Comprehensive Study Guide: Alkenes, Alkynes, Aromatic Compounds, and Stereochemistry

Chapter 3: Alkenes

IUPAC Nomenclature of Alkenes

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. Proper naming is essential for clear communication in organic chemistry.

• Parent Hydrocarbon: Identify the longest carbon chain containing the double bond; this forms the parent name.

• Numbering: Number the chain from the end nearest the double bond to give the double bond the lowest possible number.

• Suffix: Use the suffix -ene to indicate the presence of a double bond (e.g., ethene, propene).

• Multiple Double Bonds: Use prefixes such as diene, triene, etc., for multiple double bonds.

Example: CH2=CH-CH2-CH3 is named 1-butene.

Common Names for Alkenes

• Ethylene: Common name for ethene (CH2=CH2).

• Propylene: Common name for propene (CH2=CH-CH3).

• Isoprene: 2-methyl-1,3-butadiene, a key monomer in natural rubber.

Electronic Structure of Alkenes

• Each carbon in the double bond is sp2 hybridized, forming three sigma bonds and one unhybridized p orbital.

• The unhybridized p orbitals overlap to form a pi bond, which is above and below the plane of the molecule.

Cis/Trans and E/Z Isomerism

• Cis/Trans Isomers: Occur when each carbon of the double bond has two different substituents. Cis means similar groups are on the same side; trans means on opposite sides.

• Stability: Trans alkenes are generally more stable than cis due to less steric hindrance.

• E/Z Isomers: Used when there are more than two different substituents. Assign priorities using the Cahn-Ingold-Prelog rules.

When to Use: Use cis/trans for simple cases; use E/Z for complex cases with more than two different groups.

Cahn-Ingold-Prelog Rules for E/Z Nomenclature

• Assign priorities to substituents on each carbon of the double bond based on atomic number.

• Z (zusammen): Higher priority groups on the same side.

• E (entgegen): Higher priority groups on opposite sides.

Types of Organic Reactions

• Addition: Atoms are added to a double or triple bond.

• Elimination: Atoms are removed, forming a double or triple bond.

• Substitution: One atom or group replaces another.

• Rearrangement: The carbon skeleton is reorganized.

Reaction Mechanisms

• Polar Reactions: Involve movement of electron pairs (nucleophiles and electrophiles).

• Radical Reactions: Involve movement of single electrons.

• Heterolytic Cleavage: Both electrons go to one atom.

• Homolytic Cleavage: Each atom gets one electron.

Nucleophiles and Electrophiles

• Nucleophile: Electron-rich species that donates an electron pair.

• Electrophile: Electron-poor species that accepts an electron pair.

Addition of HCl to Ethylene: Mechanism

• Step 1: Protonation of ethylene to form a carbocation intermediate.

• Step 2: Chloride ion attacks the carbocation to form chloroethane.

Equation:

$\mathrm{CH_2=CH_2 + HCl \rightarrow CH_3CH_2Cl}$

Carbocation Intermediates and Energy Diagrams

• Carbocation: Positively charged carbon intermediate.

• Stability Order: methyl < primary < secondary < tertiary.

• Energy Diagram: Shows energy changes during a reaction, including transition states and intermediates.

• Activation Energy (Ea): Minimum energy required for a reaction to proceed.

Catalysts

• Lower the activation energy, increasing the rate of reaction without being consumed.

Chapter 4: Reactions of Alkenes and Alkynes

Hydrohalogenation

• Addition of HX (X = Cl, Br, I) to alkenes.

• Markovnikov's Rule: The hydrogen atom adds to the carbon with more hydrogens; the halide adds to the more substituted carbon.

Equation:

$\mathrm{CH_2=CH_2 + HBr \rightarrow CH_3CH_2Br}$

Carbocation Structure and Stability

• Carbocations are stabilized by alkyl groups via hyperconjugation and inductive effects.

• Order of stability: methyl < primary < secondary < tertiary.

Hydration

• Addition of water (H2O) across a double bond, usually acid-catalyzed.

• Follows Markovnikov's rule.

Mechanism: Protonation, nucleophilic attack by water, deprotonation.

Oxymercuration-Demercuration

• Adds H and OH across a double bond without carbocation rearrangement.

• Predict the product based on the reagents used.

Halogenation

• Addition of X2 (Cl2, Br2) to alkenes.

• Forms a cyclic halonium ion intermediate (e.g., bromonium ion).

• Results in anti stereochemistry (opposite sides).

Hydrogenation (Reduction)

• Addition of H2 across a double bond using a metal catalyst (e.g., Pd/C).

• Results in syn stereochemistry (same side addition).

Hydroxylation (Oxidation)

• Oxidation of alkenes to diols using reagents like potassium permanganate (KMnO4).

