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