terminal alkynes have the triple bond at the end; internal alkynes have it within the chain.

  • IUPAC Naming: ethyne, propyne, 2-butyne, etc.

  • Priority: Alcohol > Alkyne; Alkyne > Alkene.

13-2 Properties of Alkynes

  • Bond Strength: C≡C triple bond is stronger and shorter than C=C or C–C bonds.

  • Stability: More substituted alkynes are more stable due to hyperconjugation.

  • Acidity: Terminal alkynes are more acidic (pKa ~25) than alkenes or alkanes due to higher s-character.

13-4 Preparation of Alkynes

  • Double Dehydrohalogenation: Elimination of two equivalents of HX from dihalides.

  • Anti-Elimination: Stereospecific formation of alkynes.

13-5 Alkynyl Anion Reactions

  • Alkynyl Anion: Generated by deprotonation of terminal alkynes (e.g., with NaNH2).

  • Reactivity: Good nucleophile for SN2 reactions with alkyl halides.

13-6 Hydrogenation of Alkynes

  • Complete Hydrogenation: Forms alkanes (Pt, Pd, Ni catalysts).

  • Partial Hydrogenation: Lindlar catalyst yields cis-alkenes (syn addition); dissolving metal reduction yields trans-alkenes (anti addition).

13-7, 13-8 Electrophilic Addition to Alkynes

  • Hydrohalogenation: Follows Markovnikov's rule (H adds to less substituted C).

  • Halogenation: Anti addition of X2 to form dihalides.

  • Hydration: Addition of H-OH via oxymercuration; forms ketones or aldehydes after tautomerization.

  • Hydroboration-Oxidation: Anti-Markovnikov hydration; forms aldehydes from terminal alkynes.

13-9 Alkynyl Halide Chemistry

  • Organometallic Reactions: Formation of Grignard and organolithium reagents from alkynyl halides.

  • Heck Reaction: Palladium-catalyzed cross-coupling of alkynyl halides with alkenes.

Ch. 14: Chemistry of Conjugated Pi Systems

14-1, 14-2 Allylic Systems

  • Allylic Carbocation: Stabilized by resonance; more stable than typical carbocations.

  • Allylic Anion and Radical: Also stabilized by resonance; important in nucleophilic and radical reactions.

  • Radical Allylic Halogenation: NBS (N-bromosuccinimide) is used for selective bromination at the allylic position.

14-5 Conjugated Dienes

  • Structure: Conjugated dienes have alternating double and single bonds, allowing delocalization of π electrons.

  • Stability: Conjugated dienes are more stable than isolated dienes due to resonance.

  • Naming: trans-1,3-pentadiene, 1,5-cyclooctadiene, etc.

14-6 Electrophilic Addition to Dienes: Kinetic vs. Thermodynamic Control

  • 1,2-Addition vs. 1,4-Addition: At low temperature, kinetic product (1,2-addition) dominates; at high temperature, thermodynamic product (1,4-addition) dominates.

  • Carbocation Stability: Allylic carbocations are stabilized by resonance, affecting product distribution.

Tables

Bond Type

Bond Dissociation Energy (kcal/mol)

Bond Length (Å)

C≡C (sp–sp + 2π)

229

1.203

C=C (sp2–sp2 + π)

173

1.34

C–C (sp3–sp3)

90

1.54

Compound

s Character (%)

pKa

Acidity

CH≡C–H

50

25

Most acidic

CH2=CH–CH3

33

44

Less acidic

CH3–CH2–CH3

25

65

Least acidic

Key Equations

  • Markovnikov's Rule:

  • Bond Dissociation Energy:

  • General Formula for Alkynes:

Summary Table: Types of Alkene and Alkyne Addition Reactions

Reaction Type

Reagents

Regioselectivity

Stereoselectivity

Product

Hydrohalogenation

H-X

Markovnikov

None

Alkyl halide

Halogenation

X2

None

Anti

Dihalide

Hydration

H2O, acid

Markovnikov

None

Alcohol

Hydroboration-Oxidation

BH3, H2O2

Anti-Markovnikov

Syn

Alcohol

Ozonolysis

O3, Me2S

None

None

Aldehyde/Ketone

Additional info:

  • Notes include skipped sections and exam coverage, indicating focus on core addition reactions and mechanisms.

  • Some advanced topics (e.g., molecular orbitals of conjugated systems) are marked as not covered in the final exam but are included for completeness.

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