• Reaction conditions (acidic or basic) affect the outcome.

Polymers and Monomers

• Monomer: Small molecule that joins to form a polymer.

• Polymerization Mechanism: Initiation, propagation, termination.

Example: Polyethylene from ethylene monomers.

Conjugated Dienes

• Compounds with two double bonds separated by one single bond.

• Conjugation allows for delocalization of electrons, increasing stability.

• Undergo 1,2- and 1,4-addition reactions.

Allylic Carbocations and Resonance

• Allylic carbocations are stabilized by resonance.

• Resonance forms and hybrids must be drawn according to specific rules (move only pi electrons or lone pairs, never break single bonds).

Alkynes

• Hydrocarbons with a carbon-carbon triple bond.

• Addition reactions include:

  • H2 (with Pd/C for full reduction, Lindlar catalyst for cis-alkene)

  • HX (Markovnikov's rule applies)

  • X2 (halogenation)

  • H2O (hydration)

• Formation of acetylide anions by deprotonation with a strong base.

Chapter 5: Aromatic Compounds

Kekulé's Proposal and Resonance Structure of Benzene

• Kekulé proposed a six-membered ring with alternating single and double bonds.

• Modern understanding: Benzene is a resonance hybrid with delocalized pi electrons.

Naming Aromatic Compounds

• Use benzene as the parent name.

• Common names: toluene, aniline, phenol, etc.

• Ortho (1,2-), meta (1,3-), para (1,4-) indicate positions of substituents.

• For more than two substituents, number the ring to give the lowest set of locants.

Electrophilic Aromatic Substitution (EAS)

• Mechanism: Aromatic ring acts as a nucleophile, attacking an electrophile, forming a sigma complex, then restoring aromaticity.

• Catalyst is necessary to generate a strong electrophile.

• Substitution is favored over addition to preserve aromaticity.

Types of EAS Reactions

• Halogenation: Bromination and chlorination require a Lewis acid catalyst (e.g., FeBr3).

• Nitration: Introduction of a nitro group using HNO3/H2SO4.

• Friedel-Crafts Alkylation/Acylation: Introduction of alkyl or acyl groups using AlCl3 as a catalyst.

Substituent Effects in EAS

• Substituents affect reactivity and orientation of further substitution.

• Ortho-para activators: Electron-donating groups (e.g., -OH, -NH2).

• Ortho-para deactivators: Halogens (e.g., -Cl, -Br).

• Meta deactivators: Electron-withdrawing groups (e.g., -NO2, -COOH).

Polycyclic Aromatic Compounds and Heterocycles

• Compounds with fused aromatic rings (e.g., naphthalene, anthracene).

• Heterocycles contain heteroatoms (e.g., pyridine, furan).

Hückel's Rule (4n+2 Rule)

• Aromatic compounds must have (4n+2) pi electrons (n = integer).

Chapter 6: Stereochemistry

Chirality and Chiral Environments

• Chirality: A molecule is chiral if it is not superimposable on its mirror image.

• Chiral environments lack symmetry elements (plane of symmetry, center of symmetry).

Enantiomers

• Non-superimposable mirror images.

• Example: Lactic acid has two enantiomers (D- and L-lactic acid).

Stereocenter (Chiral Center)

• A carbon atom bonded to four different groups.

• Everyday chiral objects: hands, shoes, gloves.

Optical Activity and Polarimetry

• Chiral compounds rotate plane-polarized light.

• A polarimeter measures the angle of rotation.

Specific Rotation

• The observed rotation under standard conditions (concentration, path length, temperature, wavelength).

Racemic Mixture

• Equal mixture of two enantiomers; optically inactive due to cancellation of rotations.

R and S Configuration Assignment

• Assign priorities to groups attached to the chiral center using Cahn-Ingold-Prelog rules.

• Orient the lowest priority group away from you.

• If the sequence 1-2-3 is clockwise, configuration is R; if counterclockwise, S.

Enantiomers and Diastereomers

• Enantiomers: Non-superimposable mirror images.

• Diastereomers: Stereoisomers that are not mirror images.

Meso Compounds

• Achiral compounds with chiral centers due to an internal plane of symmetry.

• Example: Tartaric acid meso form.

Separation of Racemic Mixtures

• Convert enantiomers to diastereomers using a chiral resolving agent.

• Diastereomers have different physical properties and can be separated by standard techniques (e.g., crystallization).

• After separation, convert back to enantiomers.

Additional info: For resonance, always move electrons in pi bonds or lone pairs, never break sigma bonds. For EAS, the energy profile shows a higher activation energy for addition than for substitution, explaining why substitution is favored in aromatic systems.