Alkenes and Addition Reactions

Physical Properties of Alkenes

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. Their physical properties vary with molecular size and structure.

• Low molecular weight alkenes are nonpolar, water-insoluble, and volatile.

• Boiling and melting points increase as the number of carbon atoms in 1-alkenes increases.

• Alkenes with 2-4 carbons are gases at room temperature.

• C5-C16 alkenes are liquids at room temperature.

• C18 alkenes are solids.

Degree of Unsaturation

The degree of unsaturation indicates the number of rings and multiple bonds in a molecule. It is useful for determining the structure of alkenes.

• Unsaturated alkenes have fewer hydrogens than the corresponding alkane.

• General formula for alkenes: CnH2n

• Degree of Unsaturation (DU): Each double bond or ring increases the DU by 1.

Formula for Index of Hydrogen Deficiency (IHD):

• Where C = number of carbons, H = number of hydrogens, X = number of halogens, N = number of nitrogens.

Example: C4H8 has one degree of unsaturation (one double bond or ring).

Molecular Orbital Picture of Alkenes

Alkenes have a characteristic bonding structure due to sp2 hybridization of carbon atoms.

• Each carbon in ethylene (C2H4) is sp2 hybridized, forming a trigonal planar geometry with 120° bond angles.

• σ (sigma) bond: Formed by overlap of sp2 orbitals.

• π (pi) bond: Formed by side-to-side overlap of unhybridized p orbitals.

• Hydrogen atoms use 1s orbitals to bond with carbon.

Key Point: No free rotation around the double bond due to the π bond's orbital overlap.

Cis-Trans Isomerism in Alkenes

Disubstituted Alkenes and Stereoisomerism

Alkenes with two different groups on each carbon of the double bond can exhibit cis-trans (geometric) isomerism.

• Cis isomer: Similar groups on the same side of the double bond.

• Trans isomer: Similar groups on opposite sides of the double bond.

• Example: cis-2-butene vs. trans-2-butene

Diastreomers: Cis and trans alkenes are diastereomers, not mirror images.

E/Z Isomerism and Priority Rules

The E/Z system is used for alkenes with more than two different substituents. Priorities are assigned based on atomic number (Cahn–Ingold–Prelog rules).

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

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

Assigning Priorities:

• Compare atomic numbers of atoms directly attached to the double bond.

• If atoms are the same, move outward to the next atom.

Example: (E)-2-methyl-2-pentene vs. (Z)-2-methyl-2-pentene

Addition Reactions of Alkenes

General Mechanism

Alkenes undergo addition reactions where reagents add across the double bond, breaking the π bond.

• Syn addition: Both atoms added to the same side of the double bond.

• Anti addition: Atoms added to opposite sides.

Examples of Addition Reactions:

• Hydrogenation: Addition of H2

• Hydrohalogenation: Addition of HX (X = halogen)

• Hydration: Addition of H2O

• Halogenation: Addition of X2

• Hydroboration-oxidation: Addition of BH3 followed by H2O2

Markovnikov and Anti-Markovnikov Addition

Regioselectivity in addition reactions is determined by Markovnikov's rule.

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

• Anti-Markovnikov addition: The hydrogen adds to the less substituted carbon (e.g., hydroboration-oxidation).

Types of Addition Reactions

• Type 1: Electrophilic addition (HX, H2O/H+, HOR)

• Type 2: Halogenation or hydrogenation with bridged intermediates

• Type 3: Hydroboration-oxidation (alkene to alcohol)

Alkynes: Structure and Reactions

Structure and Hybridization

Alkynes are hydrocarbons with at least one carbon-carbon triple bond. The carbons in the triple bond are sp hybridized, resulting in a linear geometry (180° bond angle).

• General formula: CnH2n-2

• Terminal alkyne: Triple bond at the end of the chain

• Internal alkyne: Triple bond within the chain

Example: Acetylene (ethyne), H–C≡C–H

Reactions of Alkynes

Alkynes undergo addition and elimination reactions, often involving the breaking of both π bonds in the triple bond.

• Hydrohalogenation: Addition of HX to form geminal or vicinal dihalides

• Halogenation: Addition of X2 (e.g., Br2, Cl2)

• Hydration: Addition of H2O (often catalyzed by acid)

• Hydroboration-oxidation: Addition of BH3 followed by oxidation

Elimination reactions: Conversion of alkene to alkyne via halogenation and elimination steps.

Tautomerization

Tautomerization is the interconversion between enol and keto forms.

• Enol: Alcohol (OH) group attached to a carbon-carbon double bond

• Keto: Carbonyl (C=O) group attached to a carbon atom

• Keto form is generally more stable than the enol form.

Acidity of Alkynes

Terminal alkynes are more acidic than alkenes and alkanes due to the sp hybridization of the carbon atom.

• Terminal alkynes can be deprotonated by strong bases to form acetylide anions.

• Acetylide anions are strong nucleophiles and can attack electrophiles to form new carbon-carbon bonds.

Example: Deprotonation of acetylene with sodium amide (NaNH2) forms the acetylide anion.

Summary Table: Key Differences Between Alkenes and Alkynes

Additional info: The notes have been expanded to include definitions, examples, and context for all major concepts, including molecular orbital theory, stereochemistry, and reaction mechanisms. Tables have been recreated and formulas provided in LaTeX format for clarity.